topic/ h ed~ted by

FRANK A SETTLE JR chemic~[ /n/trum@ntotfofl V ~ r ~ n ~ M ~ ~ l a v ~ ~ s t ~ l u f e Ler ngton VA 24450

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Lasers in the Undergraduate Curriculum

II. Coursework Experiments and Research Projects

Jack K. Steehler Roanoke, College, Salem, VA 24153

In Part I of this series of ankles on lasers in the undergraduate chemistry curriuulum, the features of lasers were hriefly reviewrd and some guiding principles were given for the proper use of laser experimentation ( I ) . In this second part, the focus will he on spe- cific experiments suitahle for different topi- cal areas in chemistry. In each case a brief description of the experiment will be given, the applicable laser features will he pointed out, the required laser instrumentation will be specified, and references will he given. Since this article is designed to provide di- rect guidance on incorporating Lasers into lab courses, most references will he to -ti- des describing student experiments rather than references to the original research level publications. More general references can be found in the part I article. Within a given section, experiments are listed in order of increasing complexity.

High levels of student interest in laser experimentation lead naturally to their use in long-term undergraduate research pro- jects. Students pursuing such projects are enthusiastic and committed, which certain- ly enhances the quantity of results they ob- tain and the knowledge gained. This article will briefly discuss the centraland peripher- al benefits of undergraduate laser research and will list some criteria and cautions for the choice of appropriate research topics.

I t is not intended that all or even most of the suggested experiments and demonstra- tions he adopted a t a given institution. As with any tool, overuse of the laser must be avoided. Three or four laser experiments spread throughout a curriculum are certain- ly sufficient. As with moat major instrumen- tation, student laser experiments are best suited to lower enrollment upper level courses and to those lab courses where sev- eral different experiments are used simulta- neously by different groups of students. The list of experiments below illustrates the range of types and levels of experiments that can be performed, hut is by no means exhaustive.

Instrumentallon Requlred The experiments below (with one excep-

tion) require one of three possible laser sources. These sources are a helium neon laser, a nitrogen laser, or a dye laser (usually pumped by a nitrogen laser). Mbre sophisti- cated laser sources such a s Nd:YAG lasers or excimer lasers can readily he substituted for the nitrogen laser if availahle. The lower

cost lasers are well matched to the experi- ments shown here; while higher power (more expensive) lasers are typically used in more intensive research environments. As mentioned in part I, HeNe lasers cost sever- al hundred dollars, a low-power nitrogen la- ser costs about $4000, and a small dye laser costs $2500. Homemade versions of the ni- trogen or dye lasers can he constructed from literature references at some savings but are recommended for experienced technicians only. The cost savings is often negated by the loss of reliability in a homemade system.

Laser spectroscopy typically has a reputa- tion for fancy optical setups, expensive hardware, and expensive electronic detec- tion systems. These research-level require-

ments are not necessary for instructional systems or for undergraduate research. Homemade optical tables of a variety of ma- terials are quite suitable for most experi- ments. Simple post holders for lenses and sample mounts are also sufficient. Commer- cial optical mounts are very expensive, and homemade alternatives are easily found. If commercial mounts are the only option, a recommended source is the micro and mini series of components of the Newport Corpo- ration, P. 0. BOX 8020,18235 Mt. Baldy Cir- cle, Fountain Valley, CA 92728. Most ex- periments involve simple light detectors (photodiodes or inexpensive photomultipli- er tubes). Such detectors cost under $100,

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Volume 67 Number 3 March 1990 A65

although the photomultiplier tube does re. quire a high wltage power supply ($350). Photodiode detectors yield electrical arwslv that for continuous iasers can be directly measured with lab multimeters. The pulsed signals found when nitrogen or nitrogen pumped dye lasers are used require time- gated detection electronics (see Fig. 1). These data systems average the signal only when it is present, outputting that average until the next laser pulse occurs. Such pulsed data averaging is most commonly done with a gated integrator. Commercial systems are expensive ($4500, Stanford Re- search Systems, 460 California Avenue, Palo Alto, CA 94306), but lower cost options ex- ist. Evans Electronics (P. 0. Box 5055, Berkeley, CA 94705) offer a gated integrator circuit board (completely assembled) for $178. Simple TTL electronic timing circuits for the gate and a power supply are the only necessary additions, keeping the total cost below $225. Access to an oscilloscope and optional interfacing to a computer should also be considered. A final optical equip- ment item is a monochromator. Only those experiments that require wavelength scan- ning require a monochromator. Fixed (or nonselective) wavelength detection of fluo- rescence, for example, requires only a simple optical glass filter to keep the excitation w~arele!l~th from r e a c h i ~ ; ~ the detector. Such filters cost approrimamly S50.00 each ~Corion Coro.. 73 Jeftrev Aw.. Hollist<m. . . . MA Ol74fi). Typical modest-resolution ( 0 .5 . nm) monorhrumators cost nppn,ximarely

