1999ApJ...516L..41H

The Ames infrared spectral database of isolated, neutral and ionized polycyclic aromatic hydrocarbons (PAHs) shows that aromatic CH out-of-plane bending frequencies are significantly shifted upon ionization. For solo- and duet-CH groups, the shift is pronounced and consistently toward higher frequencies. The solo-CH modes are blueshifted by an average of 27 cm^–1 and the duet-CH modes by an average of 17 cm^–1. For trio- and quartet-CH groups, the ionization shifts of the out-of-plane modes are more erratic and typically more modest. As a result of these ionization shifts, the solo-CH out-of-plane modes move out of the region classically associated with these vibrations in neutral PAHs, falling instead at frequencies well above those normally attributed to out-of-plane bending vibrations of any type. In addition, for the compact PAHs studied, the duet-CH out-of-plane modes are shifted into the frequency range traditionally associated with the solo-CH modes. These results refine our understanding of the origin of the dominant interstellar infrared emission feature near 11.2 µm, whose envelope has traditionally been attributed only to the out-of-plane bending of solo-CH groups on PAHs, and provide a natural explanation for the puzzling emission feature near 11.0 µm within the framework of the PAH model. Specifically, the prevalent but variable long-wavelength wing or shoulder that is often observed near 11.4 µm likely reflects the contributions of duet-CH units in PAH cations. Also, these results indicate that the emission between 926 and 904 cm^–1 (10.8 and 11.1 µm) observed in many sources can be unambiguously attributed to the out-of-plane wagging of solo-CH units in moderately sized (fewer than 50 carbon atom) PAH cations, making this emission an unequivocal tracer of ionized interstellar PAHs.