Interstellar PAH emission in the 11-14 micron region: New insights from laboratory data and a tracer of ionized PAHs

The Ames infrared spectral database of isolated, neutral and ionized polycy clic aromatic hydrocarbons (PAHs) shows that aromatic CH out-of-plane bendi ng frequencies are significantly shifted upon ionization. For solo- and due t-CH groups, the shift is pronounced and consistently toward higher frequen cies. The solo-CH modes are blueshifted by an average of 27 cm(-1) and the duet-CH modes by an average of 17cm(-1). For trio- and quartet CH groups, t he ionization shifts of the out-of-plane modes are more erratic and typical ly 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 vibrat ions in neutral PAHs, falling instead at frequencies well above those norma lly 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 i nto the frequency range traditionally associated with the solo-CH modes. Th ese results refine our understanding of the origin of the dominant interste llar infrared emission feature near 11.2 mu m, whose envelope has tradition ally 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 n ear 11.0 mu m within the framework of the PAH model. Specifically, the prev alent but variable long-wavelength wing or shoulder that is often observed near 11.4 mu m likely reflects the contributions of duet-CH units in PAH ca tions. Also, these results indicate that the emission between 926 and 904 c m(-1) (10.8 and 11.1 mu m) observed in many sources can be unambiguously at tributed 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.