identi cation of carbon forms in soot materials of...

Astron. Astrophys. 329, 1087–1096 (1998) ASTRONOMYAND

ASTROPHYSICS

Identification of carbon forms in soot materialsof astrophysical interestA. Rotundi1, F.J.M. Rietmeijer2, L. Colangeli3, V. Mennella3, P. Palumbo1, and E. Bussoletti11 Istituto di Fisica Sperimentale, Istituto Universitario Navale, via A. De Gasperi 5, I-80133 Napoli, Italy2 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA3 Osservatorio Astronomico di Capodimonte, via Moiariello 16, I-80131 Napoli, Italy

Received 24 February 1997 / Accepted 19 August 1997

Abstract. We determined the carbon structures in condensedsoot samples of variable C/H ratio by using both scanning andtransmission electron microscopy. We identified several types ofcarbon structures. All the samples are formed mainly by “chain-like aggregates” of amorphous grains. A small amount of moreordered forms of carbon is also detected. A “poorly graphi-tized carbon” is formed on a pre-existing amorphous carbonsubstrate due to auto-annealing. “Bucky-carbons” are “proper”products of condensation. Occasionally, rare graphitic carbonribbons and single-crystal carbon platelets are observed. The“chain-like aggregates” are the only form sensitive to hydrogencontent variations and, probably, the main form responsible forthe UV spectral response. In agreement with previous analyseson similar soot samples, we conclude that the internal structureof the amorphous grains drives the overall spectroscopic prop-erties of hydrogenated soots. The relations between structuraland spectral trends, common to several kinds of soot samples,suggest that carbon ordered on a micro-scale only, rather thangraphitic carbons, is more appropriate to interpret the interstel-lar UV extinction bump position. The carbon forms, “proper”products of soot condensation processes, are similar to thosedetected in chondritic interplanetary dust particles. This couldimply that they are of primary origin, rather than derived fromparent body processing.

Key words: stars: carbon – stars: circumstellar matter – dust,extinction – methods: laboratory

1. Introduction

Fine-grained refractory particles are observed in circumstellar,interstellar and interplanetary media. However, their physico-chemical properties vary according to the actual environment.Circumstellar dust condenses directly from a cooling gas phase(e.g. Lattimer & Grossman, 1978; Nuth & Donn, 1982) before

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being ejected into the interstellar medium. Since direct monitor-ing of dust formation and evolution in these environments is im-possible, the production, processing and analysis of laboratoryanalogues represent an effective approach to understand the evo-lution of dust in astrophysical environments. Thermal anneal-ing can be considered a first approximation of dust processingwhich occurs in interstellar environments. Laboratory experi-ments show that vapor phase condensates are generally XRD(X-ray diffraction) and IR amorphous solids, while crystallinephases generally indicate post condensation annealing. On theother hand, analytical electron microscope studies of chondriticinterplanetary dust particles (IDPs) show that they initially con-sisted of amorphous ferromagnesiosilica and aluminosilica ma-terials (Rietmeijer 1992; Thomas et al. 1995) that subsequently –probably in protoplanetary bodies – became partially crystallinesolids. These chondritic IDPs are among the most carbon-rich,ultrafine-grained extraterrestrial materials. They include amor-phous and poorly-graphitized carbons and hexagonal diamond(Rietmeijer 1992). As far as the interstellar dust is concerned,the actual interpretation of the 217.5 nm bump in the interstellarextinction curve is strongly related to the intrinsic nature of solidcarbons present in space. This feature was initially attributed tothe presence of small spherical graphite grains (e.g. Gilra 1972;Draine & Lee 1984). The difficulty to produce pure graphite inastronomical environments was discussed by Mathis & Whiffen(1989). Hecht (1986) considered hydrogen-free carbon grainsas the carrier of the UV bump. Dehydrogenation process inthe diffuse medium, induced by UV radiation, cosmic rays andshocks (Sorrell 1990), may also be involved in the formation ofthe 217.5 nm peak (Hecht 1986). The existence of hydrocarbongrains in interstellar and circumstellar regions was supported bythe meteoritic record (e.g. Nuth 1985), laboratory simulations(e.g. Bussoletti at al. 1987; Sakata et al. 1983; Colangeli et al.1995) and models based on astronomical observations (e.g. Du-ley & Williams 1983). Laboratory experiments on submicronhydrogenated amorphous carbon grains, subject to thermal an-nealing, UV and ion irradiation (Mennella et al. 1995a; 1996;1997a), showed that the UV spectral response depends on the

