Investigating C-D out-of-plane vibrational modes in PAHs as a tool to study interstellar deuterium-containing PAHs

Abstract

Previous as well as recent observations by ISO, Spitzer, AKARI, SOFIA, JWST etc. have revealed various characteristics of mid-infrared emission bands between 3-20 micron. Subsequently, several forms of organics including Polycylic Aromatic Hydrocarbons (PAHs)/PAH-like molecules are proposed as carriers for these bands. Deuterated PAH (PAD) is one such substituted PAH, which is proposed as a potential candidate carrier for weak emission bands at 4.4 and 4.65 micron, detected towards few astronomical targets and are characteristics of aromatic and aliphatic C-D stretching modes in a PAD molecule, respectively. However, the 4.4 micron band is not widely detected. In order to validate PADs as carriers for mid-infrared emission bands, an additional alternative tool is crucial. If PAHs are deuterated, they should also possess an inherent signature from the C-D out-of-plane (C-Doop) vibrations, which are at the longer wavelength side. In this report, features due to C-Doop modes in PAHs bearing a single to multiple deuterium atoms are reported by performing quantum-chemical calculations. This paper reports that some of the C-Doop vibrations appear at the 14-19 micron range. Also, the strength of C-Doop modes is not proportional to the D/H ratio in PAHs. In addition, a moderate change in the spectra of deuterated PAHs is observed from that of the undeuterated counterparts, as deuteration would alternate the adjacency class of the C-H bonds and the symmetry of the molecule. We discuss the efficiency and usefulness of these bands to constrain the form of PAHs emitting mid-infrared emission bands.