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R A M A N S P E C T R O S C O P I C S T U D I E S OF C A R B O N IN E X T R A - TERRESTRIAL MATERIALS

John Macklin.* Donald Brownlee,* Sherwood Chang** and Ted Bunch**

*Departments of Chemistry and Astronomy. respectively, University of Washinton, Seattle. WA 98195 USA

**NASA Ames Research Center. Moffett Field. California 94035 USA

Micro-Raman spectra of a number of synthetic carbon materials and carbon containing extraterrestrial samples have been measured. Some of the materials included in the study are pyroletic graphite, glassy carbon, seven carbon samples prepared by heating polyvinylchloride at various temperatures, eight Allende and six Murchison meteorite samples derived from various treatments that remove minerals and other soluble substances, thin sections of unt reated Murray, Murchison and Al lende meteor i te samples and an interplanetary dust particle. Emphasis is given to measuring the detaile_~ ! shape, frequencies and relative intensities of bands at ~ 1360 and 1580 cm that are attributable to carbon in order to establish the analytical significance of the observed differences in these parameters. The point is to look for indications of known and expected st ructura l differences. For example, similarities and differences in spectra of the natural materials may correlate with differe~ces in the medium that include the carbon and reflect the manner of carbon accretion: or spectra of the samples treated at various temperatures may show the effect of thermal history on the spectra. Features in the spectra that identify minerals that are included in the samples are also noted. Raman spectra of meteor i te samples that have been heat t reated at increasingly higher temperatures will also be included to determine whether the carbon spectrum can be changed by this treatment and .at what temperature. This experiment may indicate the maximum temperature that the meteorite samples have experienced.

The frequencies of bands due to carbon in the Raman spectra of materials included in this investigation are shifted with excitation frequency in a manner that seem_] to parallel the frequency decrease of the bands at 1360, 2720 and 2950 cm ,n the Raman spectra of graphite and carbons as the excitation frequency is changed from 488.0 to 641.1 nm (Fishbach et al., 1981). The analytical uti l i ty of these frequency shifts and corresponding relative intensity changes will be explored along with their contribution to better understanding the Raman spectra.

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RAMAN SPECTRA OF EXTRATERRESTRIAL MATERIALS

The conclusions arrived at in this work are based on comparison and correlation of the details of Raman band positions, band widths, and relative intensities. Other analytical measurements and information such as carbon and mineral content are included in the correlations. The infrared spectrum of many of the materials has been measured to assist in interpreting the Raman data. The infrared spectra are particularly useful in this regard since bands due to minerals appear most intensely in these spectra while the most prominent bands in the Raman spectra are usually those assigned to carbon. A few indications are readily apparent from the spectroscopic data. For example: (1) It appears that the Raman spectrum of the insoluble carbon phase from the meteorites included in this study is not altered by treatments that remove other materials and concentrate the carbon and that the carbon spectra from different parts of the untreated meteorite samples are also the same (2) The graphitic character of the insoluble carbonaceous matter i F meteorites is not always reflected by the magnitude of the 1580 to 1360 cm band intensity ratio that is commonly considered to be a measure of the extent of graphitization; that is, the band intensity ratio is not simply a funct ion of the size of dispersed graphit ic particles. (3) The Raman spectrum of carbon in amounts as low as 0.1 to 1% in meteorite samples can be measured. It is expected that further study of the detai ls of these measurements will reflect some structural characteristics like the size and separations of layers in graphitic carbon that is thought to account for the observed carbon spectra and indicate whether other forms of carbon are included in the materials.

As the study is continued, it will be expanded to include addit ional natural terrestrial and extraterrestrial materials containing carbon, graphite .intercalation compounds and other samples prepared under controlled conditions in order to obtain materials that can be shown by the similarity of their Raman spectra to contain carbon that is s imi lar to carbon in natura l environments.

Vidano, R. P. Fishbach, D. B., Willis, L. J. and Loehr, T. M.: 1981, Solid State Communications 9, 341.

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