1088 A. Rotundi et al.: Carbon forms in soot material of astrophysical interest

hydrogen content of the analogues. Mennella et al. (1995b) in-ferred that the internal structure of the carbon grains determinesthe UV extinction properties. Similar conclusions have beendrawn by studying carbon grains produced in an atmospherewith different hydrogen content (Mennella et al. 1997b).

In order to support interpretations of astrophysical data andlaboratory work to date, it becomes important to study the mor-phology and the structure of cosmic dust analogues producedby vapour phase condensation. In this respect, the possibilityof comparing astronomical observations with laboratory spec-tra is based on the implicit assumption that analogue materialsresemble cosmic dust also in their morphological and struc-tural characteristics. This is a point not yet studied in detail.Moreover, the correct interpretation of spectroscopic featuresand their variations needs a systematic correlation with possi-ble changes in the morphology/structure of the samples analysedin laboratory. Actually, several authors (e.g. Koike et al. 1994,Papoular et al. 1996) have pointed out that soot samples, arti-ficially synthesized, usually contain a wide variety of differentcomponents, in terms of morphology and structure. Thus, it ishighly desirable to identify their characteristics and, possibly,to uncover the role they play in the determination of the sam-ple optical properties. Interestingly though, the heterogeneity ofsoot appears to be a common feature of homogeneous vapourphase condensation experiments. This evidence suggests thatdifferent components must be considered as proper products ofthe condensation process.

The main aim of the present work is to identify the car-bon forms obtained in a typical carbon production experiment.We have studied several soot samples, both thermally treatedand produced in atmospheres with variable hydrogen content.In order to assess how they evolve in function of experimentalconditions. Moreover, attention has been devoted to distinguishcarbon forms which are “proper” of condensation and thosewhich result from subsequent auto-annealing as part of this pro-cess (cf. Rietmeijer & Nuth 1991). We found several forms ofcarbon. Thus, we addressed the issue about those actually re-sponsible for the observed spectral features, with emphasis onthe so-called “UV extinction bump” (e.g. Mennella et al. 1995a).

2. Experimental

In order to analyse soots characterised by different hydrogencontent, we considered two sets of samples: a) condensed inatmosphere with variable hydrogen content and b) thermallyannealed. The carbon material was produced by arc-dischargebetween two amorphous carbon rods, at a pressure of 10 mbar,and collected on UV grade fused silica substrates (for more de-tails see Mennella et al. 1995a). In the production of the first setof samples the only variable parameter was the composition ofthe ambient atmosphere. We produced samples in atmospherescharacterized by variable proportions of argon and hydrogen: 1)Ar = 100%; 2) Ar = 99.6% and H2 = 0.4%; 3) Ar = 99.2% andH2 = 0.8%; 4) Ar = 90% and H2 = 10%; 5) Ar = 70% and H2

= 30%; 6) H2 = 100%. They are labelled respectively: ACAR,ACH2(0.4), ACH2(0.8), ACH2(10), ACH2(30), ACH2-a. The

second set consists of ACH2 samples after annealing at 415 ◦C,700 ◦C and 800 ◦C, plus the not annealed material, labelledAC415, AC700, AC800, and ACH2-b, respectively. Thermalannealing lasted 3 hours in vacuum (p< 10−5mbar), (for moredetails see Mennella et al. 1995a). The two sets of samples, i.e.ACAR to ACH2-a and AC800 to ACH2-b are comparable interms of increasing amount of hydrogen in the carbon.