$2500, while lower resolution (1 nm) ver- sions costing less than $1000 are available. Old spectrometers can also be adapted to serve as monochromators in many cases. A find indispensible item of equipment is pro- tective laser goggles (2). All laser users (0th- er than low-power HeNe laser users) should have such goggles and should use them faithfully.

Llstlng of Experiments A reasonably broad spectrum of laser ex-

periments is shown below. The experiments are divided into traditional areas of chemis- try, but these divisions are by no means ab- solute. Each listing shows in parentheses the laser properties being used and the type of laserreauired for the exoeriment. The refer-

Physical Chemistry Experiments Loser refractometry. (3) (Directionality,

HeNe.) Reaction kinetics are a "must" topic in physical chemistry laboratory courses. As long as the reactant and product differ in refractive index, the promess of the reaction can be monitored h i th; displacement of a probe laser beam traveling through the reac- tion mixture (see Fig. 2). The reaction vessel must have nonparallel entrance and exit windows,such as a test tube used off axis. In the cited reference the hydrolysis of glycidol to glycerol is studied, with focus on typical kinetic studies such as catalyst concentra- tion, reactant concentrations, temperature, ete.

Romonspectroscopy. (4) (High intensity, monaehromaticity, nitrogen laser.) The utility of Raman vibrational spectroscopy

for analysis of symmetric vibrations or for samples in aqueous solvents is reasonably well known. but is rarelv illustrated exueri- mentally. due to the hiih power lasers'and expensive high resolution monochrcmatun employd tor real lifp samples. Nirn,gen la. ser excitation with simple monochromator resolution works well for experiments de- signed to illustrate the Raman principle. Spectra of solvents like acetone and di- methyl sulfonide are readily obtained. Stu- dents in instrumental analvsis courses often haw great difficulty undentnnding how vi. lrratiunal freq~lcncies nrr rnlrulated h m Ramau speetm, nnd thlr first-hand erperi- ence is very valuable to them. A monochro- mator is required. If a dye laser is available, the inverse fourth power dependence of scattering intensity on wavelength can be illustrated using Rayleigh scattering from any transparent liquid.

High-resolution spectroscopy. (5,6) (Monochromaticity, HeNe or nitrogen la- ser.) Physical chemistry lab courses often include the analysis of well resolved gas phase spectra, typically 4 or HClIDCl. The IS absorption spectrum experiment can be expanded by detection of the laser-induced fluorescence, demonstrating selective exci- tation and emission. Vibrational and rota- tional analyses can be done, as in other ver- sions of the experiment. If a dye laser is available, emission from a variety of differ- ent selectively excited states can be com- pared (7.8). A monochromator is required.

Luminescence decay. (9) (Time resolu- tion, nitrogen laser.) Fast chemical kinetics can often be studied only with lasers. Stern- Volmer quenching of fluorescence illus- trates a moderately complex set of kinetic pathways. Fluorescence excited by the laser

can he quenched by a second molecular spe- cies in solution, shortening the fluorescence lifetime, with that lifetime being measured as a function of quencher concentration. Data analvsis leads to determination of sev- ern1 kmetlr ronPmntP In the rerprenred ex- perrmenr, a ruthenrum rmnplex (9 the fluor. aphor and aterhium complex is the quench- er. Note: moderately sophisticated data collection setups are needed to obtain the fluorescence decay curves. If such setups are not available, similar information can he ob- tained using as the quencher amolecule that also fluoresces (10,ll). The fluorescence emission ratio far the two molecules as a function of concentrations can also he ana- lyzed with Stern-Volmer models. A mono- chromator is required.

