Peripherally hydrogenated neutral polycyclic aromatic hydrocarbons as carriers of the 3 micron interstellar infrared emission complex: Results from single-photon infrared emission spectroscopy

Infrared emission spectra of five gas-phase UV laser-excited polycyclic aro matic hydrocarbons (PAHs) containing aliphatic hydrogens are compared with the main 3.3 mum and associated interstellar unidentified infrared emission bands (UIRs). We show that neutral PAHs can account for the majority of th e 3 mum emission complex while making little contribution to the other UIR bands; peripherally hydrogenated PAHs produce a better match to astrophysic al data than do those containing methyl side groups; 3.4 mum plateau emissi on is shown to be a general spectral feature of vibrationally excited PAHs containing aliphatic hydrogens, especially those containing methyl groups; and finally, hot-band and overtone emissions arising from aromatic C-H vibr ations are not observed in laboratory emission spectra, and therefore, in c ontrast to current assignments, are not expected to be observed in the UIRs .