Infrared spectroscopy of interstellar nanodiamonds from the Orgueil meteorite

Abstract

We present results from an ongoing study of the infrared (IR) and optical properties of nanodiamonds, an objective of which is to identify spectral features in the laboratory that could also be used telescopically to trace the presence of these particles in the interstellar medium (ISM). Fourier transform midand far-infrared spectra of nanodiamond residue extracted from the Orgueil (CI) chondrite were acquired. All of the mid-IR bands initially present were found to diminish, with the exception of a band at ∼1100 cm-1, following additional oxidation of the diamonds. The ∼1100 cm-1 band can be predominantly attributed to adsorbed species, especially an ether-type linkage, while the "oxidisable" features seem to be associated with less stable, surface-bonded species and residual carbonaceous material. We obtained three far-IR features but are uncertain about the origin of those at 475 and 188 cm-1. We did not obtain a feature at ∼120 cm-1 reported by another group but do not discount the possibility that the band at 188 cm-1 could be related to it. The weak absorption band at 475 cm-1 (21 μm) is especially interesting because it may be strong in emission from hot nanodiamonds and, therefore, related to the unidentified infrared feature (UIF) observed at this wavelength in the spectra of some C-rich protoplanetary nebulae.