Two-photon spectroscopy. (12) (High in- tensity, monochromaticity, nitrogen laser plus dye laser.) The concept of symmetry in controlling transition cross sections is a cen- tral idea in spectroscopy. Since one-photon s ~ e c t r a are all most students see. the idea of ditferent aelectmn rules for tao.photon ur Ibmnn spectra is often pwrrly underrtmd. DYP laser e x c m t i ~ n t ~ i t h n<mselectiw fluo. rescence detection) of the two-photon ah- sorption spectrum of Iz can help illustrate such concepts.

Loser isotope separation. (13) (High in- tensity, monochromaticity, carbon dioxide laser.) High technology applications of la- sers include isotope separation by isotope selective photochemistry. Separation of 92S and dS is done by selective dissociation of SFb. molecules with a C02 laser. The laser lines are more resonant with the J2S-con- taining molecules, resulting in selective en- richment of theJ4S isotope, as monitored by infrared spectroscopy.

LASER OUTPUT

SAMPLE RESPONSE (E.G. FLUORESCENCE)

n TlME GAm FOR AVERAGING

Figure 1. Time profile for a typical pulsed laser luminescence experiment. The time gate for aver- aging is the gate used by gated integrator.

Analytical Chemistry Experiments

Loser refroetometry. (14) (Direetionality, HeNe.) Refraction can be used as a ehro- matographic detector for molecules such as sugars that have no convenient absorption hands. Using a HeNe laser and a hollow prism (or other glassware with nonparallel entrance and exit faces. such as an Erlen- meyer flask or a test tube used offanrs) filled with sugar solutions bf different vuncentra- tions.thispossibility iseasilydemonrtrated. The deflection of the laser beam is propor- tional to the d u l i o n concentration.

L o w endpmnt detwrron. (IS] (Laser as simple licht wurce. HeNe.1 All snal\.liral or genkral chemistry iourses use volumetric ti.

PROBE U S E R

T& TUBE CONTAINING REACTION MIXTURE

I

Figure 2. Laser refractometry experiment. The beam displacement will change proportionately with changing concennations of reactant and prod- uct ~pecies in the reaction minure.

trations for one purpose or another. The use of a laser to detect the color change at the endpoint can spice up this often unpopular typeof experiment. In thecited reference an indicator with different absorhances for the acid and base forms (at the HeNe wave- length) is used, with a $20 detector system.

Loser-induced fluorescence. (16,17) (High intensity, nitrogen laser.) Fluores- cence excitation is the most common analyt- ical application of lasers. Subnanogram1 milliliter detection limits are readily oh- tained, using filter or monochromatar wave- length selection and gated integrator detec- tion. Specific experimental systems are found in the references.

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Thermal lensing. (18,19) (Directionality, HeNe laser.) Absorption spectroscopy is ap- plicable to almost all samples but is not ter- ribly sensitive. Luminescence spectroscopy is very sensitive but is limited to a small subset of possible samples. Thermal meth- ods of analysis using lasers combine the best features of both, the generality of ahsorp- tion and thesensitivityof an emissionmeth- od. Heat emitted by the nonradiative relag- ation that follows absorption of light is mea- sured. The heat is detected by measuring the change in laser beam direction caused by a thermally induced refractive index chanee. A wide varietv of auantitative ex- ~er iments exist. illustrked dv the acid-base .~ ~ ~ . ~ ~

study of nn indirntor in the refermre listed. Room-remperature phosphorescence.

(20-22) (High intensity, time resolution, ni- trogen laser.) Routine application of phos- phorescence was long limited by the need for liquid nitrogen. The use of a variety of surfaces that minimize nonradiative relax- ation have to a laree extent solved that pnhlem. Possible experimenla range frvm simple quantitation rxperimrntv to sper- trally resolved crperimentk tmonochroma- tor required) to time-resolved experiments using emission lifetimes as an additional de- gree of selectivity. Deposition of a liquid sample onto a piece of filter paper, drying, and excitation of phosphorescence with the nitrogen laser are the basic steps required.

Organic Chemistry Experiments Optical activity. (23) (Polarization,

HeNe.) The idea thatchiral molecules inter- act with polarized light in a unique way is hard to illustrate for students. A very direct demonstration of this effect is to shine a polarized HeNe laser beam through a verti- cally oriented long tube with an optically active colloid present, such as quinine in acetic acid (see Fig. 3). Scattering of light to the side of the tube is observed only when the plane of polarization of the light is per- pendicular to the viewing direction. As the viewer walks around the tube, the minima and maxima in scattering intensity (corre- sponding to different optical rotations that make the laser polarization parallel and per- pendicular (respectively) to the observing direction) move up or down, thus directly visualizing the progressive spiralling of the polarization direction as optical rotation oc- curs. This s i m ~ l e and eleeant demanstra- tion is an ercrllrnt instrurtional aid.

