Theoretical modeling of infrared emission from neutral and charged polycyclic aromatic hydrocarbons. II.

The nature of the carriers of the interstellar infrared (IR) emission featu res between 3.3 and 12.7 mum is complex. We must consider emission from a f amily of polycyclic aromatic hydrocarbons (PAHs) in a multiplicity of catio nic charge states (+1, +2, +3, and so on), along with neutral and anionic P AHs. The adopted intrinsic IR cross sections of the various modes of the PA Hs are the key to all models. Here we make a comparison between laboratory- measured cross sections and quantum chemical calculations and find that, ov erall, the agreement is very good. In this paper, we consider emission from a wide variety of specific PAH molecules, which includes all available dat a from our laboratory and quantum chemical databases. We incorporate this i nto our model to produce a theoretical analysis that is more realistic, det ailed, and comprehensive than prior studies. PAH molecular structures that we consider include symmetric condensed, symmetric noncondensed, aromatics containing pentagonal rings, linear, and methylated PAHs. The synthesized I R spectra show large variations in peak position for the small PAHs studied , while their spectral profile is uniquely characteristic of each different molecular structure. We also investigate the spectral variations with mole cular structure of a PAH population at the surface of the Orion photodissoc iation region (PDR) and include an example of how the IR spectrum of our PA H population varies dramatically as a function of depth (or radiation field ) through the PDR. We make a comparison of these results with Infrared Spac e Observatory data measured at the surface of the Orion PDR. We conclude th at the charge of PAHs in a composite population has a stronger effect on it s IR emission spectrum than its molecular structure. However, on the basis of the PAH samples considered in this paper, detailed studies of the inters tellar IR emission features can be used effectively to identify molecular c haracteristics of the interstellar PAH family. In Paper III, we extend the theme of this paper by investigating the effects of hydrogenation on a wide variety of PAHs up to size 54 carbon atoms and compare our results with ob servational profiles for the Orion PDR.