In the present work the samples were analysed by meansof two different techniques: (1) field emission scanning elec-tron microscopy and (2) transmission electron microscopy. The3-D morphology was determined using a Stereoscan 360 -Cambridge Field Emission Scanning Electron Microscope (FE-SEM), operating at a maximum accelerating voltage of 25 keVand with a spatial resolution of 2 nm. In order to check for thepresence of contaminants in the soot samples we used an EnergyDispersive X-ray (EDX) detection system, attached to the FE-SEM, that was able to detect elements down to Be (i.e. Z > 4).The characterisation of the structural units was performed us-ing a JEOL 2000FX Transmission Electron Microscope (TEM)operating at an accelerating voltage of 200 keV and with a spa-tial resolution of 0.32 nm in the TEM mode. Rietmeijer (1995)described the experimental conditions we used in this study, in-cluding those for selected area electron diffraction (SAED) anal-yses. Each condensed sample was embedded in epoxy (Spurrs).Serial ultrathin (80 nm–100 nm) sections were prepared usinga Reichert–Jung Ultramicrotome E that was operated at a dia-mond knife speed between 0.3 and 0.6 mm/s. Ultrathin sectionswere placed on a holey carbon thin-film supported by a TEMgrid and housed in a Gatan low-background, double-tilt speci-men holder for TEM analyses. For the thermally annealed sam-ples a small portion of material was scraped off the collectingsubstrates and deposited directly onto a holey-carbon thin filmsupported by a Cu-grid. The FESEM analyses were performeddirectly on the collecting substrates that were attached to FE-SEM aluminium pin stubs by a conductive silver paste. Theseanalyses were useful to define the spatial distribution of differ-ent morphologies within the samples. In order to characterisethe morphology at higher resolution, FESEM measurementswere repeated on small fractions of dust dispersed onto prop-erly smooth silicon wafers attached with conductive silver pasteto the aluminium pin stubs. The EDX measurements were per-formed separately on materials dispersed onto smooth carbonpin stubs in order to look for contaminants in the samples. In afew samples (both annealed and condensed) we detected tracesof Si. In order to identify the cause of Si contamination we cross-checked the EDX results for ACH2 samples with fragments ofboth fresh and used-up carbon rods mounted with silver pasteon Al stubs. For comparison, ACH2 soot was collected directlyduring the arc-discharge onto carbon stubs. We concluded thatsilica contamination results from scraping off soot samples fromtheir collecting substrate.

3. Observations

Typical FESEM and TEM images of examined samples are re-ported in this section to indicate the main morphological and

A. Rotundi et al.: Carbon forms in soot material of astrophysical interest 1089

Fig. 1. Field emission scanning electronmicrograph of sample ACH2 showing thefluffy morphology of the CLA smoke. In thebottom left frame a high-resolution imageshows the single grains arranged in agglom-erates.

Fig. 2. Transmission electron micrograph ofsample AC415, dispersed onto a holey carbonthin-film, showing CLA carbon in the lowerright-hand corner (black arrow), thin carbonsheets with isolated loops (PGC) that evolved lo-cally into a tangled network (open arrows), andrare GCR (solid white arrow).

structural features. FESEM images for all samples display afluffy morphology of fine-grained (diameter ∼ 7 up to about15 nm) spheres organised in agglomerates that are, in turn, ar-ranged in necklaces together with individual spheres (Fig. 1).TEM analyses of these typical Chain-like Aggregates (hereafterCLA) show round discs of amorphous carbon grains (Fig. 2) –actually spheres, as shown by FESEM analysis – with values ofsphericity S = 0.9–1.0. All the samples are dominated by CLAcarbon. In addition to the CLA texture, FESEM analyses showthat the samples contain micron-sized (up to 20 µm) patches

characterized by more compact CLA with abundant whiskersand lesser amounts of irregularly shaped platelets (Fig. 3).

