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Using the Mid-Infrared Spectrometer on board the Infrared Telescope in Spaceand the low-resolution grating spectrometer (PHT-S) on board the Infrared SpaceObservatory, we obtained 820 mid-infrared (5 to 12 $\mu$m) spectra of thediffuse interstellar medium (DIM) in the Galactic center, W51, and CarinaNebula regions. These spectra indicate that the emission is dominated by theunidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 $\mu$m.The relative band intensities (6.2/7.7 $\mu$m, 8.6/7.7 $\mu$m, and 11.2/7.7$\mu$m) were derived from these spectra, and no systematic variation in theseratios was found in our observed regions, in spite of the fact that theincident radiation intensity differs by a factor of 1500. Comparing our resultswith the polycyclic aromatic hydrocarbons (PAHs) model for the UIR bandcarriers, PAHs in the DIM have no systematic variation in their sizedistribution, their degree of dehydrogenation is independent of the strength ofUV radiation field, and they are mostly ionized. The latter finding isincompatible with past theoretical studies, in which a large fraction ofneutral PAHs is predicted in this kind of environment. A plausible resolutionof this discrepancy is that the recombination coefficients for electron andlarge PAH positive ion are by at least an order of magnitude less than thoseadopted in past theoretical studies. Because of the very low population ofneutral state molecules, photoelectric emission from interstellar PAHs isprobably not the dominant source of heating of the diffuse interstellar gas.The present results imply constant physical and chemical properties of thecarriers of the UIR emission bands in the DIM.Comment: 13 pages, 6 figures. Accepted for publication in Ap

Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium