Theoretical Modeling of Infrared Emission from Neutral and Charged Polycyclic Aromatic Hydrocarbons. I.

Since the discovery of interstellar infrared emission features in the 3.3-12.7 μm wavelength range three decades ago, the carriers of these features have been the subject of much debate. Recent observational work with the Infrared Space Observatory, experimental work, and quantum chemical calculations concerning positively charged polycyclic aromatic hydrocarbon (PAH) molecules point to the infrared fluorescence of such species. This paper presents a model of the interstellar infrared emission between 3.3 and 12.7 μm from a population of symmetric, condensed polycyclic aromatic hydrocarbons composed of up to 54 carbon atoms. We describe the infrared emission intensity in terms of the size of the emitting molecule, its charge, and its temperature probability distribution function. The model takes the charge state (anion, neutral, cation of charge state up to +3) into account self-consistently, employing the most recent quantum chemically calculated infrared cross sections of such species. This paper provides an exploratory study to illustrate the dependence of the interstellar infrared emission on the polycyclic aromatic hydrocarbon charge distribution. We conclude that the charge state of the PAH has an important effect on the emitted infrared spectrum. The 3.3 μm stretching mode and, to a lesser extent, the 11-15 μm C–H out-of-plane bending modes produce significant emission relative to the other infrared features and originate predominantly from neutral and anionic PAHs. The 6-8 μm emission from the C–C stretching modes, in contrast, originates mainly from charged PAHs with only a partial contribution from neutrals. For heavily ultraviolet irradiated regions such as the Orion Bar, multiply positively charged PAHs are the norm and contribute significantly in this wavelength region. However, because the total infrared emission is a sum over various charge states of different molecules, the ratios of the infrared emission bands do not vary much for Go/ne ≤ 103 cm3. This range includes conditions relevant to both the diffuse interstellar medium and typical reflection nebulae. Larger variations in the interstellar infrared emission features can be expected from photodissociation regions associated with dense H II regions such as the Orion Bar (Go/ne ~ 104).