One Hundred Years of the Fritz Haber InstituteBretislav Friedrich,* Dieter Hoffmann, and Jeremiah James

Dedicated to Professor Gerhard Ertl on the occasion of his 75th birthday

1. Introduction

The Kaiser Wilhelm Institute for Physical Chemistry andElectrochemistry was established in 1911 as one of the firsttwo institutes of the Kaiser Wilhelm Society (KWG). Itssuccessor, the Fritz Haber Institute (FHI), is not only one ofthe oldest and most tradition-rich institutes of the Max PlanckSociety (MPG), but also one of the most distinguished, withthe highest number of affiliated Nobel laureates of any KWG/MPG institute. These include Fritz Haber, the foundingdirector, the later directors Max von Laue, Ernst Ruska, andGerhard Ertl, and several scientists who served at theInstitute in lesser capacities, such as James Franck, EugeneWigner, and Heinrich Wieland (Table 1).

The Institute has been not only a hub of scientificexcellence and productivity but also an active participant inthe history of the 20th century. It played a central role in theGerman development of chemical warfare during WorldWar I. It was particularly hard-hit by Nazi racial policies andwas revamped into a “Nazi Model Enterprise” during theThird Reich; and finally, to remain productive during theCold War, its directors had to find ways to assert itsimportance in the territorially insular and politically preca-rious city of West Berlin.

The shifting fortunes and socio-political roles of theInstitute help to explain the striking breadth of topics thathave been researched within its walls over the past century,but so too do the diverse abilities and personalities of thescientists who have made the Institute, however briefly, their

We outline the institutional history and highlight aspects of the scientific history of the Fritz HaberInstitute (FHI) of the Max Planck Society, successor to the Kaiser Wilhelm Institute for PhysicalChemistry and Electrochemistry, from its founding in 1911 until about the turn of the 21st century.Established as one of the first two Kaiser Wilhelm Institutes, the Institute began as a much-awaitedremedy for what prominent German chemists warned was the waning of Germany�s scientific andtechnological superiority relative to the United States and to other European nations. The history ofthe Institute has largely paralleled that of 20th century Germany. It spearheaded the research anddevelopment of chemical weapons during World War I, then experienced a “golden era” during the1920s and early 1930s, in spite of financial hardships. Under the National Socialists it suffered apurge of its scientific staff and a diversion of its research into the service of the new regime,accompanied by a breakdown in its international relations. In the immediate aftermath of WorldWar II it suffered crippling material losses, from which it recovered slowly in the postwar era. In1952, the Institute took the name of its founding director and the following year joined the fledglingMax Planck Society, successor to the Kaiser Wilhelm Society. During the 1950s and 1960s, theInstitute supported diverse research into the structure of matter and electron microscopy in itsgeographically isolated and politically precarious location in West Berlin. In subsequent decades, asBerlin benefited from the policies of d�tente and later glasnost and the Max Planck Societycontinued to reassess its preferred model of a research institute, the FHI reorganized around a boardof coequal scientific directors and renewed its focus on the investigation of elementary processes onsurfaces and interfaces, topics of research that had been central to the work of Fritz Haber and thefirst “golden era” of the Institute. Throughout its one-hundred-year history, the Institute�s pace-setting research has been shaped by dozens of distinguished scientists, among them seven Nobellaureates. Here we highlight the contributions made at the Institute to the fields of gas-phase kineticsand dynamics, early quantum physics, colloid chemistry, electron microscopy, and surface chemistry,and we give an account of the key role the Institute played in implementing the Berlin ElectronSynchrotron (BESSY I and II). Current research at the Institute in surface science and catalysis aswell as molecular physics and spectroscopy is exemplified in this issue [Angew. Chem. 2011, 123,10242; Angew. Chem. Int. Ed. 2011, 50, 10064].

DOI: 10.1002/anie.201104792

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intellectual home. Dozens of distinguished scientists, includ-ing the previously mentioned seven Nobel laureates, haveshaped the pacesetting research in physical chemistry, chem-ical physics, and other areas, such as electron microscopy,performed at the Institute. Their interests have ranged fromproviding for the concrete needs of society, in times of peaceor war, to plumbing the abstract depths of quantum mechan-ics, and from the apparent simplicity of hydrogen chemistry tothe acknowledged complexity of nonlinear dynamics. Theirinvestigations reflect a distinct, intellectual facet of 20thcentury history that is inextricable from social, cultural, andpolitical history.

In order to do justice to the complex scientific andpolitical history of the FHI, the Institute�s Board of Directors,prompted by the approaching centenary of the Institute (andthe KWG/MPG), offered support in 2007 for a broadhistorical investigation of the Institute from its inception tothe present. The Centennial Group, established in response tothe Board�s initiative in the Fall of 2008 and comprised ofBretislav Friedrich, Dieter Hoffmann, Jeremiah James, andThomas Steinhauser, launched a research project to examinethe Institute�s scientific and institutional history supported byarchival evidence and set against the context of the rapidchanges in the intellectual content of the sciences to which theInstitute contributed and in the societies, both scientific andpolitical, that supported it.

In writing the scientific and institutional history of theInstitute, we have been frequently reminded of the words of adoyen of modern history of science research, Gerald Hol-

ton:[1] “[The] science researchproject of today is the temporaryculmination of a very long, hard-fought struggle by a largely invis-ible community of our ancestors.Each of us may be standing on theshoulders of giants; more often westand on the graves of our prede-cessors.” At times in the history ofthe Fritz Haber Institute, thesestruggles for the future of sciencehave been as clearly political andsocial as “simply” intellectual andhave, in themselves or throughtheir outcomes, had profound,and even fatal, repercussions.The Centennial Project in generaland its main outcome, the centen-nial volume,[2] in particular haveaimed to highlight these strugglesof the past and to pay tribute tothose who, for the most part,persevered through them.

In this Essay, largely based on the centennial volume, weprovide a brief survey of the institutional history of theInstitute and highlight contributions from the Institute to theresearch areas that featured on its agenda with particularprominence: gas-phase kinetics, early quantum physics,colloid chemistry, electron microscopy, BESSY, and surfacechemistry.

2. Outline of the Institutional History of the FHI

When the KWG was founded in 1911 it became the thirdin a series of institutional innovations during the “long 19thcentury”—after the founding of the Berlin University (1810)and of the Imperial Institute of Physics and Technology(1887)—which originated in Berlin but shaped the modernresearch establishment more broadly. The founding of theKWG in general and the Kaiser Wilhelm Institute for PhysicalChemistry and Electrochemistry in particular came about inreaction to forewarnings by numerous prominent scientistsand science policymakers about the waning of Germany�sscientific and technological superiority relative to the UnitedStates and to other European nations, especially France andBritain.[3] In hindsight, these institutions appear to have beena successful answer to this perceived challenge, in that duringthe following decades the KWG established itself as one ofthe leading research organizations both domestically andinternationally. The Institute supported the development ofphysical chemistry, which was—in comparison to the thendominant organic chemistry—not a well-established field inGermany at that time. At its founding it had more resourcesand better research equipment at its disposal than mostlaboratories at German universities, and it rapidly becameone of the leading research institutions in its field, with itspredominantly “pure” research endeavors reaching a firsthigh-point during the Weimar era.

Table 1: Nobel laureates affiliated with the KWI for Physical Chemistry and Electrochemistry or later theFritz Haber Institute of the Max Planck Society.

[*] Prof. Dr. B. Friedrich, Dr. J. JamesFritz-Haber-Institut der Max-Planck-GesellschaftFaradayweg 4–6, 14195 Berlin (Germany)E-mail: bretislav.friedrich@fhi-berlin.mpg.de

Prof. Dr. D. HoffmannMax-Planck-Institut f�r WissenschaftsgeschichteBoltzmannstrasse 22, 14195 Berlin (Germany)

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The establishment of the KWI for Physical Chemistry andElectrochemistry was made possible by an endowment fromthe Berlin Banker and philanthropist Leopold Koppel (1854–1933 Figure 1), granted on the condition that Fritz Haber(1868–1934; Figure 2), well-known for his discovery of amethod to synthesize ammonia from its elements, be made itsdirector.

Fritz Haber headed the Institute (Figure 3) from itsinception up to his resignation in protest of National Socialistpolicies and emigration from Germany in 1933.[4] His aim wasto establish a research institute for modern physical chemis-try, whose research spectrum ranged from such classic topics

as chemical processing tech-nology and electrochemistryto reaction kinetics and col-loid chemistry to aspects ofquantum physics. He was, inthis respect, akin to contem-poraries such as WaltherNernst and Gilbert N. Lewiswho helped pave the way forthe transition from classicalphysical chemistry to chem-ical physics.

The German entry intoWorld War I in August of1914 brought an end to thefounding era of the Institute,since Haber not only took

part in the widespread enthusiasm that accompanied Germanmobilization and entry into the war, but also promptlyredirected the resources of his institute toward projectsrelevant to the war. After the war he would explain that “inwar, scientists belong to their Fatherland, like anyone, in peace,they belong to humanity.”[5] The first war-related task of theInstitute was to search for ways to economize or providesubstitutes for so-called “war materials”—substances re-quired for the operation of firearms, artillery, and other warmachines. In the winter of 1914–1915 Haber proposed andbegan organizing the first gas cloud attack, which took placenear Ypres on April 22, 1915. Shortly thereafter, at therequest of the military, Haber redirected the research at theInstitute toward the needs of gas warfare, focusing first on gasmasks and defense but soon branching out into research anddevelopment of offensive measures. The Institute became thecenter of chemical warfare research and development inGermany (Figure 4). This brought about an unprecedentedexpansion of the Institute and its division into nine depart-ments employing approximately 150 scientists and engineersand 1300 support staff, the latter composed largely of women.The Institute became a prototypical example of Big Science inthe context of a military–industrial–academic complex, notonly with respect to its sheer size but, above all, with respectto the complexity of its organizational structure and inter-disciplinarity of its research methods. In the words of thehistorian Fritz Stern, Haber�s Institute during the First WorldWar became “a kind of forerunner of the ManhattanProject.”[6]

After the German defeat in World War I the Institute hadto withdraw rapidly from military involvement and a dis-cussion ensued as to its future research direction. Thefollowing fourteen years are sometime referred to as the“golden era” of the Institute, since so much pioneering andoutstanding research was done during the 1920s and early1930s. The central research areas of the Institute during the“golden era,” as set forth by Haber in 1923, were colloidchemistry and atomic structure.[7] These very broadly definedfields were meant to include investigations of surface energy,coagulation, photochemistry, reaction mechanisms, and evenaspects of combustion—some details of which are providedbelow.

Figure 3. The Kaiser Wilhelm Institute for Physical Chemistry andElectrochemistry in Berlin-Dahlem, 1913; the Director’s villa is on thefar right.

Figure 4. The Kaiser Wilhelm Institute under Haber’s direction withsurrounding barracks during World War I.

Figure 1. The banker and entrepreneur Leopold Koppel. Illustra-tion by David Vandermeulen.

Figure 2. Fritz Haber in his laborato-ry in Karlsruhe, ca. 1905.

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Paradoxically, this period ofthriving research was also markedby ongoing financial hardships ow-ing to the hyperinflation of theearly 1920s and the Great Depres-sion that began at the turn of the1930s. More than once, the contin-ued existence of the Institute wascalled into question and staffmembers could not be paid or didnot have their contracts or fundingrenewed. One consequence ofthese funding difficulties was thatin 1923 the Institute left the ad-ministration of the Koppel Foun-dation and fully joined theKWG—only then becoming astandard Kaiser Wilhelm Institute.During the 1920s, about 50 scien-tists pursued research at the Insti-tute; however, most of them werenot employed by the KWG. Theywere instead paid by various“third-party” funds, in particular the Emergency Asso-ciation of German Science (Notgemeinschaft der Deut-schen Wissenschaft), the later German Research Asso-ciation (Deutsche Forschungsgemeinschaft), industrialsponsors, and funds administered directly by Haber.Throughout the 1920s the Institute developed step bystep into a more or less stable arrangement of fourdepartments: the oldest department, for physicalchemistry, under Haber; a department for colloidchemistry, guided by Herbert Freundlich; a physicsdepartment, established under James Franck in 1919(Figure 5) and after 1924 led by Rudolf Ladenburg;and the department of Michael Polanyi, who movedfrom the next-door KWI for Fiber Chemistry toHaber�s institute in 1923. Interspersed among these

departments were a number of small “research groups”(Arbeitskreise), which were headed by highly qualifiedscholars, in general with a Habilitation, such as KarlFriedrich Bonhoeffer, Georg Ettisch, and HartmutKallmann, whom Haber expected to pursue independentresearch and direct their own assistants and graduatestudents (Figure 6).

The unusual modes of financing at the Institute led tomembers of certain social groups, who were at acomparative disadvantage when competing for regularuniversity appointments in Germany at the time, beingdisproportionately attracted by the professional oppor-tunities offered at the Institute. This applied to the KWGin general, where foreigners, women, and above allscientists with Jewish ancestry were famously “over-represented” in comparison to university staffs. Accord-ing to contemporaries, roughly half of the researchersactive at Haber�s institute during the 1920s had recog-nizably Jewish family backgrounds,[8] although many of

the scientists or their families hadofficially converted to Christian-ity, including all of the scientificmembers of the Institute.

Not least of all because ofthese “Jewish” co-workers, theInstitute came under pressurevery soon after the seizure ofpower by the National Socialistson January 30, 1933 and theenactment of the first race laws,particularly the Law for the Re-storation of the Professional CivilService, promulgated on April 7,1933. As a consequence of thegovernment demand to dismisshis “Jewish” co-workers, Haberresigned his directorship with aletter to the ministry, in which hestated: “My sense of traditionrequires of me that, in the fulfill-ment of my scientific post, I only

Figure 5. Farewell party for James Franck, Dahlem, 1920. Left to right,seated: Hertha Sponer, Albert Einstein, Ingrid Franck, James Franck, LiseMeitner, Fritz Haber, Otto Hahn; standing: Walter Grotrian, Wilhelm West-phal, Otto von Baeyer, Peter Pringsheim, Gustav Hertz.

Figure 6. Planting of the Haber Linden Tree on the occa-sion of Fritz Haber’s 60th birthday, December 9, 1928,with most of the academic staff members of the Institutepresent.

Figure 7. Farewell gathering in the Institute gardens, July 1933. First rowstanding on the right: Friedrich Epstein; seated from the right: HartmutKallmann, Michael Polanyi, Fritz Haber; seated in front of Haber: RitaCracauer; two chairs left of Haber: Herbert Freundlich.

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choose staff members according to their professional abilitiesand character, without regard to their racial composition. Youwill not expect of a man in the 65th year of his life that he changehis way of thinking, one which has led him through the past 39years of his university career. You will also understand that thepride with which he has served his native German soilthroughout his life now requires him to request immediateretirement.”[9] This was one of the very few documentedinstances of resistance by German academia to Nazi rule.