Organ,cphororhemtcrrj. (High intensity. monochromaticity, nitrogen h e r . ) A wide variety of organic photochemical reactions can he adapted for use with a laser source. Rather than replacement of broadband "lightbulb" sources, a comparison of the broadband experiment to the single-wave- length laser experiment should be attempt- ed. Possible reactions include halogena- tions, isomerizatians (e.g., the cis to trans conversion of dimethyl fumarate to di- methyl maleate) (24), and others (25). Yields and product purities are often found to differ in such comparative experiments. If a dye laser is available, the wavelength dependence of such reactions can be deter- mined.

inorganic Chemistry Experiment Magnetic susceptibility. (26) (Direction-

POLARIZED PROBE U S E R

Figure 3. Demonstration of optical activity. The doubieended arrows indicate the polarization of the laser beam at different spatial positions. Ob- servation horn the side shows scattered light only when the polarization direction is perpendicular to the observation direction, as shown by the maxima and minima indicated. This figure is patterned aner ref 23, which gives the procedure for preparing the optically active colloidal solution.

ality, Hetie.) Magnetic properties of inor- ganir compoundr are often measured with complex Couy or Fsraday balances. The ha. sic principle is a weight change occurring in the presence of amagnetic field. In place of a balance, a pendulum arrangement can be used, with the mass change monitored by measuring the period of pendulum oscilla-

tion. A HeNe laser crossing the path of the suspending wire of the pendulum can readi- ly give this period. Simple photodiode de- tection is all that is required.

Biochemist!y Experiment Lanthanide ion probes. (27) (High inten-

sity, monochromaticity, nitrogen laser.) Many enzymes require particular metal ions for activity. For example, the binding of cal- cium to different sites can be prohed by re- placing the calcium with a fluorescent Ian- thanide ion. Spectral intensities and wave- length maxima can yield information about the binding equilibria and binding site envi- ronments. Such experiments should be viewed as longer term projects rather than as single-lab-period experiments.

Lasers in Undergraduate Research In many institutions, most chemistry ma-

jors participate in a longer term research project, for a variety of well-known peda- gogical reasons. In the case of a laser spec- troscopy project, there is alist of concepts or topical areas that are involved beyond the idea of undergraduate research itself. Stu- dents undertaking such projects absorb, one way or another, the key features of avariety of areas. These areas include, in order of importance, (1) spectroscopy, (2) kinetics, (3) quantum mechanics, (4) electronics, (5) computers, (6) optics, and (7) machine shop. This range of topics is ideal far most stu- dents interested in the fields of physical chemistry, analytical chemistry, or organic photochemistry. The first three are partieu-

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inftrumentation larly well related to chemistry and are con- cepts students have little intuitive feel for after a first exposure in the classroom set- ting. Any discussion of laser spectroscopy incorporates these concepts, including the basic discussion of how lasers work. Jah- lonski energy-level diagrams, kinetic rates of absorption and emission events, and the notion of allowed versus forbidden transi- tions are involved in laser operation and la- ser spectroscopy applications. Students who have a hard time grasping these ideas in the formal equation format of physical chemis- try classwork are delighted to finally under- stand them through their laser research pro- jects. The remaining items on the list are less important, though they do cover many of the practical scientific tools students are expected to pick up somewhere along the way.

Since laser spectroscopy can be compii- cated (and usually is s t the university re- search level), i t is useful to give some guid- ance concerning the appropriate level and timeframe for undergraduate projects. These guidelines are derived from experi- ence with a dozen short- and long-term pro- jects over the last few years. In s nutshell, undergraduate projects should be (1) fo- cused on chemhtry, (2) tightly focused, (3) designed for an existing, working setup, (4) designed for modest data-analysis require- ments, and (5) designed with reasonable ex- pectations. If these requirements are met, and adjustments are made for the individual backgrounds and skill levels of individual students, a highly satisfying and instructive experience can he expected.