Systematic TEM analyses of the condensed and thermallyannealed samples allowed us to characterise five structures, be-sides the CLA. Poorly graphitized carbon (PGC) is arrangedas isolated domains of loops or rings on smooth carbon sheets,which consist of fused condensate grains and form irregularpatches within the CLA smoke. Occasionally, they are up toseveral micrometers in size. Typically, the isolated ring-likestructures are delineated by a single graphite basal plane lat-


Fig. 3. Field emission scanning electron mi-crograph (left) showing in the ACAR samplea micron-sized patch characterized by morecompacted CLA (upper right) with abundantwhiskers, i.e. “bucky-tubes” (lower right).

Fig. 4. High-resolution transmission elec-tron micrograph of single loops in an ul-trathin section of sample ACAR. In thisimage, rare loops (PGC) consist of sev-eral graphitic lattice fringes (arrow) while aclosed “buckytube” is shown on the right.The grey background in the transmissionelectron micrographs of ultrathin sectionsrepresents the embedding epoxy. The holeycarbon thin-film supporting the ultrathin sec-tions is also visible.

tice fringe, clearly visible as a darker edge of the grains (Fig. 2,4). The shape of these structures suggests that fusion of loopsoccurred prior to formation of the lattice fringe. When the den-sity of loops becomes sufficiently high, the result is a networkof tangled loops (Fig. 2, 5). While this network of thin ringsis common to all samples, a network of coarser loops (Fig. 6)is also present in most samples. The thickness of the singlegraphite layer forming the loops is markedly distinct in bothnetworks. The coarser network may also occur in micrometer-sized patches.

All samples also contain compact irregular masses of well-defined concentric circular units and tubes, or “bucky-onions”,with diameters in the range of 10–40 nm (average = 22 nm), and“bucky-tubes” up to about 100 nm × 10 nm in size (Fig. 3 and7). Typically, these tubes have a high aspect ratio, distinct fromstubby “bucky-tubes” that usually occur with “bucky-carbon”forms. The typical onion shell morphology is formed by distinctbasal graphitic lattice fringes. Narrow ribbons (about 5 to 15 nm)of similarly well-developed fringes occur among the “bucky-carbons”. They tend to form concentric units with either circularor polyhedral shapes (Fig. 8). While a well-defined size cut-off


Fig. 5. Transmission electron micrograph ofthe fine-grained tangled network (PGC) inan ultrathin section of sample ACH2(0.8).The darker areas on the right side and in theupper left-hand corner of the figure are denseconcentrations of “bucky-carbons”.

Fig. 6. Transmission electron micrograph ofthe coarse-grained tangled network (open ar-row) in an ultrathin section of sample ACARwith a fine-grained network in the lowerright-hand portion of the figure (PGC).

is not evident, the smallest units are about 30–40 nm in diameter.Spherical grains, with several (up to about 5) graphitic layersalong the grain boundary, are also present among these “bucky-carbons” in most samples.

In addition, TEM analyses revealed broad (∼ 10 up to ∼125 nm) Graphitic Carbon Ribbons (GCR) with graphitic basallattice fringes (Fig. 2, 9). They are very rare and were detectedonly on one TEM grid of AC415, AC700 and ACH2(0.4) sam-ples. Extremely rare Single-crystal Carbon Platelets (SCP) oc-cur as micrometer-sized sheets embedded in the dominant CLAcarbon material (see Rietmeijer et al. 1997). The sheets con-

sist of well-defined, hexagonal single-crystal platelets about onenanometer thick. The SAED data support a hexagonal carbonphase. The SCP were observed only on one TEM grid of ACAR,AC800 and ACH2(0.4).

In summary, the carbon forms identified in our samplesare: CLA (Chain-like Aggregates), PGC (Poorly GraphitizedCarbon), GCR (Graphitic Carbon Ribbons), “bucky-carbons”(“bucky-tubes” and “bucky-onions”) and SCP (Single-crystalCarbon Platelets).

