Infrared, ultraviolet, and electron paramagnetic resonance measurements on presolar diamonds:: Implications for optical features and origin

Infrared (IR) and ultraviolet (UV) absorption spectra were obtained for diamonds from the Allende and Murchison meteorites. In addition, and for the first time, electron paramagnetic resonance spectra were measured. The IR and UV data confirm the suspicion of Russell et al. (1996) that N in presolar diamonds predominantly appears in the form of dispersed N atoms, as is the case for terrestrial type Ib diamonds. In accordance with other observations, our electron paramagnetic resonance measurements suggest a high H content in presolar diamonds. The presolar diamonds most likely originated in a H-rich region, an environment in which nanometer-sized diamonds may be more stable than graphite (Badziag et al., 1990). This adds to the evidence-previously based mainly on the twin microstructures of presolar diamonds (Daulton et al., 1996) and the absence of graphite with the same isotopic composition as presolar diamonds (Anders and Zinner, 1993)-for a homogeneous nucleation of presolar diamonds from a gas phase. Based on our results for detection of diamonds in space, we suggest searching for the N-induced IR and UV absorption features of type Ib diamonds. Other characteristic diamond features that could also be used to detect diamonds in space are the (-CHn) IR absorption features due to II-coated diamonds, as they are described by Allamandola et nl. (1993) and the IR multiphonon absorption features of the diamond lattice. The multiphonon features are very weak (Edwards, 1985), but their intensity increases somewhat with increasing temperature (Collins and Fan, 1954), so perhaps a search for them is not totally hopeless.