Any undergraduate project must focus on chemistry. The goal is to deepen the stu- dent's understanding of chemical concepts and thought processes. Projects that focus on construction of optical mounts or elec- tmniccircuits will rarely meet this goal. The short time blocks students typically have available require efficiently designed pro- jects, whether they be several-day special projects or year-long senior thesis projects. Students usually have free time that does not exceed about three hours, perhapa twice per week. Experiments that are operational the moment a student enten the lab result in definite progress being made in each three-hour period. Canstruction projects, with the inevitable delays from unanticipat- ed needs for materials or tools do not fit such schedules well. In almost all cases, full responsibility for experimental setup should he carried by the faculty member.

Well-defined projects are tightly focused Projects. Steady student progress through a project requires initial comprehension of both the broad scope of the project and the specific objectives to be addressed. In most cases, i t is only a t the completion of a semes- ter long project that students can cantem- plate the big picture of a research area and distill from that picture a set of specific ex- periments which are cohesive, reasonable, and do-able.

A research supervisor should also realize that data analysis is just as difficult as per- forming the experiments and allow suffi- cient time for that effort. In most cases, in- dependent research projects are the first times the students do calculations and dis-

cussions without a recipe or formal ques- tions to guide them. The necessary skills must be taught in the same depth as experi- mental skills are taught. Even for outstand- ing students, this portion of a project is time-intensive and demanding.

Example Projects One example of a short-term (several lab

periods) project in laser spectroscopy is the investigation of the concentration depen- dence of one or several analytes in a lumi- nescence project. The sample preparation step often takes a full lab period, collection of data takes a full lab period, and analysis and plotting of data can take the equivalent of two additional periods. Such projects can be tied into the important concepts of detee- tion limits, dynamic range, and self-quench- ing, among others.

Another short-term project might be an investigation of laser detection of latent fin- gerprints (28). The concept of crime solving excites almost all students. Solid experi- mental design skills are learned through "in-lab" controlled experiments comparing powder visualization of fingerprints, direct laser visualization of fingerprints, and dye- based enhancements of laser visualization methods. Investigations of different sur- faces, different time lapses, and different laser wavelengths can serve as alternative projects.

A longer term (ane-semester) example would he a survey of the dependence of a luminescence signal on chemical structure. Ten to 15 related compounds made up in equivalent concentrations can be quantita- tively compared based on the intensity of the signals generated. If selective wave- length detection is used, fewer compounds should be done, because determination of the optimal emission wavelength for each compound takes significantly more time than if the emission wavelengths are known or if nonselective optical filters are used. The focus of this project should be on the chemical structure correlation with emis- sion wavelength or intensity. Observed ef- fects can be correlated with known substitu- ent sterie or electronic effects.

The high level of student interest in lasers is a strong motivation for such projects, but the laser itself is neither the focus of the project nor the main Lesson of the project. In almost all cases, as in most research pro- jects, the sticking points become those of obtaining reproducible results, interpreting results, and determining the chemical sig- nificance of the results. In none of the dozen or so projects I have directed has the routine use of the laser and electronic detection sys- tems been more than a first-day distraction or stumbling block. As it should be, the chemistry is seen as the focus of the effort, with the laser being a routine tool ("Turn it on and it works!"). Yet the fun of working with laser beams is retained.

Conclusion The goal of these two articles has been to

remind you of the fascination that lasers have for students. Incorporated into theun- dergraduate curriculum, laser experiments can motivate students to learn mare chemis-

A70 Journal of Chemical Education

try and to enjoy the learning experience more. Using lasers is no more complex than any of the other instruments chemists use routinely. At the level of the most difficult laser experiment mentioned here, the ex- perimental setups are no more complex than a 60-MHz NMR instrument. Commercial lasers are highly reliable, turnkey systems. Given the high level of excitement I have found lasers to add to mv awn teachine. I ... hope I haveconvinced y<>u togive them n try in your wtting, whether y w use the rrm. plest experiment rnentloned, or the most complex. Remember also that these experi- ments only touch the surface of what is pos- sible. Good luck!

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12. crieneinen, H. P. "Tho Nitrogen Laser-Pumped Dye Laser: An Ideal Light Source for College Experi- m e n e 1984, p 12. Laser Science, Inc., 80 Pmspect St..Cambridge. MA02139.

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1" 4" . . . , . . . 28. Menrel, E. R. A n d Chem 1989,61,557A.

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