16-20 mu m spectra of carbon nanoparticles

A comparison between spectra of carbon nanoparticles in the 16-20 mm range and those of interstellar sources shows that the major emission features attributed to aromatic hydrocarbons in this region are present in laboratory spectra. These features are tentatively identified with bending and wagging modes of polycyclic aromatic hydrocarbons containing aliphatic hydrocarbon side groups. Detailed comparison of laboratory spectra with emission spectra of NGC 7023 shows that variations in the relative intensity of features at 16.4, 16.6, 17.2, 17.4, 17.8, and 18.9 mu m arise, in part, from changes in chemical composition. The feature at 18.9 mu m appears to be unrelated to that of other bands in laboratory spectra, mimicking the behavior of this feature in NGC 7023. The line widths of these features in laboratory spectra are typically 5-15 cm(-1) and are consistent with vibrational de-localization on a timescale of similar to 10(-12) s. This indicates that the emitters of these bands in interstellar sources could contain in excess of 10(3) carbon atoms.