Freundlich and Polanyi were of a similar opinion andstepped down from their posts, shortly before Haber officiallyresigned (Figure 7). During the following months not onlywere the remaining Jewish members of the scientific andsupport staff expelled; the remaining non-Jewish scientific co-workers were also dismissed.[10] This was the first step in thetotal restructuring of the Institute by Nazi and military circles.At first, the plan was to turn the Institute into a center forresearch on the chemistry of war materials under thedirectorship of Gerhart Jander, but this plan failed. Instead,in 1935, Peter Adolf Thiessen, a recognized physical chemistbut also a long-time member of the National Socialist party,was appointed director of the Institute by the Senate of theKWG “in an unusual manner”—as KWG President MaxPlanck described it. The appointment was proposed andstrongly pushed by the ministries and the military, thusundermining the scientific and institutional autonomy of theKWG. Nevertheless, the KWG accepted this poltical impo-sition, and Thiessen, who would become one of the mostprominent and successful science administrators in the ThirdReich, was free during the following decade to develop theInstitute into a “Nazi Model Enterprise.”

Under Thiessen, research at the Institute moved awayfrom the fundamental interests of the Haber era and focusedon applied science. At the same time, for largely politicalreasons, the Institute experienced a breakdown in its interna-tional relations. Sincere efforts were made to achieve theresearch goals established by the Nazi government, to activelycontribute to the drive toward German economic self-sufficiency, and to fulfill the demands of the National Socialist

regime for new armaments, including chemical weapons.[11]

These efforts established the framework for both applied andbasic research at the Institute, so that no clear distinction canbe made between exclusively military research and researchinto broader technical problems. Nevertheless, the Institute�sachievements in applied research and the highly technicalnature of the work done there were widely recognized andpraised by the scientific community of the time. One focalpoint of research at the Institute was the structural analysis offibers, glasses, synthetic materials, and metals. In this con-nection, pioneering work was done using modern analyticalmethods such as novel X-ray analysis techniques, electrondiffraction, and electron microscopy.

In the immediate aftermath of World War II theInstitute suffered crippling material losses, as it was givenspecial priority in the dismantling and pillaging efforts ofthe Soviets. Its high-quality, cutting-edge equipment, wasthe best to be found among the institutes available forplundering in Berlin. Furthermore, Thiessen was “invit-ed” to continue his research in the Soviet Union, wherehe worked on isotope separation as part of the Sovietatomic bomb project. Over and above the material losses,the resumption of research proved exceedingly difficultbecause scientific activities were effectively halted byAllied legal restrictions. Nevertheless, the Institutebecame a refuge for scientists in Berlin who lacked otherinstitutional ties and needed to retool professionally.Among them was Hartmut Kallmann, who had survivedthe Holocaust living in Berlin thanks to a “privilegedmixed marriage” and took up his research at the Institutevery soon after the downfall of the Nazi regime. In 1948,he and some of his students were able to develop the firstscintillation counter, a device that continues to play an

Figure 8. Institute grounds circa 1939. The dedication to Haber around theHaber Linden has been removed.

Figure 9. Karl Friedrich Bonhoeffer at the unveiling of the Habercommemorative plaque on December 9, 1952.

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important role in the detection of charged particles, partic-ularly electrons.[12] The Institute, however, was struggling tosurvive. Given the nebulous future of the KWG and theprecarious situation of the divided Berlin, its governmentaland other institutional affiliations were highly unstable andhence its financing was also uncertain. Paradoxically, the ColdWar helped to safeguard the Institute�s existence, first as partof a newly founded German Research University—a kind ofinstitute for advanced study—intended to contribute toAmerican plans for the reform of the German research andeducation establishment, and then in 1953 as a bridgebetween West Berlin and West Germany through the MaxPlanck Society (MPG), the fledgling, Gçttingen-based, suc-cessor to the Kaiser Wilhelm Society. After Karl FriedrichBonhoeffer, who led the Institute from 1948 to 1951 (Fig-ure 9), finally decided to depart for Gçttingen to assume thedirectorship of a new MPI for Physical Chemistry, Maxvon Laue (1879–1960), already 72 years old at the time, tookover as Director (Figure 10). Admission of the Institute intothe MPG in 1953 coincided closely with Laue�s appointmentand with the renaming of the Institute for its foundingdirector, making it the Fritz Haber Institute of the MaxPlanck Society.

Drawing upon his personal prestige as the Nobel prizewinning member of the collaboration that discovered X-raydiffraction in crystals, as well as scientific and technicalexpertise at the Institute dating back to the end of theThiessen era, Laue strove to develop the Institute into acenter for research into the microstructure of matter, usingprimarily diffraction techniques. At his disposal among thescientific staff were Iwan Stranski (crystal structure andgrowth), Kurt �berreiter (macromolecular structure), KurtMoli�re (electron diffraction), and his closest allies GerhardBorrmann (X-ray diffraction and absorption) and RolfHosemann (small-angle X-ray diffraction). In addition, ErnstRuska, who had previously been affiliated with the Instituteonly part-time, became head of the Department for ElectronMicroscopy—later to become a quasi-independent institute—shortly after the admission of the FHI into the MPG. Lauewas a highly successful director in that he stabilized andexpanded funding for the Institute and managed tokeep it productive and relevant in spite of thechallenges posed by its location in West Berlin,particularly with respect to attracting outside scientificstaff. Laue did not succeed, however, in developing thekind of coherent focus on structure research at theInstitute he had initially envisioned. Extensive researchin fields related to material structure was conducted atthe Institute, but in part thanks to the many rapidchanges in leadership and staff that had accompaniedthe immediate postwar era (1945–1953), few collabo-rations and essentially no permanent organizationalstructures held together the various departments andresearch groups of the Institute.

Laue stepped down as director of the Institute inMarch of 1959, at almost 80 years old, to make way forRudolf Brill, who had been trained in X-ray diffractionat the former KWI for Fiber Chemistry and made thefield a mainstay of his research career thereafter, and

therefore had the potential tosteer the Institute deeper intostructure research. However, Brillhad at least as much trouble asLaue in unifying research in thevarious departments, and the In-stitute continued to operate muchas it had before, divided into sixlargely independent departmentswhose research related more orless closely to the microstructureof matter. But Brill would overseeone of the single largest expan-sions in infrastructure in the his-tory of the Institute, albeit as aresult of an initiative begun underLaue. In part because of the spe-cial importance a prestigious research institute like the FHIhad for West Berlin during its Cold War isolation, andinarguably in response to the growing prestige of Ernst Ruska(see below), the FHI received a large grant in 1957 for theconstruction of a new building for the Institute for ElectronMicroscopy, as well as a library, lecture hall, and adminis-trative offices, which were inaugurated in 1963.

For the future of the Institute though, the ways in whichBrill broke with the tradition of Laue were more importantthan the ways in which he continued it. In particular, Brillchose surface catalysis, a field that had essentially vanishedfrom the Institute with Haber, as the main topic of research inthe department he led. He established two related researchgroups, headed by Hans Dietrich (structure of catalyticcompounds) and Jochen Block (electron emission spectros-copy). This, coupled with the retirement of Stranski in 1967,reduced the emphasis on structure research and maderesearch at the Institute appear less focused than it hadbefore; however, in the end, it would turn out to be the firststep toward the present incarnation of the FHI. As Brill�sretirement in 1969 approached, the Chemical-Physical-Tech-nical (CPT) Section of the MPG formed a committee to

Figure 11. Installation of Heinz Gerischer as FHI Director, December 9, 1968.From left to right, first row: A. Butenandt (with chain of office), H. Gerischer,R. Gerischer; second row: K. H. Herrmann, K. �berreiter, I. Stranski. Fourthrow on the right: R. Hosemann.

Figure 10. Max von Laueat an Institute outing, late1950s.

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consider the future of the FHI, as well as a committee tosearch for Brill�s successor. Working in close contact, theydecided to invite Heinz Gerischer (1919–1994) to become thenew director of the Institute, in part because of his willingnessto expand upon the lines of research introduced by Brill anddevelop the FHI into a center for surface physics andchemistry. Gerischer took office in November 1969 (Fig-ure 11).

During Gerischer�s directorship the Institute would notonly transform into a center for research on surface scienceand catalysis, but would also make the transition to acollective scientific administration (Kollegiale Leitung),which was increasingly becoming the norm for MPG insti-tutes. This intellectual and administrative restructuring wouldtake considerable organizational efforts and most of the 1970sto complete. During the transition phase, from approximately1974 to 1981, the FHI was divided into three subinstitutes:Physical Chemistry, Structure Research, and Electron Mi-croscopy. The physical chemistry institute was somewhatdeceptively named in that it included the core of researchersworking on the new topics of surface science and catalysis, ledby Gerischer, Moli�re, and Block. The structure institutehoused the remainder of the researchers, Hosemann and�berreiter, who adhered more or less strongly to Laue�searlier vision of the FHI, and the electron microscopyinstitute remained under Ruska up to the end of 1974.Thereafter, careful consideration of how to integrate electronmicroscopy with the idea of the FHI as a center for surfacescience research resulted in the appointment in 1976 of ElmarZeitler as Ruska�s successor. Several important junior ap-pointments also occurred during the 1970s, notably that ofAlexander Bradshaw (see below); however, until the retire-ments of �berreiter, Moli�re, and Hosemann, all in 1980, nonew positions were open for senior researchers, so that newlines of research that would come to define the FHI duringthe 1980s and beyond often developed within individualresearch groups rather than defining entire departments. Thekey exception was Gerischer�s department, which was builtaround cutting-edge research into electrode processes, solarcells, and new methods of exploring surface structures, whichwere experiencing a boom thanks to advances in ultra-high-vacuum technologies. In addition, work in Block�s depart-ment on electron emission methods also maintained a focus

on the examination of surface structures and surface reac-tions.

As of January 1, 1981 the restructuring of the FHI waseffectively complete, and the Institute took on the form it hasessentially maintained ever since. A new charter established aBoard of Scientific Directors, the earlier subinstitutes wereeliminated in favor of a simpler department-based structure,and an Advisory Board (Fachbeirat) of internationallyrecognized experts was established to counsel the directorson questions of research policy. As of 1981, there were onlyfour departments at the Institute: Physical Chemistry (Ger-ischer), Surface Reactions (Block), Surface Physics (Brad-shaw), and Electron Microscopy (Zeitler). Surface science ingeneral and heterogeneous catalysis in particular was a topicof research in all of these departments, albeit to a somewhatlesser degree under Zeitler than the others. With the structureof the Institute settled, the way was open for a number of keyappointments that advanced existing fields and rounded outthe research into surface science and catalysis. In 1985, in asomewhat surprising turn of events, the Institute managed toconvince one of the main protagonists of modern surfacechemistry research, Gerhard Ertl, to leave his post in Munichand move to the FHI as successor to his mentor HeinzGerischer, who would soon retire. Three years later, a fifthdivision, the Theory Department, opened under the directionof Mathias Scheffler.

It was a new heyday for the FHI. In 1986, Ernst Ruskareceived half of a shared Nobel Prize in Physics, “for hisfundamental work in electron optics, and for the design of thefirst electron microscope” (Figure 12). The presence ofdistinguished surface science and catalysis researchers, suchas Gerhard Ertl, and the much improved conditions in Berlinmade it easier for the Institute to again attract top-notchscientists, and the present board of directors gradually tookshape as Robert Schlçgl (1994), Hans-Joachim Freund (1996),Gerard Meijer (2002), and Martin Wolf (2008) joinedMatthias Scheffler at the Institute (Figure 13). But beforethis process was quite complete the award of another Nobelprize would be celebrated at the Institute. In 2007, on Ertl�s71st birthday, his career was crowned with the award of anunshared Nobel Prize in Chemistry, “for his studies ofchemical processes on solid surfaces” (Figure 14).

Figure 13. The current directors of the FHI in front of HaberVilla, 2011. From left to right: R. Schlçgl, M. Scheffler, H.-J.Freund, G. Meijer, and M. Wolf.

Figure 12. Ernst Ruska (left) and Elmar Zeitlerat the departmental Nobel Prize celebration onOctober 30, 1986.

Figure 14. A spontaneous gathering in thegarden of the Fritz Haber Institute onOctober 10, 2007, after the announcementof the 2007 Nobel Prize in Chemistry.

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3. Key Scientific Contributions from the FHI

3.1. Gas-Phase Kinetics and Dynamics

Ammonia Synthesis. With the completion of hishabilitation on “Experimental Investigations into theDecomposition and Combustion of Hydrocarbons” atthe Technische Hochschule Karlsruhe early in 1896,Fritz Haber turned his attention to the still-youngdiscipline of physical chemistry and distinguishedhimself as a rising star in the field. His work in appliedthermochemistry, the thermodynamics of industrial gasreactions, and gas analysis attracted particular atten-tion. Early in the summer of 1909, Haber made hisgreatest scientific achievement. Through a collabora-tion with BASF, Haber developed an industriallypromising catalytic process for the synthesis of ammo-nia from its constituent elements. Carl Bosch and AlwinMittasch at BASF then refined the process into atechnology for the large-scale production of ammonia,thus capitalizing on its agricultural and military signifi-cance.[13]

Research in the name of the Kaiser Wilhelm Institute forPhysical Chemistry and Electrochemistry commenced in theautumn of 1911. As they lacked laboratories of their own inBerlin, Haber and his colleagues initially pursued theirresearch as guests at various Berlin research centers, includingthe Imperial Institute of Physics and Technology in Berlin-Charlottenburg. The first scientific activities undertaken atthe Institute were culminations of projects begun in Karls-ruhe. Gerhard Just continued research begun with Haber intoelectron emission during gas–metal reactions, while F. Hillersfollowed Haber�s lead in research on the inner cone ofhydrocarbon flames. Haber also reaffirmed his interest inelectrochemistry through an investigation of the effects ofcurrents passing through the walls of gas containers on theelectrochemical behavior of gases. In the realm of gaschemistry, Haber and Fritz Kerschbaum collaborated indeveloping a general observation made by the Americanphysical chemist Irving Langmuir into a specific method formeasuring very low pressures using the oscillations of a quartzfiber.

Nevertheless, the bulk of Haber�s scientific publications inthe years leading up to the First World War continued torelate to ammonia synthesis. In addition to an array of articlesdetailing new research on the subject, most of them co-writtenwith Setsuro Tamaru, Haber also published the results ofresearch projects undertaken with Robert LeRossignol,Haber�s chief assistant during initial development of thesynthesis process, and Harold Cecil Greenwood, both ofwhom had returned to Britain rather than follow Haber toDahlem. These articles concerned, primarily, the thermody-namics of the synthesis reaction and measurements of thespecific heat of ammonia. In them, Haber broadened hisinitially applications-oriented perspective on ammonia syn-thesis, reexamining the reaction in light of fundamentalquestions in physical chemistry, such as the equilibriumconstant and its relation to the heat capacities of the reactantsand products. Haber also enlisted Albert Einstein, who

arrived in Berlin shortly before the outbreak of the FirstWorld War, to aid the Institute in quantum theory researchand provided office space for him (Figure 16).

