Polycyclic Aromatic Hydrocarbons with Straight Edges and the 7.6/6.2 and 8.6/6.2 Intensity Ratios in Reflection Nebulae

Abstract

Using density functional theory, we have investigated the mid-infrared spectral characteristics of a series of polycyclic aromatic hydrocarbons (PAHs) that have straight edges and that contain an even or odd number of carbons. For several even and odd-carbon PAHs, the 8.6/6.2 and 7.6/6.2 intensity ratios computed in emission after the absorption of a 8 eV photon match the observed ratios obtained for three reflection nebulae (RNe), namely NGC 1333, NGC 7023, and NGC 2023. Odd-carbon PAHs are favored, particularly for NGC 1333. Both cations and anions are present, with the cations being predominant. Relevant PAHs span sizes ranging from 46 to 113 carbons for NGC 7023 and NGC 2023 and from 38 to 127 carbons for NGC 1333, and have symmetries ranging from D 2 h to C s . Our work suggests that even- and odd-carbon PAHs with straight edges are viable candidates for the PAH emission seen toward irradiated photodissociation regions.