We made qualitative estimates of relative abundances basedon 40–50 individual ultrathin sections of each sample. The CLA


Fig. 7. Transmission electron micrograph ofconcentric circular “bucky-onions” and closed“bucky-tubes” with variable aspect ratios (black ar-rows) in an ultrathin section of sample ACAR.

smoke is the predominant carbon in all the samples. We derivedthat CLA constitutes about 70% in area of each sample. In thesamples condensed in an atmosphere containing hydrogen, sub-micrometer patches of “bucky-carbons” are more common thantangled network carbons (PGC), while in the ACAR sample theopposite trend is observed.

4. Discussion

The occurrence of different types of textures in carbon soot sam-ples with different hydrogen content, obtained by condensationand thermal annealing, is directly related to, and indigenous to,their formation from the gas phase. The formation of the CLA,dominating each sample, is typical of vapour phase condensa-tion and occurs in both carbon and silicate smokes (Stephens& Kothari 1978). In fact, the condensation process produces aquenched liquid which cools rapidly through the glass transitiontemperature to the ambient temperature.

The tubes and concentric circular units are similar to“bucky-tubes” and “bucky-onions” produced by arc-dischargeof graphite in a He atmosphere (Harris et al. 1993) and invapor-deposited amorphous carbon films (Iijima 1980). We in-terpret these structures as products of the condensation process“proper”, such as the CLA. Coagulation and fusion of grainsforming CLA during auto-annealing produces the contiguousamorphous carbon sheets that develop graphitic loops, i.e. thePGC component. The loops often delineate the original con-densate grain boundaries. Variations in the density of theseloops locally result in compact patches of the PGC structure.The “proper” condensation process and auto-annealing are in-timately linked. They occur in the condensation chamber asa result of the overall soot production process. The “bucky-carbons” (i.e. “bucky-tubes” and “bucky-onions”) and the PGCloops have fundamentally different origins. In fact, the formercondense as they are observed, while the latter are formed ona pre-existing substrate (amorphous carbon) during thermal en-ergy relaxation. Yet all these structures are forms of carbon


Fig. 8. Transmission electron micrograph of openloops on the left-hand side, both concentric circularand polyhedral “bucky-carbons” (arrows) from thetop-left to the bottom-right corners of the figure, in-cluding a complex polyhedral loop structure, in anultrathin section of sample ACAR.

intrinsic to the vapor phase condensation and they are not pecu-liar of the used experimental production method. We note that“bucky-tubes” and “bucky-onions” and the loops on smoothcarbon sheets in our samples are similar to those reported forpoorly-graphitized carbons in chondritic IDPs (Rietmeijer andMackinnon 1987, Rietmeijer 1992), shown in Fig. 10, and inacid residues of carbonaceous chondrites (Smith and Buseck1981, Lumpkin 1981).

The formation of very rare pre-graphitic ribbons (GCR) inour samples could be due to local thermal stresses, which causegraphitization, occurring after grain formation and/or duringthermal annealing. This hypothesis is supported by the similarityto wavey ribbons that develop during heat treatment of pre-graphitic graphitisable carbons (e.g. Bonijoly et al. 1982). Theorigin of the very rare SCP structure remains to be addressed.Again, it could be the result of local peculiar chemico-physicalconditions (e.g. lack of hydrogen atoms and high temperatureof the substrate).

Taking into account the discussion reported above, a firstconclusion we can draw is that all forms of carbon we observeare intrinsic to the condensation process and are present in allthe examined samples, but GCR and SCP, which are indeedoccasionally detected. Actually, PGC and “bucky-carbons” are

pre-graphitic/graphitised structures which should contain a verylow amount of hydrogen, if at all. Thus, they should not be sen-sitive to hydrogen content during formation and/or to thermalannealing. This is, in fact, what we conclude from our observa-tions, as these forms of carbon do not show any variation withinthe two sets of examined samples. On the contrary CLA, theamorphous component, should be sensitive to the presence ofhydrogen. Indeed, CLA shows structural variations dependenton the C/H ratio, which deserve an in depth analysis, subject ofa forthcoming paper (Rotundi et al. 1997).