Methane Detector (Firedamp Whistle). Similarly centralto research at the Institute in its early years was thedevelopment of a firedamp (methane) detector for use incoal mines.[14] Up to that point, the safety lamp developed bySir Humphry Davy at the beginning of the 19th century wasthe preferred safety and warning apparatus. However, thelamps themselves posed something of a risk, since a defectivelamp could set off an explosion. The Kaiser witnessed theeffects of one such disaster during a visit to Krupp at VillaH�gel in Essen in the summer of 1912, and he used theopportunity of the opening of the Kaiser Wilhelm Institutes torequest that German chemists develop a safer, more reliabledetector. Haber had been informed in advance of the Kaiser�sinterest in such a device, and at the inauguration, he presenteda gas interferometer he had developed in collaboration withthe Zeiss company. However, the interferometer was aprecision measurement device, not a rugged methane detec-tor; hence the practical problem remained unsolved. Togetherwith his assistant Richard Leiser, Haber dedicated himselfduring the next year to fulfilling the Kaiser�s request. In sodoing, he was entering into competition with colleagues atnumerous other chemical institutes, among them the Directorof the neighboring KWI for Chemistry, Ernst Beckmann.Unlike his colleagues, who based their designs primarily uponspectroscopy or analytic chemistry, Haber began with acous-tics, specifically the fact that the tone of a whistle dependsupon the speed of sound in the gas that fills it. Using this fact,Haber and Leiser developed a firedamp whistle, whose pitchwould change when filled with methane (Figure 17). How-ever, the whistle required precision machining that made itsproduction costly, and it was not robust enough to withstandlong periods of uninterrupted use. Moreover, it could not beproperly calibrated on site. But even though the device wasneither a rousing practical success, nor a generator of greatprofits, it did add to Haber�s scientific reputation. Haber�sinterest in firedamp detectors also illustrates the extent to

Figure 15. Fritz Haber with his colleagues (left to right) Gerhard Just (?),Setsuro Tamaru, and Richard Leiser, circa 1913.

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which he oriented his re-search around the technicalproblems of his times—a fea-ture of his scientific activitiesthat would take on particularsignificance during WorldWar I.

Haber–Born Cycle. Hab-er spent some of the limitedtime he personally felt able todedicate to pure research inthe first years after the wardeveloping new models of thestructure of solids. The bestknown result of these effortsis the Haber–Born cycle, athermodynamic analysis ofthe formation of ionic crystalsinto component steps corre-

sponding to energies (ionization energy, electron affinity, etc.)whose sum gives the total energy of formation of the crystal; itwas frequently used to calculate lattice energies, the one stepin the cycle that cannot generally be measured directly. TheHaber–Born cooperation resulted unexpectedly from thefrequent trips Max Born made to Berlin to visit James Franck.Born was initially wary of Haber, as Born was opposed tochemical warfare, but Haber managed to win his confidenceand arrange a brief collaboration with long-lasting results.The Born–Haber research had clear antecedents in the workof Born and Alfred Land� on crystal lattice energies, butHaber too made roughly contemporaneous attempts tocalculate macroscopic properties of crystals on the basis ofatomic scale models, albeit with a focus upon metal structureand markedly less lasting success.[15]

Chemiluminescence. Haber�s definitive contribution tothe research direction of the Institute in the early 1920s,however, would be an article with Walter Zisch on theemission of light during combustion reactions. Haber andZisch studied the spectra emitted by ordinary flames, as wellas those produced during reactions of alkali-metal and halidegases, in particular sodium and chlorine. They observed thatthese reactions emitted light that could not be ascribed to theheat of the reaction and that could be seen even when thereaction mixture was not hot enough to glow visibly accordingto the laws of blackbody radiation. To control the reactiontemperature, Haber and Zisch allowed the gases to react onlyat very low pressures, creating “highly dilute flames”. Theyposited that the process that produced this anomalous light,and which they took to be representative of chemilumines-cence in general, was the inverse of a photochemical reaction.That is, they argued that light was emitted after electronicexcitation of the reactants or products by the reaction, whichthen returned to their ground state, emitting light through aprocess analogous to fluorescence. The phenomenon wasmarkedly more complex than fluorescence in that the nucleiand the electrons could interact in ways not fully understoodat the time and the emitted light could originate from anelectronic transition in any one of the reactants, intermedi-ates, or products present in the reaction vessel. Hence, Haber

and Zisch argued, one could not expect a one-to-onecorrespondence between the number of molecules of productformed and the number of light quanta emitted, as one mightexpect from a straightforward inversion of the photochemicaldecomposition mechanism, but one could very likely detectunstable intermediates and gain insights into reaction mech-anisms by studying chemiluminescent spectra. Both thecomplexity of the reaction mechanisms suggested by theHaber and Zisch article and the possible insights one mightgain into reaction mechanisms by careful spectral observa-tions became launching points for new lines of research at theInstitute. This would include research by Hans Kautsky onchemiluminescence in colloids and experimental work by thePolanyi group on reaction mechanisms in the gas phase, aswell as Haber�s own return to combustion research after1926.[16]

Several independent strands of research emerged withinHaber�s “physical chemistry” department while Haber wasunderway with Department M (the code name for the gold-from-seawater project). One of the most prestigious beganwith the arrival of Karl Friedrich Bonhoeffer at the Institutein 1923 and his subsequent investigations of the chemistry ofactivated, that is, atomic, hydrogen. Given that atomichydrogen was the only substance for which physicists feltthey might have an acceptable quantum theoretical model atthe time, it was a topic with manifest, if nebulous, potential forbridging physics and chemistry. From the outset, Bonhoeffertook advantage of the expertise in low-pressure-gas chemistryavailable at the Institute. As he branched out to research onsimple hydrogen-containing compounds, he also benefitedfrom its spectroscopic facilities and from the assistance ofLadislaus Farkas, with whose help he established a connectionbetween the diffuse bands in the electronic spectra ofammonia and predissocciation, and interpreted the bands�

Figure 16. Albert Einstein andFritz Haber in the stairwell ofthe KWI, 1914.

Figure 17. Fritz Haber and Richard Leiser with the firedamp whistle,1913.

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widths in terms of the energy–time uncertainty relation.[17]

Bonhoeffer�s research took a distinct turn, however, after thearrival of a new collaborator, Paul Harteck, in 1928. Harteckhad habilitated under Max Bodenstein in Berlin, and thenspent two years working as an assistant to Arnold Eucken inBreslau, at the time a center for experimental research onlow-temperature specific heats, where first unsuccessfulsearches for the allotropic forms of hydrogen had beenundertaken. Upon Harteck�s arrival at the Institute, he andBonhoeffer would adopt this line of research and achievesuccess (see below).

Combustion. After 1926, Haber returned personally tothe study of combustion reactions. Inspired by a practicalinterest in improving existing combustion fuels and discover-ing new ones, his new research concentrated on the mecha-nism of combustion reactions rather than the light theyemitted. In spite of the shift in focus, Haber and hiscollaborators continued to rely on many of the laboratorytechniques and apparatus that had enabled the earlier Haberand Zisch research, including the use of spectroscopy toidentify reaction intermediates. Karl Bonhoeffer joined himearly in this research, followed by Ladislaus Farkas, PaulGoldfinger, Hans Dietrich Graf von Schweinitz, and HubertAlyea (Figure 18). Their results demonstrated the importanceof free radicals in combustion reactions and led, in the case ofthe combustion of hydrogen, to an interim reaction scheme,the so-called “Haber Chain.” These insights attracted consid-erable interest from fellow chemists; however, they were notof immediate industrial importance. Instead, like Haber�soriginal ammonia synthesis, they deepened scientists� under-standing of the principles behind commercially significantchemical reactions, but left the practical application of theseinsights to industrial chemists and chemical engineers.[18]

3.2. Early Quantum Physics

Statistical Mechanics. In the wake of the First SolvayConference (1911), many leading physicists had begunembracing the quantum hypothesis as a key to solvingoutstanding problems in the theory of matter. The quantumapproach proved successful in tackling the thermal propertiesof the solid state, resulting in the nearly definitive theories ofDebye (1912) and of Born and von Karman (1912–1913).However, the application of the old quantum theory to gaseswas hindered by conceptual difficulties, which were due inlarge part to the lack of a straightforward way of reconcilingfrequency-dependent quantization techniques with the aperi-odic behavior of gas molecules. A breakthrough came fromunlikely quarters. Otto Sackur (1880–1914; Figure 19) hadbeen trained as a physical chemist but had been attracted tothe new field of quantum physics. In 1911, he discovered anexpression for the absolute translational entropy of amonoatomic gas. A Dutch high-school student, Hugo MartinTetrode, obtained the same result at about the same timeindependently. The resulting Sackur–Tetrode equation ren-dered entropy as an extensive variable (in contrast to theclassical expression, cf. the Gibbs paradox) and expressed thethermodynamically undetermined constant in terms of mo-lecular parameters and Boltzmann�s and Planck�s constants.This result was of great heuristic value because it suggestedthe possibility of deriving the thermodynamic quantities of agas quantum mechanically. At the same time, the Sackur–Tetrode equation offered a convenient means to evaluate theequilibrium constant of gas-phase reactions, thus foreshadow-ing a unification of quantum theory, thermodynamics, andphysical chemistry.[19]

The key steps in the development of the Sackur–Tetrodeequation were the partitioning of the phase space intoelementary cells and the use of Planck�s constant to fix thecells� volume. Following up on this feat, Sackur attempted todevelop a general quantum theory of the ideal gas with,however, only partial success—and with a dose of naivet�.However, Sackur�s bold attempt to deploy the quantumhypothesis in classical statistical mechanics prepared the pathfor Planck�s later theory of a quantum gas. Sackur launchedhis research at the intersection of physical chemistry, thermo-dynamics, and quantum theory while a Privatdozent inBreslau, with hopes for a more senior academic appointment.His hopes were fulfilled at the end of1913, when, thanks in part to mediationby Clara Immerwahr, Haber�s first wife,Sackur received a call to Haber�s insti-tute. In 1914 he was promoted to the rankof department head. After the outbreakof World War I, Sackur was enlisted inmilitary research at Haber�s institute, buton the side succeeded to carry on with hisexperiments on the behavior of gases atlow temperatures. He was killed in alaboratory accident in 1914 at the youngage of 31.

The Franck–Hertz and the ComptonEffects. In the Physics Department, un-

Figure 18. Fritz Haber amongst colleagues, standing: Paul Goldfinger,unknown; seated: Hans Dietrich Graf von Schweinitz, LadislausFarkas, circa 1930.

Figure 19. OttoSackur, circa 1913.

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der the direction of James Franck, in the years immediatelyfollowing the war Walter Grotrian, Paul Knipping, and ErichEinsporn concentrated primarily on the careful measurementof absorption spectra and ionization energies and thecorrelation of these measurements with the Bohr–Sommer-feld model of the atom. It was an extension of a line ofresearch that Franck and Hertz had begun while at BerlinUniversity before the war. There they devised the venerable“Franck–Hertz experiment,” which demonstrated that elec-tron collisions with mercury va-por atoms were elastic only up toa certain threshold energy, andthat beyond this threshold inelas-tic collisions led to ionization andelectronic excitation of the atom-s.[20a] The specific ionization andexcitation energies they observedcorresponded with predictionsbased on Niels Bohr�s quantummodel of the atom, providing itstrong experimental support.Though performed at an ostensi-bly chemical institute in the con-text of physical chemistry, theirpostwar efforts were similarlycentral to quantum physics, astheir careful measurements ofspectra and ionization energies“enabled the confirmation ofBohr�s theory to a high degreeof precision.” Nevertheless, theirresults also formed part of thebasis for several later investiga-tions at the Institute, including, asalready discussed, the pivotal1922 study by Haber and Zisch,[16]

as well as the work of HansBeutler and others[20b] on thequantum mechanics of atomiccollisions. In this respect, thefocus of the Franck group on thespectra of mercury vapor was particularly important, as theirexemplary results encouraged later researchers at the Insti-tute to choose mercury vapor as a model system.

In an interregnum at the Physics Department betweenJames Franck and Rudolf Ladenburg, Paul Knipping pub-lished retrospective articles on the discovery and practice ofX-ray diffraction and descriptions of a new apparatus forionization measurements.[20c] More importantly, the X-rayapparatus and spectroscopic equipment at the Instituteoffered members of the Physics Department the opportunityto branch out into research on the Compton effect, the shift inthe wavelength of X-rays caused by inelastic scattering froman electron. The discovery of the Compton effect in 1923caused quite a stir in the physics community, as it providedstrong support for the particulate nature of X-rays and, byextension, of light.[21] However, Compton�s results provedsomewhat difficult to replicate. Noted Harvard X-ray phys-icist William Duane tried and failed, but Hartmut Kallmann

of the Physics Department in collaboration with HermannMark, an expert on X-ray analysis at the neighboring KWI forFiber Chemistry, were able to reproduce the phenomenonand to make careful measurements of the relationshipbetween the scattering angle and the shift in wavelength.[22]

Kallmann also performed a more rigorously physical analysisof the ongoing research at the Institute into the excitation ofgas spectra through chemical reactions, cf. Haber and Zischcirca 1913.[23] Still, there is nothing to indicate that these were

aspects of a department-wide lineof research, and Kallmann, thoughnominally attached to the PhysicsDepartment, pursued his researchinterests essentially independentlyeven before he became head of hisown research group in 1928.

Allotropic Forms of Hydrogen.Bonhoeffer and Harteck set outtogether to confirm the existence oftwo recently posited, distinct formsof molecular hydrogen: ortho-hy-drogen, with nuclear spins orientedparallel to one another, and para-hydrogen, with mutually opposingnuclear spins. Ever since Euckenfirst measured the specific heat ofhydrogen gas at low temperaturesin 1912, its anomalous temperaturedependence had posed a challengefor quantum theories of specificheat. In 1927, working in closecorrespondence, Werner Heisen-berg and Friedrich Hund, eachindependently published articles inwhich they suggested that hydrogenexisted in distinct ortho and paraallotropic forms. Furthermore, theyargued that these two forms shouldexist in a ratio of 3 to 1 at hightemperatures, but as Hund pointedout, should contribute differently

to the specific heat of the gas. Later that year, DavidDennison combined these insights into a theory that fullyaccounted for the observed specific heat of hydrogen, basedin part on the premise that at low temperatures the pararather than the ortho form would be favored, but thetransition between the two states would be slow.[24] It wasthis transition that would allow Bonhoeffer and Harteck totest the new theory, but it was a challenging task, Harteck�sBreslau training in low-temperature methods notwithstand-ing. In a letter from October 28, 1928, Bonhoeffer wrote:[25a]

“we have set our minds upon an experiment that should showthat ordinary hydrogen…is a mixture, as the theorists belie-ve…but it isn�t working at present, and I have lost half my hairto the futile drudgery”.

Fritz London, then an assistant to Erwin Schrçdinger atthe Berlin University, came to their aid and suggested keepingthe hydrogen at low temperature as long as possible tofacilitate the transition. In conjunction with Bonhoeffer�s

Figure 20. The Institute’s hydrogen team, circa 1930. Leftto right: Adalbert Farkas, Paul Harteck, Ladislaus Farkas,Karl Friedrich Bonhoeffer.

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proposal that they measure the heat conduction rather thanthe specific heat of the gas, the former being proportional tothe latter but easier to measure, they were able to overcomethe technical challenges of the experiment. In March 1929they published their results[25b] supporting the new theory,narrowly beating to press a nearly simultaneous announce-ment from Arnold Eucken. They also discovered, much to thebenefit of later hydrogen research, that activated charcoalcatalyzes the otherwise painfully slow conversion of ortho- topara-hydrogen. The importance of their research would bealluded to in the Nobel citation for Werner Heisenberg, whoreceived the 1932 physics prize “for the creation of quantummechanics, the application of which has, inter alia, led to thediscovery of two allotropic forms of hydrogen.” It also led totheir being nominated at least once, in 1937, for the NobelPrize in Chemistry. Adalbert and Ladislaus Farkas soonjoined Bonhoeffer and Harteck inthe investigation of hydrogen (Fig-ure 20), using similar low-temper-ature methods to explore not onlyfurther properties of ortho- andpara-hydrogen but also the proper-ties of heavy hydrogen, that is,deuterium. This work distinguishedthe Institute as one of the worldleaders in hydrogen research, andthe results of investigations at theInstitute formed the foundation forone of the first monographs on thesubject, Orthohydrogen, Parahydro-gen and Heavy Hydrogen, publishedby Adalbert Farkas in 1935[25c] fol-lowing his emigration in 1933. In thecase of Harteck, the hydrogen re-search also marked a first step in thedirection of nuclear research, as he later recalled in con-nection with his choice to study with Rutherford at theCavendish laboratory:[26] “exactly like thermodynamics wasand still is of importance for chemistry, similarly in theforeseeable future nuclear physics should open interesting andfundamental fields for a physical chemist.”

