1998A&A...336L..41H

We present the results of a mid-infrared laboratory study of meteoritic nanodiamonds, a natural `nitro-gen-rich' terrestrial diamond, and a `nitrogen-poor' diamond before and after irradiation with fast neutrons. Characteristic one-phonon modes, absent in the perfect diamond structure, are evident in the `nitrogen-rich' and neutron-irradiated diamonds that we analysed. One feature common to these bulk materials occurs at ∼21µm. The precise origin of this feature is unknown, although it may arise from molecular-type vibrations associated with lattice defects created by nitrogen impurities or neutron irradiation. We suggest that interstellar nanodiamonds also possess this feature and that they are the carrier of the unidentified ∼21µm infrared emission feature observed in the dust shells around some carbon-rich proto-planetary nebulae.