It is now important to understand the role of different sootstructures in determining their optical properties. For this pur-pose it is important to link the results obtained in the presentwork with previous spectroscopic studies on similar samples.In particular, analyses about the “UV extinction bump” shifthave been perfomed as a function of both thermal annealing(Mennella et al. 1995 a,b) and variable hydrogen content duringcondensation (Mennella et al. 1997b). The two sets of sampleshave shown that the UV peak position shifts towards longerwavelengths as the hydrogen amount in the soot decreases (seeFig. 11). It is interesting to point out here that: a) the ACH2sample (produced in hydrogen atmosphere) does not displayany obvious UV peak; b) the analyses here reported show that


Fig. 9. Transmission electron micrograph ofgraphitic carbon ribbons (GCR) (open arrow)in an ultrathin section of sample ACH2(0.4)along with concentric circular and polyhedral“bucky-onions” and “bucky-tubes”.

the ACH2 sample contains all the different carbon structuresthat are common to all our samples; c) as long as the hydrogencontent decreases in the samples, the overall relative amount ofthe different carbon forms does not appear to change, while theUV peak shifts. According to this experimental evidence, wecan deduce that the observed spectral modifications should bedue to changes occurring in the carbon form more sensitive tohydrogen content variation. The amorphous and most abundantcomponent, CLA, suffers from C/H ratio changes and, there-fore, is the first candidate to be responsible for the spectral be-haviour. The evidence that the ACH2 sample does not show anybump, although it includes pre-graphitic/graphitised structures,suggests that a possible contribution to the UV bump by theseforms is overwhelmed by the dominant CLA.

An interesting comparison of the previous results can beperformed with the data reported by de Heer & Ugarte (1993),who have studied the heat treatment of carbon soot and the im-plications about the 217.5 nm interstellar bump interpretation.They have produced soot by the carbon arc method in inert at-mosphere and, then, have annealed the dust product at different

temperature steps from 1700 ◦C to 2400 ◦C. We note that the“as produced” soot presents a morphology/structure similar toour CLA (see their Fig. 1a). Moreover, this material shows abroad UV bump falling around 264 nm, in line with the 240 nmpeak of ACAR (Mennella et al. 1997b) and the 259 nm peak ofAC800 (Mennella et al. 1995b) hydrogen poor/free samples. Onthe other hand, the annealing temperatures used by de Heer &Ugarte (1993) are so high that they induce a complete modifica-tion of the whole samples. The soot particles develop graphiticshells, growing in number with the temperature. As a follow upthe UV peak does not shift and only the bump width is affectedby the structural variations.

In the framework of our interpretation, the evidences re-ported above confirm that pre-graphitic/graphitic structures can-not account for the UV peak shift observed in our experiments.This point further supports the conclusion that in our case CLAis the main form responsible for the observed spectral variations.

As a matter of fact, our results and those reported by de Heer& Ugarte (1993) refer to different stages of evolution of soots


Fig. 10. A transmission electron micrographof poorly graphitized carbon in an ultra-thin section of an interplanetary dust particleshowing the thin carbon sheets with scat-tered loops (open arrow), “bucky-onions”(black arrow) and “bucky-tubes” closedat both ends. Reproduced from Rietmeijer(1992) by the courtesy of Geochim. et Cos-mochim. Acta.

200 400 600 8000

2

4

6

wavelength (nm)

Fig. 11. Normalised UV extinction spectra of soot samples produced byarc discharge in hydrogen/argon quenching atmospheres and thermallyprocessed. Curves, from top to bottom, refer to ACH2, ACH2(0.8),ACH2(0.4), ACAR and AC800 samples.

and could be considered in a common scenario. Actually, oursamples, condensed in atmosphere with variable hydrogen con-tent or thermally annealed up to 800 ◦C, concern a soot evolu-tionary phase in which the hydrogenated amorphous component(CLA) drives the overall properties. On the contrary, de Heer &Ugarte (1993) have started from a material hydrogen free andhave analysed substantial large scale structural modifications

induced by heavy thermal treatments. In terms of spectroscopicbehaviour, we infer two main steps in the progressive evolutionof the UV bump: a) it shifts up to around 260 nm, due to the CLAmicrostructure evolution; b) it is stable at this wavelength, witha variable width caused by structure evolution at large scale.