Later decisions concerning how he pursued this line ofresearch would earn Harteck the dubious distinction of beinginterned at Farm Hall in England immediately after theSecond World War, in connection with his participation in theGerman uranium project.[27]

Symmetry. Yet further afield from the Physics Depart-ment under Franck and Ladenburg, but still in keeping withthe interest in spectroscopy at the Institute, were the enduringcontributions of Eugen Wigner (1902–1995) to quantumtheory (Figure 21). In 1926–1927, while still dividing hisresearch efforts between Reginald Herzog�s KWI for FiberChemistry and Haber�s Institute, Wigner became the firstscientist to employ group-theoretical considerations in theinterpretation of the selection rules of atomic spectroscopy.He accomplished this by analyzing the transformationproperties of energy eigenstates of a system with respect tooperations that leave the system physically unchanged, forexample, spatial rotations, mirror inversions, exchange of

identical electrons. Wigner had developed his skills withgroup theory and symmetry transformations while workingwith Karl Weissenberg on crystallography, a field in whichthese mathematical tools had been commonplace since theend of the 19th century. Symmetry groups, however, had notyet made similar inroads into other branches of physics, andmany physicists were initially hostile to their importation intoquantum theory, even referring to them as the “Gruppenpest,”the group plague.[28]

Nevertheless, the encounter Wigner arranged betweengroup theory and the old quantum-theoretical notion ofselection rules had a profound and long-lasting impact onquantum theory.[29] The connection between selection rulesand group theory endowed quantum theory with a new typeof symmetry argument, in which selection rules, rather thanconservation laws, were regarded as the observable signature

of an underlying physical symme-try. Interpreting experimental datain terms of selection rules, there-fore, led to a redefinition of thetraditional conserved quantities,notably angular momentum. In abrief paper published in 1927,Wigner drew attention to the new,quantum form of conservationlaws, articulating what is todayreferred to as the quantum versionof Noether�s theorem. Wigner not-ed that in quantum mechanics onewas only allowed to ask about theprobability distribution of the val-ues of physical quantities and con-cluded:[30] “It is therefore necessaryto formulate also the laws of con-servation in this sense. They will

then have the form, for example: The probability that theenergy will have the value E does not change with time.”

When asked in the early 1930s by Max von Laue whatgroup-theoretical result derived so far was the most importantone, Wigner replied: the explanation of the Laporte rule (theconcept of parity) and the quantum theory of vector addition(angular momentum).[31] Partly in recognition of the power ofthese new theoretical devices, Wigner would receive the 1963Nobel Prize in Physics “for his contributions to the theory ofthe atomic nucleus and the elementary particles, particularlythrough the discovery and application of fundamental sym-metry principles”.

Dispersion. While Wigner was taking the first steps tointegrate group and quantum theory, a new line of exper-imental research was developing within the Physics Depart-ment, as Rudolf Ladenburg (1882–1952; Figure 22), incollaboration with Hans Kopfermann and Agathe Carst,undertook a series of experiments intended to test the newquantum theory of dispersion. Dispersion played a centralrole in the development of quantum theory in general, and inthe formulation of the matrix mechanics by Werner Heisen-berg in particular,[32a] and during his time in Breslau, Laden-burg had made important contributions to the transformationof classical dispersion theory into its quantum counterpart. In

Figure 21. Eugene Wigner (right) with Werner Heisen-berg, 1928.

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Breslau, Ladenburgwas assisted in this re-search by his friendand colleague FritzReiche. In Dahlem,the task fell primarilyto Hans Kopfermann(1895–1963; Fig-ure 23), who arrivedat the Institute in1926, immediately af-ter completing his ha-bilitation in Gçttingenunder James Franck.Ladenburg and Kop-fermann (and laterCarst) compared thepredictions of the lat-est versions of thequantum theory withnovel experiments ondispersion in excitedgases, and as Haber

reported to a meeting of the Prussian Academy in June of1926: “Using the method of interference bands, anomalousdispersion was confirmed, and in some cases measured, inseveral lines of the He, Ne, Hg and H [spectra], when the gaseswere excited by a continuous current. On the basis of thequantum theoretical dispersion formula of Ladenburg andKramers and the F-summation rule of Reiche–Thomas thesemeasurements were used to determine the probabilities ofvarious quantum transitions, as well as the number of atoms inthe excited states and their dependence upon current and thetemperature and pressure of the gas.”

The continuation of this line of research also led to a seriesof articles publishedbetween 1928 and 1930,[32b] in which theypresented the first evidence of “negative dispersion,” whatphysicists now call stimulatedemission. According to the quan-tum dispersion theory, as formu-lated independently by RalphKronig (1926) and HendrikKramers (1927), one could cre-ate a sample material that, whenilluminated by light of the ap-propriate frequency, actuallyemitted more light of that fre-quency than it absorbed. This isnow recognized as the crucialphenomenon behind the opera-tion of lasers, and some histor-ians of science have even arguedthat with just a bit more luckLadenburg and Kopfermannmight have observed the firstlaser pulse.[33a] After G�ntherWolfsohn took over as Laden-burg�s assistant in 1930, Kopfer-mann turned his attention to the

hyperfine structure of atomic spectra. His investigations ofthe spectra of different isotopes during the following yearcontributed to the discovery of the “isotopic shift,” the effectsof the nucleus on the energy of the surrounding electrons.Exploring the properties of the nucleus through its interac-tions with electron orbitals would develop into Kopfermann�sresearch specialty when he later moved to professorships atKiel and then at Heidelberg, and he wrote in 1940 one of thestandard early works on the topic, Kernmomente.[33b] As withPaul Harteck, this clear move toward nuclear studies laid thegroundwork for his later participation in uranium researchunder the National Socialist regime.

Theoretical Chemistry. One of the research areas that hasbeen pursued with particular vigor and success at both theKaiser-Wilhelm-Gesellschaft and Max-Planck-Gesellschaftincarnations of the institute is Theoretical Chemistry. Thetheoretical work at the Kaiser-Wilhelm-Institut f�r physikali-sche Chemie und Elektrochemie during the 1920s and early1930s was closely linked to Michael Polanyi�s pioneeringresearch in experimental chemical kinetics (Figure 24).[34]

With his mutually “trusting but critical” team of youngtheorists, which included Eugene Wigner, Fritz London, andHenry Eyring, Polanyi laid conceptual foundations for kinetictheory consistent with the new quantum mechanics andforeshadowed the coming of chemical reaction dynamics,which would only arrive in the early 1960s in America.

Polanyi had puzzled over the implications of quantummechanics for the kinetics of chemical reactions since about1920. He recognized that the kinetic theories at hand couldnot be quite right, as the ratio of the forward and backwardreaction rates failed to equal the equilibrium constantpostulated by thermodynamics. In 1925, he and Wignerresolved the conundrum for two-body capture and its reverse,one-body decay, by invoking the uncertainty principle, in aform gleaned from spectroscopy by Niels Bohr.[35a] Theirtheory not only reconciled kinetics with thermodynamics forthe capture/decay process but also foreshadowed what was

later to become the Breit–Wignerformula (1936), which captures thekinetics of both molecular andnuclear near-resonant collisions.

Of paramount importance wasthe work on the dynamics of thesimplest chemical exchange reac-tion, H + H2QH2 + H, which wasprompted by the above-mentioneddiscovery at the institute of para-hydrogen and the study of its inter-conversion with ortho-hydrogen.This work established a way oflooking at the process of makingand breaking of chemical bondswhich, for thermal and hyperther-mal reactions, prevails until thisday: a ball, representing the nucleiof the constituent atoms, rolls onthe potential energy surface, givenby the eigenenergy of the electrons(Figure 25). En route from the

Figure 22. Rudolf Ladenburg, circa1930.

Figure 23. Hans Kopfermann, circa 1928.

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valley of the reactants to the valley of the products, the ballfollows a path restricted by the reaction�s energy disposal,comprising translational, vibrational, and rotational compo-nents. This view of the reaction entails a separation betweenthe nuclear and electronic motions, known today as the Born–Oppenheimer approximation. The potential energy surfacewas calculated by Fritz London, who moved to Berlin fromZurich. There, in 1927, he and Walter Heitler invokedquantum mechanics to explain the baffling two-electron,covalent bond through the exchange interaction.[35b] WhereasLondon and Heitler�s ground-breaking feat in quantumchemistry validated Gilbert Newton Lewis�s 1916 hypothesison the role of electron pairs in chemical bonding, London�stackling of the H3 system breathed new life into SvanteArrhenius�s 1889 concept of activation energy, by reinterpret-ing it as the summit-to-be-conquered between the electroniceigenenergy valleys of the reactants and products. Polanyi andEyring subsequently enhanced the accuracy of London�scalculations of the potential energy surface, by making use ofdata on the electronic energyobtained independently fromspectroscopy. This approach,dubbed semiempirical, was yetanother methodological advancewhich has since proved invalua-ble in a variety of contexts.

The rate at which the ballmakes its transit over the sum-mit—and hence the rate of thereaction—was evaluated in 1932in Polanyi�s group by Hans Pelzerand Wigner who made use ofstatistical mechanics and theLondon-Eyring-Polanyi semiem-pirical potential energy surface.This was the first take on the“transition-state” or “activatedcomplex” theory of chemical reactions, which was later(1935) developed by Eyring and his collaborators at Prince-ton, and further refined by others.

Molecular Beams. Macroscopic reaction rates as ob-served, for example, in the experiments of Michael Polanyiand others during the 1920s and 1930s, represented averagesover zillions of elementary collisions, whose identity andnature remained largely unknown—as did their relation tothe molecular forces involved. This situation has been greatlyremedied through the use of molecular beams whose deploy-ment has made it possible to break with the bulk past andlaunch a new era in reaction kinetics based on the direct studyof the dynamics of the underlying elementary collisions.Although the transition to the chemical/molecular dynamicsera would materialize fully only three decades later and onthe American continent, molecular beam methods have theirroots in Europe, originating in part at Haber�s institute. In1921, Hartmut Kallmann and Fritz Reiche proposed amolecular beam experiment designed to determine whetherindividual polar molecules—as opposed to polar molecules inthe bulk—carry an electric dipole moment.[37] A beam ofpolar molecules was to be sent through an inhomogeneous

electric field and its deflection monitored. Kallmann andReiche presumed that, while the beam�s dilution wouldpreclude any bulk interaction among the molecules, thedirectionality of the molecules in the beam would make theirdeflection, if any, measurable. Kallmann and Reiche therebytapped into a key feature of the molecular beam method, aslater characterized by Otto Stern, who extolled the method�s“simplicity and directness”, emphasizing that it “enables us tomake measurements on isolated neutral atoms or moleculeswith macroscopic tools .. [and thereby] is especially valuablefor testing and demonstrating directly fundamental assump-tions of theory”.[38a]

Kallmann and Reiche�s paper prompted Stern to publishhis proposal for what was to become the Stern–Gerlachexperiment to test whether space quantization was real.[38b] Itsdemonstration, carried out in Frankfurt in 1922 by Stern andWalther Gerlach, ranks among the dozen or so canonicalexperiments that ushered in the heroic age of quantumphysics.[39]

Kallmann�s further molec-ular beam exploits include theproduction of keV cation andanion beams as well as ofneutral beams obtained fromion beams by collisional elec-tron transfer.[40] Kallmann alsomade use of electron transferfrom multiply charged anionsin his design of a multistage ionaccelerator: an anion of initialcharge of magnitude q was sentthrough an array of accelera-tion stages separated by thesame voltage difference V. Af-ter being accelerated to anenergy qV, the ion was passedthrough a thin foil where it lost

an electron so that the magnitude of its charge dropped to q’<q. Although decelerated again on its flight out from theacceleration stage to the next, the ion gained a net energy(q�q’)V. By repeating this process in subsequent accelerationstages (which entailed further stripping of the ion of itselectrons), MeV energies could be attained with a voltage Von the order of only 100 kV.[41] This is a key element of the“tandem” principle which is at the core of accelerator physics.

Physisorption. Another key advance in molecular theoryconcerned physisorption. While at various stations duringWorld War I, Polanyi proposed that there is an attractiveforce between gaseous atoms or molecules and a solid surface,whose spatial dependence he described empirically, in termsof a potential function. At the time, only two types of forceswere admitted: electromagnetic and valence. However, eachwas believed to be able to adsorb only a single layer of atomsor molecules, a limitation which was not inherent to Polanyi�shypothesis. Opposed early on by, among others, Fritz Haberand Albert Einstein, Polanyi�s theory was vindicated in 1930by London, who showed that the hypothetical attraction isdue to the dispersion forces. These arise from the mutualattraction of induced atomic or molecular dipoles generated

Figure 24. Polanyi’s Physical Chemistry Department, 1933.

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by the fluctuations of the electron density and is demanded byquantum mechanics.[42a]

The interconnectedness of the research areas at theInstitute notwithstanding, at times the topics of investigationmay appear to have led members of the Institute beyond thebounds of physical chemistry, at least as we now see them. Butdisciplinary boundaries, however clear they may appear inpedagogy and funding practices, are often difficult to discernin ongoing research. For example, spectroscopy was, in veryparticular respects, of clear interest to chemists in the 1920s; itremains so today. But judging precisely in which respectsspectroscopy is “chemistry” and in which respects “physics”requires a thorough understanding of the context, if it ispossible at all. Haber, like many of his contemporaries inphysical chemistry, did not feel it necessary to wait for andobey such clear disciplinary demarcations when choosingtopics of research, and in the context of early 20th centuryphysical chemistry this strategy often led to highly regardedresults, as illustrated in the case of the Haber Institute, whoselong-standing members often received enticing offers offaculty positions at prestigious universities.

3.3. Colloid Chemistry

Colloid chemistry, the unfailing mainstay of HerbertFreundlich�s department during the Weimar years (Fig-ure 26), traces its roots back to the discovery in the 1860s bythe Scottish chemist Thomas Graham that certain aqueous

solutions pass through a semipermeable membraneonly with difficulty, if at all. These solutions hetermed “colloids,” to distinguish them from “crys-talloids” which pass with ease through such amembrane.[42b] Graham presented his research oncolloids as a fascinating aspect of the specializedfield of solution chemistry. A more modern under-standing of colloids, extending the definition ofcolloids to any substance in which one chemicalcompound is microscopically distributed throughanother regardless of the phases of either substance,and focusing on the role of surface forces in colloidbehavior, only took shape some four decades later.It was largely the result of a campaign, spearheadedby Wolfgang Ostwald, son of the famous founder ofphysical chemistry, to establish colloid research asan independent and fundamental chemical disci-pline. Colloid chemistry was primarily an experi-mental endeavor, and in addition to redefiningcolloids with respect to both their material proper-ties and their scientific significance, Ostwald and hisallies developed and refined a host of instrumentsand techniques to advance the new discipline, manyof which, such as the ultramicroscope and electro-phoresis, remain familiar to chemists today.

Herbert Freundlich (1880–1941) belonged tothe vanguard of the campaign for colloid chemistry.He completed his doctorate under Wilhelm Ost-wald in Leipzig just a year before Wolfgang Ostwalddid the same. Two years later Freundlich began

publishing in the flagship journal of colloid chemistry, KolloidZeitschrift. The journal was edited by the younger Ostwald forover three decades and would host the overwhelmingmajority of publications by Freundlich and his collaboratorsduring that period. Like many colloid chemists, Freundlichalso emphasized the importance of colloids to biology, and hesupported research in this vein in his department of theInstitute. However, even when researching the properties ofbiological compounds, Freundlich framed his experiments asinvestigations into the general principles of colloid behavior,fully in keeping with the tenor of the Ostwald campaign.

Freundlich first made a name for himself in capillary andadsorption chemistry, investigating the thermodynamics ofliquid–solid and gas–solid interfaces and the differencesbetween various adsorption phenomena, in modern termsthe distinction between chemisorption and physisorption.Following the broader definition of colloids proffered byOstwald, Freundlich saw this research as bearing directlyupon colloid chemistry, in that it aimed at clarifying thegeneral principles of surface interactions. In his research onadsorption, Freundlich relied heavily upon the earlier work ofthe Yale University physicist Josiah Willard Gibbs to developa quantitative account of the phenomena early researchershad only described qualitatively. Perhaps the most enduringlegacies of this research are the Freundlich isotherm, relatinggas adsorption to pressure at a given temperature, andFreundlich�s textbook on capillary chemistry, which wentthrough four editions and remained a standard reference inthe field for many years. However, of greater immediate

Figure 25. Potential energy surface of the H +H2QH2 +H reaction for a colinearcollision geometry as reported by Henry Eyring and Michael Polanyi in 1931.[36]

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import for Freundlich�s role at the Institute was hischoice of experimental systems. Freundlich studiedthe adsorption of non-electrolytes and weak elec-trolytes on activated charcoal, which became a keycomponent in German gas mask filters—hence hisinvitation to leave behind the Technische Hoch-schule Braunschweig and join Haber�s Institute in1916.

Immediately after the war, Freundlich had twoprimary assistants, Alexander Nathansohn andHans Kautsky. Nathansohn was a rarity, an inde-pendent chemist who developed practical industrialapplications. He worked with Freundlich on “wetmetallurgy,” that is, the application of knowledgeconcerning metal solutions and colloids to refiningprocedures. Together, they developed a patentedmethod for separating lead from tin in mixed oreswith high sulfur content. However, Nathansohn alsoencouraged, if not sparked, Freundlich�s interest in theinteraction of colloids with light. In addition to an articlewith Nathansohn on photochemical reactions in colloids,[43]

Freundlich published an article on the electrocapillarity ofcolored solutions in collaboration with Marie Wreschner.[44]

Freundlich and Wreschner included industrially significantdyestuffs in their research, but they maintained a focus on thegeneral phenomena of electrocapillarity, using a techniqueHaber developed in collaboration with Klemensiewicz whileat Karlsruhe to measure the potential of glass electrodes, thenrelating this potential to the migration of dyestuff ions insolution.

Freundlich himself went on to attempt a general thermo-dynamic account of electrocapillarity, similar to his earlierwork on adsorption. Further exploration of the opticalproperties of colloids devolved to Hans Kautsky, who wassoon joined by Hans Zocher. Kautsky had begun work underFreundlich during the war, before completing his doctorate.Biochemistry would become the best acknowledged benefi-ciary of Kautsky�s research. An extension of the experimentaltechniques Kautsky began developing at the Institute led tothe first recognition of the Hirsch–Kautsky effect, thecharacteristic quenching of chlorophyll fluorescence. HansZocher�s research, on the other hand, focused on optical andmagnetic anisotropy in colloid systems, including the stream-ing or flow birefringence identified by Georg Quincke at theturn of the century. Zocher�s recognition of the relationbetween the asymmetry of colloid particles, the anisotropy oftheir structure when stressed, and their birefringence is oftencited as one of the earliest steps toward the development ofliquid-crystal technologies. As with Kautsky and lumines-cence, Zocher published his most widely cited works in thisfield after his departure from Haber�s institute, in Zocher�scase while a professor at the Technical University in Pragueduring the 1930s. However, both lines of research clearlyoriginated at Haber�s institute and relied, in their formativestages, upon the work of institute colleagues. Kautsky drew, inparticular, upon the work of Haber and Zisch on lumines-cence, while Zocher had immediate access to X-ray crystallo-graphic studies of colloid structure, as well as a rudimentary

theory of the structure of particulate colloids developed byEugene Wigner and Andor Szegvari.

Upon his arrival in 1921, Georg Ettisch added a new facetto research in the Freundlich department. Ettisch, whoremained at the Institute until forced to leave in 1933,embodied Freundlich�s belief in the biological significance ofcolloid chemistry. Whereas Freundlich restricted himself tousing biologically significant compounds in studies of generalcolloid phenomena, such as adsorption or coagulation, Ettischendeavored to relate these general phenomena to specificbiological functions. This was a widespread field of researchduring the 1920s, but its pursuit often led to tensions betweenchemists and medical researchers, as chemists frequentlyshowed little respect for clinical research protocols andposited simple mechanisms to explain complex biologicalphenomena on the basis of exclusively laboratory research.Ettisch and his collaborators published careful studies ofcoagulation and the colloid behavior of blood serum andsimilar substances, but their research was not pivotal to therealization that proteins and other substances vital to thestructure and sustenance of life were in fact macromolecules.

Instead, what colloid research at the Institute becamewidely known for, in addition to photochemistry and capillarychemistry, was the study of thixotropy, the reversible con-version of a semirigid gel to a fluid sol through shaking,stirring, or similar prolonged exposure to shearing forces. Inthe first years after the war, Freundlich returned to the studyof coagulation only in passing, but he took a renewed interestin the topic after two junior researchers in his department,Emma Schalek and Andor Szegvari, observed reversible sol–gel transitions in iron oxide colloids. Freundlich coined theterm thixotropy to describe the phenomenon and made thestudy of reversible transition phenomena a mainstay of hisresearch for the remainder of his career. Initially, the ironoxide observations inspired several short-lived investigationsthat met with varying degrees of success, including a study ofthe structures of iron and aluminum hydroxides, bothingredients in thixotropic colloids, carried out by JohannBçhm, who married Emma Schalek in 1925.[45] Freundlich,however, soon embarked on a more systematic study ofcoagulation times and possible mechanisms for thixotropy. In

Figure 26. Herbert Freundlich’s Colloid Chemistry Department, end of the 1920s.

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1928, Karl Sçllner joined Freundlich in this research, andtogether they extended the study of thixotropy from tran-sitions induced by mechanical stress to those induced byultrasound. This research did not lead Freundlich and Sçllnerto fundamental new insights into cohesive forces, but it did laythe groundwork for our present understanding of a phenom-enon vital to numerous industrial products including solderpastes and certain adhesives.

During the Nazi era, most of the departments of theInstitute focused on the structural analysis of fibers, glasses,synthetic materials, and metals. The director himself, PeterAdolf Thiessen (1899–1990; Figure 27), headed a relativelylarge department where one of the main interests was thestructure of soaps and soap gels, which acted as a model forthe colloid properties of long-chain molecules. The intentionwas then to transfer these findings to a broad range ofsubstances, such as higher carbohydrates, dyes, rubber,cellulose, and other high-mo-lecular-weight polymers.Thiessen maintained: “Oncewe have established the pro-cesses involved in gel forma-tion, it will be possible toclarify and explain the behav-ior of technologically signifi-cant mixtures, and to cultivatethose properties [of them thathave] practical value”.[46] Inaddition to X-ray analysisand ultramicroscopy, the de-partment also used optical andthermodynamic methods tostudy key interactions be-tween the hydrophilic and hy-drophobic parts of the rodlikemolecular structures. JoachimStauff observed a bilayer insoap films, which was heldtogether by water moleculesthat forced themselves between the hydrophilic carboxylgroups, which were directed inward, toward one another,while the lipophilic hydrocarbon chains of the soap moleculesformed an external barrier (Figure 28). He was thus the firstto recognize clearly the fundamental principle behind thestructure of cell membranes and many similar aggregates.

A newly available Agfa color film was later used todocument the formation of stable aggregates in soaps throughcolor changes in polarized light. Here, too, Thiessen�s groupsearched for relationships between the properties and thestructure of the molecules, and between their spatial arrange-ment and the twisting, shifting, and stretching of theirconstituent parts. But the processes involved in aggregateand micelle formation are quite complex, and they were,therefore, unable to establish any universally applicablequantitative laws describing the phenomenon.

Beginning in 1934, August Winkel directed an independ-ent Department for Colloid Chemistry, through which hefurthered the research on aerosols, smokes, and fogs that hehad previously pursued under Gerhart Jander, the first, albeit

provisional, director of the Institute under the Nazis. Winkelemphasized the relevance of research on these mixed-phasesystems to meteorology and to occupational health, forexample, protection from inhaled particulates through theuse of smoke and dust filters. He also noted, markedly morereticently, the possible military applications of such research,which, in addition to smoke screens, included distribution ofpoison gases, which were generally aerosols of toxic liquids.The Jander department also carried out research on filters toguard against chemical weapons and on filter-breakingcompounds. Filtration research focused mainly on adsorptionfilters and porous materials, but also extended to industrialelectro-filtering, which was important in the recovery ofscarce raw and manufacturing materials.

Modern analytical methods were used to study particles assmall as 0.1 mm; amongst the main methods were lightabsorption spectroscopy, X-ray crystallography, electron

diffraction and, most impor-tant of all, ultramicroscopy.Conductivity measurementsalso ranked amongst the keyanalytical tools in Winkel�sdepartment, as they had un-der the leadership of histeacher, Jander. In this re-spect the polarography tech-niques developed by JaroslavHeyrovsky in Prague wereparticularly important.These techniques were alsoused in analyses of the struc-tures of organic molecules, inwhich differences in the re-duction potential of individ-ual functional groups, forexample, keto, carbonyl, orcarboxyl groups, were relat-ed to the chemical structuressurrounding the group.[51]

3.4. Electron Microscopy

Two of the founding fathers of electron microscopy woulddirect departments at the Institute. Erwin W. M�ller (1911–1977) would leave in 1951, but Ernst Ruska (1906–1988)would direct a department at the Institute for over twenty-fiveyears (1949–1975), and thereby define a significant portion ofits research profile. In his Diploma Thesis, completed underthe direction of Max Knoll, Ernst Ruska worked on thefocusing of electron beams in cathode ray tubes. By 1928, hehad already obtained a magnification factor of 17.5 with anelectron-optical arrangement, and in 1932 he published,jointly with Knoll, the first description of an electron micro-scope with electromagnetic lenses.[49] Its magnification cameclose to that of an optical microscope, which was firstexceeded with the next prototype in late 1933. This markedthe birth of the “ultra-microscopes”.

Figure 27. Peter Adolf Thiessen, standing on the left, points outconstruction details in one of the workshops, circa 1939.

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Four years later, Ruska persuaded Siemens AG to set up alaboratory dedicated to “ultra-microscopy.” There he devel-oped, jointly with his brother-in-law, Bodo von Borries, thefirst commercial magneto-optical electron microscope, with amagnification factor of over 30000 and a resolution of about30 nm. By the end of World War II, 40 such transmissionelectron microscopes had been produced and put in theservice of physical and biomedical research. Ruska�s youngerbrother Helmut would be particularly important to the earlydevelopment of the technique for biomedical research, and in1938 the Ruskas (Figure 29) obtained the first images ofviruses.

In 1949 a department was established for Ruska at theInstitute, although he continued to work for Siemens and alsoheld an honorary professorship at the Freie Universit�t and alectureship at the Technische Universit�t Berlin, where hewould become adjunct professor in 1959. Director Max vonLaue agreed to expand Ruska�s department in 1952, so that hemight work full-time at the Institute, allowing him to focus ontechnical improvements in electron microscopes withoutregard to the economic factors that were of import toSiemens. Nonetheless, Ruska brought considerable fundingfrom Siemens with him to the FHI;[50] from 1954 on, hisdepartment, and later the Institute, received no less than150 000 DM per year. He also brought several members of hisscientific and technical staff with him from Siemens, includingleading scientists K�the M�ller and Wolfgang Dieter Riecke.

The industrial origins of Ruska�s methods and staff had asignificant impact on the manner of work done in hisdepartment. The Ruska group did not focus so much onscientific issues, as the solving of problems related to theconstruction of ever more powerful electron microscopes.Their goal was to improve resolution up to the theoreticallypossible atomic scale. Up to 1951, Ruska pursued this goal inthe context of a friendly competition with his fellow depart-ment head Erwin M�ller. To improve resolution, department

members investigated electrostatic as well as magnetic lenssystems, and studied not only transmission electron micros-copy, which was the center of most researchers� attention atthe time, but also other applications of electron optics. Oneexample of this broad line of investigation was the doctoralresearch of Wilfried Engel. In the years between 1960 and1968, Engel developed a high-resolution, emission electronmicroscope in which the image-generating electrons could beproduced either thermally, using UV light, or by bombard-ment with neutral gas atoms.[48]

Ruska�s pet project was the development of a completelynew, short-focal-length, electromagnetic, single-field con-denser objective where the object lay at the focus of amagnetic lens—a now-standard arrangement in electronmicroscopy. Both the structural details and the adjustableparameters of the microscope had to be recalibrated tosupport this sensitive, high-performance system. The resultwas the DEEKO 100 (Figure 30). Completed in 1965, it was atransmission electron microscope with an accelerating voltageof 100 kV that enabled magnification of 800000:1. Its highperformance was the result of a delicate technical precisionthat required highly stable and reliable voltage sources as wellas electronic measurement and control instruments. Theconsiderable length of the instrument also made it susceptibleto vibrations. It could only achieve its highest resolution,2.5 �ngstrçm, during the night, when ground vibrations wereat a minimum. In light of this problem, Riecke and his teamconducted initial experiments on vibration-damping suspen-sion systems. Meanwhile, Hans G�nther Heide, one ofRuska�s most talented construction technicians, took a differ-ent approach and disconnected the adjustment mechanismfrom the support table. This markedly improved the perfor-mance of the microscope, but ground vibrations wouldcontinue to be a troublesome source of “noise” to whichRuska and his colleagues had to attend when developing high-performance instruments.

Ruska was on the verge of retirement (January 1, 1975)when a new building was completed for his Institute forElectron Microscopy (IFE). Ruska’s focus on ever higherresolutions continued to shape the IFE until the very end ofhis term of service. In 1972 construction was supposed tobegin on a new microscope with a 250 kV single-fieldcondenser objective. This DEEKO 250 was supposed to makepossible a resolution of approximately 1 �ngstrçm. However,sundry difficulties slowed its development, so that it onlybecame operational under Ruska�s successor in 1980. Thepotential of the DEEKO 100, on the other hand, wasexploited to the very theoretical limits of its performance.Building on his experience at Siemens, Karl-Heinz Herrmanndeveloped helpful image enhancement techniques.[52] Anoth-er landmark development of the early 1970s was theconstruction by Hans G�nther Heide of a helium-cooled,cryogenic single-field condenser objective that could be usedfor temperature-stabilized observations in the range between6 and 300 K. With cryo-electron microscopy one could sharplyreduce thermal vibrations as well as radiation damage, so thateven sensitive biological specimens could be effectivelyimaged. The first trials with bacteria were undertaken at theInstitute in cooperation with veterinarian Siegfried Grund.[53]

Figure 28. A bilayer in soap films as reported by Joachim Stauff,1939.[47]

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A committee was established to search for a successor toRuska who might steer the IFE toward experimentalapplications of electron microscopy. They eventually cameto agree on Elmar Zeitler who was working in Chicago at thetime. Zeitler had studied physics in W�rzburg and receivedhis doctorate in 1953 under Helmuth Kulenkampff. Aftersome work in industry and a stint as a guest at the KarolinskaInstitute for Cell Research in Stockholm, Zeitler moved toWashington D.C. as Assistant Chief for Biophysics at theArmed Forces Institute of Pathology. Then, in 1971, heaccepted an appointment at the University of Chicago asProfessor of Physics and of Biophysics and member of theEnrico Fermi Institute.

Crucial to the decision in favor of Zeitler was his work onthe quantitative interpretation of electron microscopy imagesand on the scanning transmission electron microscope(STEM), which was developed by Albert Crewe duringZeitler�s time at Chicago. While Ruska understood electronmicroscopy by analogy to light microscopy, Zeitler saw it inthe context of other measurement techniques. Shortly beforehis appointment to the FHI, he became the founding editor ofthe new journal Ultramicroscopy. His wide-ranging interests,which covered all aspects of electron microscopy from itsfundamental theories to its subtlest applications, was seen asan indicator of his ability to integrate electron microscopywith ongoing research in the other subsections of the FHI.This was deeply desired and much discussed, but easier saidthan done.

One subject of particular interest to Zeitler was the studyof the microstructures of biological samples. But heavy

bombardment with electrons gen-erally destroyed these structuresbefore a satisfactory image couldbe generated. An array of novelmethods that led to significantimprovements in image generationhelped. One of these improve-ments was the optimized adjust-ment procedure, developed by fiveformer co-workers of Ruska,prominent among them theoristPeter Schiske, and jocularly dub-bed the “F�nf-M�nner-Arbeit”(“five-man paper”). In addition,Friedrich Zemlin developed a ta-bleau that offered an overview ofpossible image frames and greatlysimplified the calibration of opticsfor practical applications;[54] con-sulting such a tableau is nowstandard procedure when a micro-scope is operated near its perfor-mance limits, and the tableau bearsZemlin�s name. With the imagequality of the DEEKO 100 en-hanced through such methods itwas possible for the first time, incooperation with Peter Otten-smeyer in Toronto, to generate a

comparatively low-noise image of protamine proteins.[55]

When Zeitler arrived in Berlin, Heinz-G�nter Wittmann,director of the newly founded MPI for Molecular Genetics inBerlin-Dahlem since 1964, was beginning to build up aninternational center for ribosome research. In 1979, Ada E.Yonath joined Wittmann at the MPI for Molecular Genetics.Wittmann wanted to take advantage of the electron micros-copy expertise on hand at the IFE, since the crystallizationprocedure necessary for the X-ray analysis of ribosomeproteins had proven a formidable challenge. However withthe help of digital image processing and a peculiarity of thesample preparation process that guaranteed the ribosomesorganized themselves into coplanar groups of four that couldbe arranged into “class average images,” it became possible togenerate images of the roughly 20 nm recognizable images ofthe ribosomes. Electron crystallographic data, much of itgathered with new low-temperature methods, would strength-en Yonath in her basic assumptions concerning the form andfunction of ribosomes. For example, in 1995, a ribosomaloutput channel for proteins could be discerned, an achieve-ment in which Marin van Heel from Zeitler�s department andhis student Holger Stark played key roles.[56] In the end, eventhough it was X-ray analysis that enabled Yonath to finalizeher structural explanation of ribosomes, for which shereceived the 2009 Nobel Prize in Chemistry, part of herachievement can be traced back to electron microscopy workdone at the FHI.

Electron crystallography of the smallest specimens wouldbe the special focus of the research group under Marin vanHeel, who came to Berlin in 1982 from Groningen University

Figure 29. Ernst (left) and Helmut Ruska at theelectron microscope, circa 1957.

Figure 30. The DEEKO 100. Cross-section fromthe title page of a brochure.

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and remained at the FHI until 1996, when he accepted aprofessorship at Imperial College London. Van Heel devel-oped computer programs for the automatic classification ofindividual image frames, by use of which sequential viewsfrom multiple perspectives could be made the basis fortomographic representations[57] of single particles or mole-cules.[58] This enabled the elucidation of the structures of anarray of receptors, enzymes, and functional protein com-plexes, such as the portal protein of the bacteriophage SPP1.

One key component for advancing this line of work was asuperconducting lens developed in the research labs ofSiemens AG in Munich by Isolde Dietrich�s group.[59] Withthis new tool it would become possible to produce “molecularmaps” of proteins. Installation of the lens brought with it areduction in signal noise, as well as enabling the constantcooling of the sample to 4.2 K. This allowed the use of lowerbeam currents with longer exposure times, which reduceddamage to specimens. The lens would be included in twodifferent electron microscopes: first, the home-built “Sulei-ka” (Supraleitender K�hlapparat), then SOPHIE (Super-conducting Objective in a Philips electron micro-scope), built with the help of Philips and a specialgrant for international academic–industrial col-laboration from the European Union.

At the same time, projects related to surface-sensitive methods supported stronger bondsbetween Zeitler�s department and other depart-ments at the Institute and contributed to itsrepertoire of surface science techniques, oneexample of which was the so-called “PEEM-chen.” In large part because of the inadequacy ofUHV technology, Ernst Bauer of the TechnischeHochschule Clausthal was only able to imple-ment the long-familiar notion of a low-energyelectron microscope (LEEM) in 1985.[60] Thistechnique allowed the observation of adsorbatedistribution on single-crystal surfaces in real time and at highcontrast. Alexander Bradshaw investigated related phenom-ena, and a cooperation grew up, initiated by Bradshaw andZeitler, between their respective co-workers Wilfried Engeland Marty Kordesch, aimed at recreating the revolutionaryinstrument with the help of Bauer. In the resulting instru-ment, a photoelectron emission microscope (PEEM), thephotoelectric effect stimulates the emission of slow electronsfrom the specimen, which are then used to form an image ofits surface. The dynamic nature of the images produced by theinstrument impressed both Bradshaw and Gerhard Ertl;[61] forexample, it enabled one to see the accrual of carbon inethylene adsorbed on platinum. The decision was reached tobuild a more sophisticated, user-friendly “PEEMchen,” atwhich point Ertl�s co-worker Harm Hinrich Rotermundjoined the PEEM group. Rotermund was able to developimpressive depictions of spatio-temporal changes in surfacestructures.[62] The “PEEMchen” would be patented on behalfof Engel by the MPG and offered for sale by STAIBInstruments, making it widely available for the study ofsurface reactions. This line of work is still being continuedthrough the SMART project at BESSY II.

Another quite successful project of this sort was theintroduction of electron energy loss spectroscopy (EELS)into the Electron Microscopy Department. When Zeitlerarrived at the Institute, many of the microscopes were eitherobsolete or inoperable. Making the best use of what wasavailable, he encouraged the inclusion of a radiation sourcealready at hand in the construction of an energy lossspectrograph, in which a sector magnet spectrally resolvedthe electron beam and a system of lenses projected this energydistribution without rotation across the entire breadth ofspectra which could be selected from different energyranges.[63] The arrangement proved to be a high-performanceEELS apparatus; in 1982, Zeitler, Engel, and Herman Sauerachieved an unrivaled lateral resolution of 0.2 nm with anenergy resolution of 0.2 eV. Since it provided a means forspatially resolved chemical analysis of minimal samples ofsubstances adsorbed on solid surfaces, EELS was a valuablesurface science technique.

Rik Brydson, a student of John Meurig Thomas, one of thefathers of modern heterogeneous catalysis research, traveled

to Dahlem regularly for several years after 1986so that he could carry out high-resolution EELSanalyses of the structures of minerals such asrhodizite,[64] rutile, and anatase.

3.5. BESSY

In addition to leading a research group at theFHI, London- and Munich-trained AlexanderBradshaw (*1944; Figure 31) took on a distinc-tive, public role in forwarding new lines ofresearch in the late 1970s; he championed effortsto make synchrotron radiation more accessible toresearchers at the Institute and in Berlin gener-ally. Around the time of Bradshaw�s move to the

FHI, users of synchrotron radiation worldwide were consid-ering plans for new “dedicated” sources. Late in 1976, in partin response to plans for a new 300 MeV synchrotronpresented by Burkhard Wende of the Berlin branch of theFederal Institute of Physics and Technology (Physikalisch-Technische Bundesanstalt, PTB), Bradshaw, together withHelmut Baumg�rtel of the Freie Universit�t and with thesupport of FHI Director Heinz Gerischer, began lobbying fora larger storage ring that could offer beam time to regionalusers, including the FHI, for experiments in solid-state andsurface sciences as well as in gas-phase spectroscopy. Shortlythereafter, an Expert Committee under the direction ofManuel Cardona was set up to evaluate proposals fordedicated synchrotron radiation sources in Germany. Thecommission considered a proposal by Ernst-Eckhart Koch,Christoph Kunz, and Gottfried M�lhaupt from the DESYaccelerator center to build a 700 MeV storage ring at DESYand a recommendation from a working group within theFederal Ministry of Research and Technology (BMFT) thatwas studying the potential use of X-ray lithography inmicrochip production, as well as the Berlin proposals. Thecommission advised that two storage rings would be optimal,an X-ray facility in Hamburg and a separate ultraviolet

Figure 31. Alexander Brad-shaw, 1977.

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source, originally slated for Bonn. Political and fundingconsiderations, however, tipped the balance in favor of thesecond site being Berlin, and just over two years later theBerlin Electron Synchrotron (Berliner Elektronenspeicher-ring-Gesellschaft f�r Synchrotronstrahlung, BESSY) wasestablished as a limited-liability company (GmbH) for theconstruction and operation of an 800 MeV electron storagering in Berlin. BESSY had eight original shareholders, fourelectronics companies: Siemens, Telefunken, Eurosil, andValvo (Philips); and four research organizations: the MaxPlanck Society, the Fraunhofer Society, the Hahn MeitnerInstitute, and DESY, but the majority of funds for itsconstruction came from the German federal government.

In light of the special interest in synchrotron radiationshown by Bradshaw and Gerischer, the CPT Section of theMPG agreed to a proposal from the Board of the FHI inFebruary of 1978 that the Scientific Director of BESSYshould also be appointed a scientific member of the FHI. Asearch committee formed within the CPT Section and choseBradshaw for the post, and he took over as Scientific Directorat the beginning of 1981, 18 months before the facility begannormal operations, and just after he was promoted toScientific Member of the FHI and head of the Departmentof Surface Physics. Bradshaw remained Scientific Director atBESSY until the end of 1985, then returned to the post forroughly a year in 1988, following the death of his successorErnst-Eckhard Koch. Bradshaw credits the success of useroperation at BESSY in the early years to a small butextremely capable in-house group, headed by William Peat-man.

The challenges of administering BESSY occupied much ofBradshaw�s time during the early 1980s. Nevertheless, Brad-shaw and his FHI group managed to make several significantscientific contributions during the 1980s, in particular tosynchrotron instrumentation. Together with Eberhard Dietzand Walther Braun, Bradshaw built a high-flux, high-energytoroidal grating monochromator (HE-TGM-1) for BESSYthat began operation in 1984. In collaboration with ManuelCardona, among others, Bradshaw also developed a VUVellipsometer that enabled novel research into the opticalproperties of solids and surfaces. The first experiments withthe infrared component of synchrotron radiation, to which

Erhard Schweizer and Ernst Lippert were key contributors,were also made at this time. At the close of the decade, theBradshaw group, in particular Josef Feldhaus, then took aleading role in the construction of the X1B undulatorbeamline at the National Synchrotron Light Source inBrookhaven, New York.

Bradshaw also remained active in promoting new syn-chrotron facilities. Even before his first term as scientificdirector at BESSY came to an end, he had begun lobbying forthe construction of a new “third-generation” synchrotron inBerlin. Since the construction of BESSY (hereafter BESSY I;Figure 32), physicists had developed “wigglers” and “undu-lators” that could increase the spectral brilliance of synchro-tron radiation several orders of magnitude by inducingperiodic, “sideways” oscillations of the electron beam in theotherwise straight sections of the storage ring. Just three yearsafter BESSY I became operational, Bradshaw and colleaguesGottfried M�lhaupt, William Peatman, Walter Braun, andFranz Sch�fers sent a proposal to the BESSY SupervisoryBoard for a 1.5 GeV storage ring at the site in Berlin-Wilmersdorf using BESSY I as the injector.[65] The ideabehind “BESSY II” was to cover roughly the same soft X-rayspectral range of the BESSY I bending magnets but with themuch brighter undulator radiation. The first publishedproposal for BESSY II appeared at the end of the year andincluded among its contributors not only Alexander Brad-shaw and Ernst-Eckhard Koch, but also Karsten Horn, DieterKolb, and Josef Feldhaus of the Fritz Haber Institute andHans-Joachim Freund, who was then at Erlangen but wouldjoin the FHI as Director of the Department of ChemicalPhysics in 1996. In 1989, when the Berlin Wall fell andGerman reunification quickly followed, a whole host ofpossible new sites for the accelerator became available. Analternative site was chosen in Berlin-Adlershof (Figure 33),home of many institutes of the former Academy of Sciencesof the German Democratic Republic, and the plan to useBESSY I as an injector was abandoned.[66] No parallelappointments similar to those spanning the FHI and BESSY Iwere made, but the ties between synchrotron radiationsources and research at the FHI remained firmly intact,above all through the efforts of Bradshaw and members of hisDepartment of Surface Physics, but also through the work ofHans-Joachim Freund and Robert Schlçgl and their respec-tive departments.

Bradshaw embarked upon at least three new lines ofresearch that specifically took advantage of the availability ofsynchrotron radiation sources. The first, a long-runningapplication of “energy scan” photoelectron diffraction tothe study of adsorbed molecules and molecular fragments incollaboration with Phillip Woodruff of the University ofWarwick. Although photoelectron diffraction had beenknown for more than 15 years when Bradshaw and Woodruffbegan their collaboration, they were able to provide novelquantitative structural information for over a hundredadsorption systems (to date) by taking full advantage ofsynchrotron radiation and efficient, innovative simulationcodes written by Volker Fritzsche. So-called direct methodswere also pioneered in the group at this time, particularly byPhilip Hofmann. The second line of research undertaken by

Figure 32. BESSY I in Berlin-Wilmersdorf, 1986.

