Infrared Spectroscopy of Diamondoid Molecules: New Insights into the Presence of Nanodiamonds in the Interstellar Medium

Although they are relatively different in band shape, infrared features around 3.4-3.5 μm in the emission spectra of HD 97048 and Elias 1 and in the absorption spectra of various dense clouds have both been attributed to diamondoid molecules/particles. This assignment is based mainly on infrared spectra of hydrogenated diamond thin films and of diamond nanocrystals of known average size. Here we present an analysis of the astrophysical implications of recently reported solid-state 2.5-12.5 μm spectra of individual diamondoid molecules, up to the size of hexamantane (C26H30). These spectra provide the first experimental measurements of the infrared frequencies of this class of molecules. In addition, laboratory gas-phase infrared emission spectra of the three smallest members of the diamondoid family are reported, as well as theoretical spectra for some larger species. The present data set allows us to relate spectral signatures to the molecular size and structure. The spectra of tetrahedral diamondoids are found to be qualitatively different from those of lower symmetry species, which possibly explains the differences between the astrophysical emission and absorption spectra. Interestingly, the 3.53 μm band is clearly observed in the spectra of these small molecular diamondoids, whereas previous studies on nanodiamond particles found this band only for species larger than ≈50 nm. Our results support the assignment of the 3.43 and 3.53 μm emission features in HD 97048 and Elias 1 to diamondoids of a few nanometers in size as well as the suggestion that smaller diamondoid molecules contribute to the 3.47 μm interstellar absorption band.