ELECTRONIC AND VIBRATIONAL SPECTROSCOPY OF DIAMONDOIDS AND THE INTERSTELLAR INFRARED BANDS BETWEEN 3.35 AND 3.55 μm

The electronic and vibrational spectroscopic properties and ionization energies of diamondoids (nano-diamonds, microdiamonds) are computed using density functional theory (DFT). Spectra of both the neutral and cationic forms of diamondoids, ranging in size from C10H16 to C38H42, and the IR spectrum of a diamondoid-PAH hybrid molecule are presented. For the 23 neutral species, the C-H stretching bands fall near 3.47 μm and are the strongest in the spec­ tra. Diamondoid ionization energies ( IEs) are found to be quite large (about 8 eV). The electronic excitation energies of the neutral species are of the same order as the IEs and have very small oscillator strengths ( f-values). For the ca­ tions, the C-H stretching peak positions differ somewhat from the neutrals and their average integrated band strengths are about half those of the neutrals. The computed electronic excitation energies for the cations are much smaller than those of the neutral species, with f-values that are very small. The neutral diamondoids will absorb most strongly near 3.47 μm, which is very close to the position of an absorption band associated with dense clouds that has been ten­ tatively attributed to the tertiary C-H stretch of diamond-like carbon. The spectroscopic properties described here imply that 3 μm emission from highly vibrationally excited diamondoid cations and neutral species should be most intense in regions with strong radiation fields. These results, in conjunction with the observation that the 3.5 μm emission feature originates very close to the exciting star, strongly supports its assignment in HD 97048 and Elias 1 to diamondoid species. While previous work makes an excellent case for large neutral diamondoids, our work shows that some cation contribution cannot be excluded. The very small f-values explain why so few sources show this emission. Subject headings: infrared: ISM — ISM: individual (HD 97048, Elias 1) — ISM: molecules — methods: numerical — ultraviolet: ISM Online material: color figures