An evident astrophysical implication of the considerationsreported above is that as long as soot tends towards a more or-dered structure it produces a UV peak around 260 nm, whichis incompatible with the position of the interstellar extinctionbump. This conclusion appears in line with the suggestions re-ported by Mennella et al. (1995b; 1996; 1997a): the interpreta-tion of the interstellar feature must be found in the microstruc-ture of disordered carbons rather than in organised graphiticmaterials.

5. Conclusions

The aim of the present work has been to monitor morpholo-gies and structures in carbon soots characterized by differentC/H ratios. We have identified several types of textures. TheChain-Like Aggregates (CLA) dominate all the samples and arestrictly related to the condensation process. The Poorly Graphi-tized Carbon (PGC) is due to auto-annealing and formed on apre-existing amorphous carbon substrate. The “bucky-carbons”are “proper” products of the condensation process and are notinfluenced by the different C/H ratio. Very rare graphitic carbonribbons and single-crystal carbon platelets could be produceddue to local variations in the chemico-physical conditions.

From the perfomed analyses we can conclude that in oursamples the (most abundant) Chain-Like Aggregates (CLA) are:a) the only form present in our soots which is actually sensitiveto hydrogen content variations; b) the most likely carrier thatcauses the UV bump and the form responsible for its shift. In


agreement with previous experimental data (e.g. Mennella etal. 1995b; 1996; 1997a) the present results confirm that theinternal structure of the CLA drives the overall spectroscopicproperties of hydrogenated soots. Indeed, a preliminary analysisof CLA evidenced a structural evolution as a function of theC/H ratio. A more detailed study of the CLA microstructure ishighly desirable and will be the subject of a forthcoming paper(Rotundi et al. 1997).

The relation between structural and spectral trends, evi-denced by soots condensed in atmosphere with variable hy-drogen content (Mennella et al. 1997b) or thermally annealed(Mennella et al. 1995b, de Heer & Ugarte 1993), has interest-ing astrophysical implications. We derive that graphitic carbonsare not appropriate to interpret the interstellar UV extinctionbump position, while some other form of carbon, ordered on amicro-scale only, could be appropriate.

Moreover, from our results we can conclude that the vari-ous forms of carbon present in soot samples are not experimen-tal artifacts but proper products of the condensation process.Thus, even though astronomical observations remain inconclu-sive with regard to the mineralogical make up of carbon in nat-ural environments, our work shows that interstellar dust mightinclude many different elemental carbons, as well as C/H car-bons. In this frame, we recall that structures similar to the PGCand “bucky-carbons” occur in chondritic IDPs. Our results rep-resent experimental evidence that various forms of carbon foundin IDPs are formed directly during condensation. This resultsupports a possible primary origin, rather than a parent bodyprocessing of IDPs.

In conclusion, although much more work is needed to clarifythe actual nature of cosmic carbon, the results presented in thispaper shed light not only on the structure of soots condensed inlaboratory but also on potential carriers of cosmic carbon dustfeatures.

Acknowledgements. We thank, S. Inarta, N. Staiano and E. Zona fortheir technical assistance during sample preparation and analyses. Wethank R. Trentarose for his technical assistance during images prepa-ration. We warmly thank Prof. A. Blanco and Prof. S. Fonti for theircontribution in the preparation of one of the samples. This work wassupported by ASI, MURST, CNR and GIFCO grants. The TEM anal-yses were performed in the Electron Microbeam Analysis Facility ofthe Department of Earth and Planetary Sciences at UNM, where JimKarner and Fleur Rietmeijer-Engelsman provided technical assistance.The SEM and EDX analyses were performed at the Cosmic PhysicsLaboratory of Naples. FJMR was supported by NASA grants NAGW-3626 and NAGW-3646.

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