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Bradshaw and his colleagues involved a series of photo-ionization studies of free molecules in the core-level region athitherto unavailable spectral resolution. This research madeextensive use of the previously mentioned X1B beamline atBrookhaven. Among other novelties, these investigationsdemonstrated the importance of vibronic coupling in mole-cules containing equivalent cores and provided the firstmeasurements of the influence of shape resonances anddouble excitations on the vibrational fine structure of corelevel lines of various small molecules. Bradshaw�s third majorarea of research in the 1990s was in low-energy electronmicroscopy and photoelectron microscopy with WinfriedEngel and Elmar Zeitler of the Department of ElectronMicroscopy. After extensive laboratory studies on, amongother things, reaction-diffusion fronts in heterogeneousreactions, this work resulted in a proposal for a photoelectronspectro-microscope for BESSY II (the SMART project)presented together with Eberhard Umbach, then of Univer-sity of W�rzburg and later an external member of the FHI,and with Hans-Joachim Freund.

These areas of research were, however, only the latestexpressions of a long-standing tradition of pursuing atomicand molecular physics using synchrotron radiation within theDepartment of Surface Physics. The group of Ulrich Heinz-mann performed pioneering studies of photoionization pro-cesses using circularly polarized radiation and spin-polarizeddetection of the photoelectrons, before Heinzmann wasappointed to a Chair in Bielefeld in 1985. Ernst-EckhardKoch, Heinzmann�s successor at the FHI, also undertookseminal experiments on molecular crystals before his un-timely death in 1988.

3.6. Surface Science

Electrons penetrate less deeply into a sample than X-rays,and soon after the diffraction of electrons by crystals was firstrecognized, in 1927, scientists realized that this limitedpenetrating power made electron diffraction analysis aparticularly powerful tool for analyzing surface structures.However, the expensive equipment and elaborate mathemat-ical methods required by the technique meant it was rarelyused in chemical research. Beginning in the National Socialistperiod, researchers at the Institute performed electrondiffraction studies using fast electrons (HEED), whosediffraction patterns were not as exclusively dependent uponsurface features but which gave rise to fewer experimentaldifficulties. They developed both new sample preparationtechniques and new methods of diffraction pattern analysiswhich allowed them to translate their data into an atomic ormolecular structure. Since surface structures play a substan-tial role in the adsorption processes involved in heteroge-neous catalysis, Theodore Schoon and his colleagues used thenew technique to investigate microcrystalline platinum cata-lysts and similarly active iron(III) oxide. Parallel researchfocused on the porosity and gas permeability of catalyticmaterials and on the systematic collection of data relating toindustrial catalysts. Beginning in 1941, Schoon had access tothe new electron microscope developed at Siemens by Ernst

Ruska,[67] who was in personal contact with Peter AdolfThiessen.[68] Schoon also used the new instrument to study thesize and shape of the particles in rubber fillers, especiallysoots, and the effects of the fillers on the properties of therubber mixtures. But Schoon�s studies were just a beginning.Electron diffraction analysis would remain a standard re-search technique at the Institute for over forty years, largelythanks to the efforts of Kurt Moli�re, who first arrived at theInstitute in the late 1930s and was a department head from thetime the FHI joined the MPG until his retirement in 1980.

In 1964, FHI Director Rudolf Brill created a researchgroup dedicated to field electron emission spectroscopy. Itwas initially headed by Werner A. Schmidt, who moved toBrill�s department from the remainder of Erwin M�ller�sgroup, which had been integrated into Gerhard Borrmann�sdepartment. In 1966, Jochen H. Block took over the group,joining what would become a 50-year tradition of fieldelectron emission spectroscopy at the Institute spanning fromend of the 1940s well into the 1990s (Figure 34). Brill soughtto groom Block for leadership, recognizing that the latter wasinterested primarily in spectroscopic methods, especiallyfield-ion mass spectroscopy, that could be used for surfaceanalysis, and hence might complement Brill�s work onheterogeneous catalysis.

Block had completed his doctorate in 1954 under thedirection of Georg-Maria Schwab, the father of Germancatalysis research, and he remained in Munich through 1960to complete his habilitation. He worked subsequently for theEuropean Research Association in Brussels and spent a yearin the United States as a consultant for Union Carbide. Blockand his group conducted experiments with field electronmicroscopes and mass spectrometers, seeking both to developmore advanced research techniques and to explain themechanisms of surface catalytic reactions. For example, theirmass spectrometer findings, together with infrared spectra,appeared to support Brill�s hypothesis that during the Haber–Bosch process higher nitrogen hydrides are formed on thecatalyst surface that then break down into ammonia beforedesorption.[69]

Later, under Gerischer, Block was able to form the core ofwhat would become the Department for Surface Reactionsusing the resources available from his earlier research group.Soon Kurt Becker and his research group also joined Block inthe new department. Becker persisted in his research into

Figure 33. BESSY II in Berlin-Adlershof.

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heterogeneous catalysis with catalysts such as zeolites. Oneprominent application of these aluminosilicates was ascatalysts in the petrochemical industry. Studies at the Instituteconcentrated primarily on their structure, stability, andreactivity. However, in addition to seeking a better under-standing of mechanisms of catalysis, members of the Institutealso carried out experiments on reaction kinetics and catalystpoisoning. Becker�s group found that the limits on the lifespanof petrochemical zeolite catalysts were set by self-poisoningwith polymerized olefins, a product of the reactions theycatalyzed.[70] Hellmut G. Karge, who retired in 1996, was oneof the exceptionally productive members of this group. Hemodified zeolites and other catalysts using ion-exchangemethods and thereby improved their longevity and selectivity.

The main focus of Block�s interests was the behavior ofsurfaces in strong electric fields, which he explored using fieldemission phenomena, especially field-ion microscopy andfield-ion mass spectrometry.Field desorption permittedinferences regarding theelectronic properties of sur-faces and surface adsorbates,and the atomic-scale resolu-tion of the technique allowedvery precise local analysis ofcrystallographically well-de-fined surfaces. However, itrequired that the substratebe manufacturable in theform of thin, sharp needles.Also, since the photoexcita-tion of field-ion formationusing light, synchrotron radi-ation, or laser pulses (photofield-emission) evinced nopenetrating power, it was treated as a technique that actedonly at the surface. Institute members used this technique tostudy samples of tungsten, silver, and aluminum hydroxide,upon which H2, O2, H2S, or ethylene had been adsorbed,across a range of temperatures.[71] Later experiments alsoexamined superconductors. One of Block�s abiding collabo-rators in these studies was Wolfgang Drachsel, who later led aresearch group in Hans-Joachim Freund�s department andwas active at the Institute until 2004.

Another central and persistent focus of research inBlock�s department was heterogeneous catalysis, a field inwhich Block showed an early interest as cofounder and firstchairman of the DECHEMA (Gesellschaft f�r ChemischeTechnik und Biotechnologie) section for catalysis. As a test ofthe applicability of field emission methods in catalysisresearch, the Block group examined simple systems such asthe adsorption of noble gases or the chemisorption of CO,both of which manifested significant deviations, while in theapparatus, from their reactivity outside an electric field.Theoretical work based on models from “high-field chemis-try” and completed, in part, through a close collaborationwith Hans J�rgen Kreuzer, backed up these experiments.Kreuzer was Professor for Theoretical Physics at DalhousieUniversity in Halifax, Canada, and was named external

member of the FHI in 1998. In later experiments, the surface-specific adsorption and chemisorption of various smallmolecules on metal surfaces were explored, phenomena thatwere of decisive import for heterogeneous catalysis. In orderto study these systems, new methods were needed thatincreased the sensitivity of the apparatus. Hence, the field-ionmicroscope was equipped with a kind of atom probe anddeveloped into a field-ion energy spectrometer that couldgenerate data relating to the position and the energy ofindividual surface atoms at defined points on the microscopetip. One model system for induced field desorption studieswas the formation of singly and multiply charged hydrogenions. With linear H3

+ ions in strong fields, it was shown thatthe H3ad species was positioned upright and linearly againstthe probe surface.[72] Among the collaborators who workedextensively with Block on the kinetics of reactions on metalsurfaces was Norbert Kruse, who had already been in Berlin

and affiliated with the FHIalmost 10 years in 1977, andwho is now Professor forChemical Physics at Univer-sit� Libre in Brussels.

Block placed greatweight on modern technicalfacilities, and in the Depart-ment for Surface Reactionsseveral other instrumentalmethods were advanced, pri-marily ones related to elec-tron field emission or ionemission. Pulse methodswere developed that allowedfor time-of-flight mass spec-trometry,[73] including a var-iant of the technique in

which a laser-stimulated photoemitter replaced the high-voltage pulse generator for exact mass determination;[74] forthe necessary UV radiation Department members relied onHASYLAB at DESY and on BESSY (see above). Throughthe combination of pulsed desorption with time-of-flight massspectrometry and digitally processed image displays it waspossible to determine exactly the formation sites of variousmolecules.

In 1983, Elmar Zeitler, then the executive director of theFHI, traveled to Munich to sound out whether Gerhard Ertl(*1936) might accept an offer from the MPG to become adirector at the FHI; this was an unlikely outcome in the eyesof many. Munich offered Ertl generous support for hisresearch, and he was, and remains, a strong supporter of theuniversities and their students. However, Ertl accepted theoffer, in part guided by what he later called an “emotionalreason:” coming to the FHI enabled him to become thesuccessor of his mentor Heinz Gerischer, who remained ascientific role model for Ertl (Figure 35). Moreover, hisprevious experiences in Stuttgart helped convince Ertl that an“optimal arrangement” was possible, in which a Max PlanckDirector works extensively with doctoral students and juniorresearchers, contributing in this way to the mission of theuniversities, but remaining free of routine teaching and

Figure 34. From left to right: Jochen H. Block, Heinz Gerischer, ErwinW. M�ller, October 1, 1976.

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administrative duties. It was this arrangement that Ertl set outto establish in Berlin. Ertl was appointed Scientific Memberof the MPG and Director at the FHI as of April 1, 1985.Rather than immediately taking over from Gerischer, Ertlshared the Directorship of the Physical Chemistry Depart-ment with him for the next two years, while commutingbetween Berlin and Munich, to which he was still bound byprevious commitments. The transition from Gerischer to Ertlwas completed with Gerischer�s retirement on April 1, 1987,at which point Ertl became the sole director of the Depart-ment.

Ertl had studied physics from 1956 to 1961 at theTechnical University of his native city of Stuttgart where hewrote a Diploma Thesis on fast chemical reactions under theguidance of Heinz Gerischer, who was then at the Max PlanckInstitute for Metals Research in Stuttgart. His thesis, inspiredby Manfred Eigen�s relaxation technique and Gerischer�sflash, presented a kinetic study of therecombination of protons and hydroxideions induced by rapid heating of water witha microwave pulse. Ertl followed Gerisch-er to the Technische Universit�t M�nchenin 1961 as his PhD student, and it was therethat he turned to the investigation ofsurface reactions, with Gerischer�s bless-ing. At that time, the ultra-high vacuumneeded in order to keep a surface cleanlong enough to study surface reactionscould only be achieved with sealed-glasssystems evacuated by mercury diffusionpumps and baked to 450 8C. Similarly, low-energy electron diffraction instruments(LEED) capable of providing informationabout the atomic structure of surfaces werestill exceptional and large apparatus, and Ertl had to relyupon a grant from the German Science Foundation (DFG) topurchase the first such apparatus for the Munich laboratory in1965. This investment launched what Ertl later called “the realsurface science era.”

In his PhD and thereafter, Ertl combined structuralinformation with thermodynamic and kinetic data to charac-terize and explain the course of chemical reactions onsurfaces. In these early studies, one can already discernaspects of Ertl�s characteristic approach to surface science. Inhis own words, he “always attempted to tackle chemicalquestions with physical methods.” But unlike many of hiscolleagues, there was no one method to which Ertl remaineddevoted. There was, however, one question that engrossedhim: “How do chemical reactions proceed.”[75] In his inves-tigations of the surface reactions of small molecules, Ertlintroduced novel experimental techniques, such as scanningtunneling microscopy (STM) and photo-emission electronmicroscopy (PEEM), to systematically study the adsorptionand chemisorption phenomena at well-defined, single-crystalsurfaces. Such fundamental studies proved indispensible forour present understanding of heterogeneous catalysis. Keywere Ertl�s mechanistic studies relating to the Haber–Boschprocess, carried out in Munich, as well as the investigations ofthe oxidation of CO on the platinum group metals from his

FHI period. Under certain conditions, the latter reactionexhibits an oscillatory behavior that could be used to learnfundamental lessons about coupled systems in surface reac-tion kinetics and beyond. Many of Ertl�s groundbreakingstudies relied on experimental techniques that were matchedto the particular needs and conditions of an experiment andwere often deployed in novel combinations with othertechniques.

The present and future of the Institute looked quitepromising when Ertl arrived in Dahlem. Although theDepartment of Physical Chemistry under Gerischer wasalready fairly large, Ertl brought roughly two dozen addi-tional co-workers with him from Munich, most of themdoctoral students. Relatively autonomous were the researchgroup on biophysical dynamics led by Josef Holzwarth, whocame to the FHI along with Gerischer, and the group of FrankWillig, whose research dealt with the picosecond dynamics of

photo-electrochemical systems. Bothgroups occupied new labs in one of thebuildings of the Department of ElectronMicroscopy. In subsequent years, Ertl�sdepartment shrunk somewhat, but itremained the most populous at theInstitute, and doctoral students contin-ued to constitute the bulk of its scientificworkforce. Throughout his tenure, Ertldeveloped a broad, multifaceted re-search program in his department thatrevolved around the model catalyticprocess of the oxidation of carbon mon-oxide on various catalysts. The bountifulresults of Ertl�s research made an indel-ible mark on surface science, catalysis,and studies of the dynamics of complex

systems and self-organization. The main activities of Ertl�sdepartment, apart from further studies of the mechanisms ofcatalytic reactions (with Karl Jacobi and Herbert Over et al.)were:· investigation of nonlinear dynamics and spatio-temporal

pattern formation, including theory, in surface reactions(with Alexander Mikhailov and Harm Rotermund et al.);

· investigations of the above in electrochemical systems(with Markus Eiswirth and Katharina Krischer et al.);

· imaging of surface processes on the atomic scale byscanning tunneling microscopy (with Joost Wintterlinet al.);

· studies of the dynamics of fast surface processes byfemtosecond pump–probe laser techniques (with MartinWolf et al.);

· various aspects of electrochemistry (with Bruno Pettingerand Rolf Schuster et al.);

· exoelectron emission in surface reactions.

At a 1974 catalysis symposium, a doyen of the field,Paul Hugh Emmett, noted that: “The experimental work ofthe past 50 years leads to the conclusion that the rate-limitingstep in ammonia synthesis over iron catalysts is the chem-isorption of nitrogen. The question as to whether the nitrogenspecies involved is molecular or atomic is still not conclusively

Figure 35. Gerhard Ertl (left) and HeinzGerischer, 1981.

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resolved.”[76] Emmett�s lament spurred Gerhard Ertl�s resolveto meet the challenge that it embodied.[77] Ertl recognized thatthe techniques of surface science then available in hislaboratory at the University of Munich (Ludwig-Maximil-lians-Universit�t) might suffice to identify the elementarysteps of nitrogen fixation, and thereby to provide a basis for aquantitative description of the reaction that is at the core ofthe Haber–Bosch process. The surface science approach totackling problems in catalysis had been advocated already inthe 1920s by Irving Langmuir, but could be implemented onlyin the 1960s, when ultrahigh vacuum technology and surfacesensitive physical methods, such as low-energy electrondiffraction (LEED), became available.

