Laser Raman Induced Degradation of Macromolecular Carbon in Coals and Meteorites

Abstract

Macromolecular carbon (MMC) is a polymeric solid form of carbon formed from numerous natural and anthropogenic processes. MMC is found in geologic samples, such as coals, meteorites, and metamorphic rocks, and the microstructure of the MMC provides details of the thermal history of the sample. Raman spectroscopy allows for a simple, quick examination of the MMC structure and thermal history of the rock. However, accurate Raman analysis necessitates that the Raman incident laser does not photodamage the MMC being measured. Here we examine, in detail, the effect of 532 and 488 nm laser irradiation on the structure of MMC in coals and meteorites. We show that photodegradation does affect the primary “G” and “D” Raman band peak positions, widths, absolute intensities, and relative intensities and we quantify changes with progressive irradiation. These results provide insight into the MMC structural changes that occur with photodegradation. We find that significant photodegradation occurs even with low laser powers (<200 μW). A literature survey of Raman MMC measurements shows that almost all publications use incident laser irradiances that would result in significant photodegradation of MMC within the accumulation time of a single Raman spectrum. Plain Language Summary Macromolecular carbon (MMC) is a high molecular weight, disordered from of carbon commonly observed in meteorites and terrestrial rocks. The structure of MMC is sensitive to the thermal alteration experienced by the rock or meteorite. Using Raman spectroscopy, one can obtain information on the highest temperature a terrestrial rock has experienced and obtain information on the thermal alteration of meteorites. We found that the Raman measurement itself effects the MMC structure and that this may result in misinterpretation of the measurement.