The need for Ertl�s approach becomes evident once asample of an industrial catalyst comes under closer scrutiny.Figure 36 shows a high-activity catalyst with a rather largespecific surface area, composed of nanometer-sized activeparticles. Under reaction conditions, these are reduced intometallic iron covered by a submonolayer of potassium (andoxygen) which acts as an “electronic” promoter. The config-uration of active particles is stabilized against sintering by aframework consisting of alumina (Al2O3) and quick lime(CaO), which act as “structural” promoters. The activecomponent possesses a variety of crystal planes and defects,all of which bear on its reactivity.

Generally, atoms that make up the surface layer of a solidhave fewer neighbors than atoms within the bulk, so they arechemically unsaturated. Therefore, surface atoms can formnew bonds with molecules impinging on the surface from thegas or liquid phases (chemisorption), and modify and evenbreak the existing bonds of the impinging molecules (dis-sociative chemisorption). The species formed on the surfacemay then jump from one site to an adjacent site and reactthere with other species. The products thus formed maysubsequently detach from the surface (desorption) and leave.If the solid surface in question is that of a catalyst immersed ina flow reactor, it can partake in the chemisorption/desorptioncycle continuously without being consumed.

At low surface concentrations, the chemisorbed speciesundergo a random walk; while at higher ones, the adsorbedparticles separate into two ordered two-dimensional phases, aquasi-solid and a quasi-gas. Ertl and co-workers as well asothers have found that the formation of such structured

adsorbate phases with long-range periodicity is quite commonand that their structural parameters can be determined byelectron diffraction techniques, such as LEED. The dissocia-tive chemisorption of nitrogen on various single-crystalsurfaces of iron exemplifies the formation of such structuredphases which, in this case, stem from chemisorbed nitrogenatoms. The probability of the dissociative chemisorption ofnitrogen over iron is very low, typically on the order 10�6, andputs a cap on the total rate of ammonia synthesis. Figure 37shows how the surface concentration of nitrogen atomschemisorbed on various iron single-crystal surfaces varieswith the number of nitrogen molecules impinging on thesurface per unit surface area and time at a given temperature.The influence of the surface structure is quite pronounced.The most densely packed (110) surface is least active, whilethe open (111) plane exhibits the highest dissociativechemisorption probability and is indeed responsible for theoverall activity of the industrial catalyst. This activity isfurther enhanced by the presence of the potassium atoms inthe role of the electronic promoter, which increases thedissociation probability of the adsorbed nitrogen moleculesN2,ad. The same behavior was also found for dissociativenitrogen adsorption at high pressure, indicating that theresults are scalable in terms of pressure—there is no “pressuregap” in the case of this reaction.

After tackling the ammonia synthesis problem for about adecade, Ertl and co-workers were able to meet Emmett�schallenge: They showed that a combination of the kineticparameters associated with the individual reaction stepsshown in Figure 38 furnishes a steady-state yield of ammoniafrom the elements that, for a range of conditions, accuratelyreproduces the real-life yields measured at industrial plants.This agreement has demonstrated that, in the case of theammonia synthesis, the “surface science” approach to catal-ysis is capable of providing no less than a quantitativedescription of an industrial process.

Like the ammonia synthesis, catalytic oxidation of carbonmonoxide is at the core of a major technological process,namely the removal of toxic substances from automobile

Figure 36. Electron micrograph of a sample of an iron-based catalyst,developed by Alwin Mittasch around 1911, widely used in the Haber–Bosch process. The sample has a specific surface area of about20 m2 g�1.

Figure 37. The variation of the relative coverage, y, of nitrogen atomschemisorbed at 693 K on various iron single-crystal surfaces withexposure to gaseous N2 (1 L = 1.33 � 10�6 millibarseconds is about theexposure which would suffice to form a complete monolayer if eachincident molecule were adsorbed).

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exhaust fumes. In a catalytic converter, the exhaust fumesinteract with the surface of fine-grained particles of metalsfrom the platinum group. While the carbon monoxide (CO)molecules are adsorbed by the surface of the catalyst, theoxygen molecules (O2) contained in the fumes are dissocia-tively chemisorbed, furnishing the Oad species. These atomsreact with the chemisorbed CO to form carbon dioxidemolecules (CO2), which instantaneously desorb into the gasphase.

Under typical steady-state flow conditions, the rate ofproduct formation is time independent. However, as noted byEwald Wicke and co-workers in 1970, if the reactants arerarefied, the rate of product formation may become time-dependent and exhibit temporal oscillations similar to thoseof a Belousov–Zhabotinsky reaction in solutions (see Fig-ure 39). This behavior, explored beginning in the 1950s by IlyaPrigogine and by Hermann Haken in the framework ofsynergetics, is characteristic of open systems far removedfrom equilibrium, which may develop dissipative structures.Ertl: “A particularly spectacular example of such a behavior isthe variation with time of the number of furs from hares andlynxes delivered [by hunters] to the Hudson�s Bay Company.

The oscillating populations of both species are coupled to eachother with a certain phase shift. The reason seems to be quiteobvious: If the lynxes find enough food (= hares), theirpopulation grows, while that of the hares decays as soon astheir birth rate cannot compensate for their loss any more.When the supply of hares drops, the lynxes begin to starve andtheir population also decays so that that of the hares canrecover.”[78]

Gerhard Ertl�s research has demonstrated that a rathersimple system (a chemical reaction occurring between twodiatomic molecules on a well-defined single-crystal surfacewith fixed external parameters and well-established mecha-nism) can be used to study and model quite a complexbehavior. The conclusions which it allows us to draw aboutopen systems far from equilibrium transcend catalysis andsurface science and provide clues about laws believed togovern the whole of nature.

We would like to thank the Board of Directors of the FritzHaber Institute, who, in preparation for the 100th anniversaryof the founding of the Institute, initiated the Centennial Project,amongst whose goals was the production of this article. TheInstitute and its Directors Hans-Joachim Freund, GerardMeijer, Matthias Scheffler, Robert Schlçgl, and Martin Wolfhave generously supported the Centennial Project over the pastthree years and followed it with an abiding interest. Our thanksare also due to our numerous colleagues who providedindispensable help and assistance in the course of theCentennial Project. Thanks the Archive of the Max PlanckSociety and the Fritz Haber Institute for providing most of thephotos used in this article. Our special thanks are due to DavidVandermeulen for allowing us to use his rendition of L. Koppel(Figure 1). This aid and assistance notwithstanding, thematerial included in this article has been selected by theauthors alone and presented in the manner we felt appropriate.We alone are answerable for the interpretations of historicalevents it offers, as well as any lacunas or inaccuracies that mayhave escaped our notice.

Received: July 10, 2011Published online: September 28, 2011

[1] G. Holton, Pais Prize Lecture: Of What Use Is the History ofScience? (American Physical Society Forum on the History ofPhysics 2008, http://www.aps.org/units/fhp/newsletters/fall2008/pais.cfm).

[2] J. James, T. Steinhauser, D. Hoffmann, B. Friedrich, OneHundred Years at the Intersection of Chemistry and Physics:Fritz Haber Institute of the Max Planck Society 1911 – 2011,Walter de Gruyter, Berlin, 2011; German version: Hundert Jahrean der Schnittstelle von Chemie und Physik, Walter de Gruyter,Berlin, 2011.

[3] J. A. Johnson, The Kaiser�s Chemists. Science and Modernizationin Imperial Germany, University of North Carolina Press,Chapel Hill, 1990.

[4] For Haber�s biography, see M. Szçllçsi-Janze, Fritz Haber 1868 –1934. Eine Biographie, C. H. Beck, M�nchen, 1998 ; D. J.Stoltzenberg, Fritz Haber. Chemiker, Nobelpreistr�ger, Deutsch-er, Jude. Eine Biographie, VCH, Weinheim, 1994.

Figure 38. Mechanism and energy diagram of ammonia synthesis oniron.

Figure 39. Temporal dependence of the CO oxidation rate over a well-defined platinum (110) surface. At the time marked by the arrow, theO2 pressure was raised stepwise from 2.0 to 2.7 � 10�4 mbar. As aconsequence, the rate slowly increased and then developed periodicvariations with a large and constant amplitude.

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[5] “Die Chemische Industrie und der Krieg”: F. Haber, Chem. Ind.1920, 43, 252.

[6] “Freunde im Widerspruch. Haber und Einstein” in F. Stern,Verspielte Grçße, Verlag C. H. Beck, M�nchen, 1996. .

[7] “Das Zeitalter der Chemie” in F. Haber, Fritz Haber: F�nfVortr�ge aus den Jahren 1920 – 1923, Springer, Berlin, 1924.

[8] See for example, correspondence between Robert Havemannand his parents, Archive of the Robert Havermann Society,Berlin; also correspondence between Paul Harteck and KarlFriedrich Bonhoeffer, Paul Harteck Papers, Rensselaer Poly-technic Institute, Troy, New York.

[9] Letter from Haber to Minister Rust, April 30, 1933, Max-Plank-Gesellschaft Archive, Abt. I, Rep. 1a, Nr. 541.

[10] R. R�rup, M. Sch�ring, Schicksale und Karrieren. Gedenkbuchf�r die von den Nationalsozialisten aus der Kaiser-Wilhelm-Gesellschaft vertriebenen Forscherinnen und Forscher, Wallstein,Gçttingen, 2008.

[11] F. Schmaltz, Kampfstoff-Forschung im Nationalsozialismus. ZurKooperation von Kaiser-Wilhelm-Instituten, Milit�r und Indus-trie (Geschichte der Kaiser-Wilhelm-Gesellschaft im Nationalso-zialismus 11), Wallstein, Gçttingen, 2005 ; H. Maier, Forschungals Waffe. R�stungsforschung in der Kaiser-Wilhelm-Gesellschaftund das Kaiser-Wilhelm-Institut f�r Metallforschung 1900 bis1945/48, Wallstein, Gçttingen, 2007.

[12] “F�nfzig Jahre Szintillationsz�hler”: I. Broser, Phys. Bl. 1998, 54,935 – 937.

[13] “�ber Wissenschaft und Wirtschaft: Fritz Habers Zusammenar-beit mit der BASF 1908 bis 1911”: C. Reinhardt in Naturwissen-schaft und Technik in der Geschichte. 25 Jahre Lehrstuhl f�rGeschichte der Naturwissenschaft und Technik am HistorischenInstitut der Universit�t Stuttgart Franz Steiner Verlag, Stuttgart,1993, pp. 287 – 315.

[14] M. Szçllçsi-Janze, Fritz Haber 1868 – 1934. Eine Biographie,C.H. Beck, M�nchen, 1998, p. 237.

[15] M. Born, My Life: Recollections of a Nobel Laureate, Scribner,New York, 1978, p. 261.

[16] “Anregung von Gasspektren durch chemische Reaktionen”: F.Haber, W. Zisch, Z. Phys. 1922, 9, 302 – 326.

[17] “Zur Deutung der diffusen Molek�lspektren”: K. F. Bonhoeffer,L. Farkas, Z. Phys. Chem. 1927, 134, 337 – 344.

[18] Cf. D. J. Stoltzenberg, F. Haber, Chemiker, Nobelpreistr�ger,Deutscher, Jude. Eine Biographie, VCH, Weinheim, 1994,pp. 506 – 519.

[19] “Putting the Quantum to Work. Otto Sackur�s PioneeringExploits in the Quantum Theory of Gases”: M. Badino, B.Friedrich, Studies in the History of Quantum Physics 2011, inpress.

[20] a) “�ber Zusammenstçße zwischen Elektronen und den Mole-k�len des Quecksilberdampfes und die Ionisierungsspannungdesselben”: J. Franck, G. Hertz, Verh. Dtsch. Phys. Ges. 1914, 16,457 – 467; b) “Reaktionsleuchten und Reaktionsgeschwindig-keit”: H. Beutler, M. Polanyi, Naturwissenschaften 1925, 13,711 – 713; “Drehimpuls und Wirkungsquerschnitt bei chemi-schen Elementarprozessen”: H. Beutler, E. Rabinowitsch, Z.Elektrochem. 1929, 35, 623 – 625; c) “Registrierapparat zurautomatischer Aufnahme von Ionisierungs- und anderen Kurv-en”: P. Knipping, Z. Instrumentenkunde 1923, 43, 241—256.

[21] Roger Stuewer, The Compton Effect: Turning Point in Physics,Science History Publications, New York, 1975.

[22] “Zur Grçße und Winkelabh�ngigkeit des Comptoneffektes”: H.Kallmann, H. Mark, Naturwissenschaften 1925, 13, 297 – 298.

[23] “Anregung von Gasspektren durch chemische Reaktionen”: H.Fr�nz, H. Kallmann, Z. Phys. 1925, 34, 924 – 950.

[24] Cf. for example, “Astonishing Successes� and Bitter Disap-pointment�. The Specific Heat of Hydrogen in QuantumTheory”: C. Gearhart, Arch. Hist. Exact Sci. 2010, 64, 113 – 202.

[25] a) P. Harteck Papers, Rensselaer Polytechnic Institute, Troy,New York. RAC Rockefeller Archive Center, Sleepy Hollow,New York, 1:1; b) “Weitere Versuche mit Parawasserstoff”: K.F.Bonhoeffer, P. Harteck, Die Naturwissenschaften 1929, 17, 321 –322; c) A. Farkas, Orthohydrogen, Parahydrogen and HeavyHydrogen, Cambridge University Press, Cambridge, 1935.

[26] P. Harteck, My Outstanding Teachers, October 1983, PHP 8:8.[27] M. Walker, German National Socialism and the Quest for

Nuclear Power, 1939 – 49, Cambridge University Press, Cam-bridge, 1989.

[28] “From the Periphery. The Genesis of Eugene P. Wigner�sApplication of Group Theory to Qauntum Mechanics”: M.Chayut, Found. Chem. 2001, 3, 55 – 78; “The emergence ofselection rules and their encounter with group theory: 1913 –1927”: A. Borrelli, Stud. Hist. Philos. Modern Phys. 2009, 40,327 – 337.

[29] A. Borrelli, B. Friedrich, E. Wigner and the bliss of the“Gruppenpest”, Second International Conference on the Historyof Quantum Physics (HQ2), Utrecht, July 14 – 17, 2008, Utrecht,the Netherlands.

[30] “�ber die Erhaltungss�tze in der Quantenmechanik”: E.Wigner, Nachr. Ges. Wiss. Goettingen Math.-Phys. Kl. 1927,375 – 381.

[31] E. P. Wigner, Group Theory, Academic Press, New York, 1959.[32] a) “On the Verge of Umdeutung: John Van Vleck and the

Correspondence Principle. I and II”: M. Jansen, T. Duncan,Arch. Hist. Exact Sci. 2007, 61, 553 – 624; M. Jansen, T. Duncan,Arch. Hist. Exact Sci. 2007, 61, 625 – 671; b) e.g., “Experimen-teller Nachweis der ’negativen Dispersion’”: R. Ladenburg, H.Kopfermann, Z. Phys. Chem. Abt. A 1928, 139, 375 – 385.

[33] a) “A History of Optical and Optoelectronic Physics in theTwentieth Century”: R. G. W. Brown, E. R. Pike in TwentiethCentury Physics (Eds.: L. M. Brown, A. Pais, B. Pippard),Institute of Physics Publishing, Philadelphia, 1995, pp. 1385 –1504; b) H. Kopfermann, Kernmomente, Verlagsgesellschaft,Leipzig, 1940.

[34] “Zum Problem der Reaktionsgeschwindigkeit”: M. Polanyi, Z.Elektrochem. 1920, 26, 161 – 171.

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