Modeling astronomically observed interstellar infrared spectra by ionized carbon pentagon-hexagon molecules (c9h7) n+

TL;DR

This study models interstellar infrared spectra using ionized pentagon-hexagon molecules, demonstrating their potential as carriers of observed PAH spectra through density functional theory calculations.

Contribution

It introduces a simplified pentagon-hexagon molecular model (C9H7)n+ for reproducing interstellar IR spectra, expanding previous work on void-induced coronene.

Findings

Ionized C9H7n+ molecules can nearly reproduce observed IR spectra.

Combination of different ionization states can match the total spectrum.

Ionized naphthalene shows little spectral coincidence with observations.

Abstract

Modeling a promising carrier of the astronomically observed polycyclic aromatic hydrocarbon (PAH), infrared (IR) spectra of ionized molecules (C9H7) n+ were calculated based on density functional theory (DFT). In a previous study, it was found that void induced coronene C23H12++ could reproduce observed spectra from 3 to 15 micron, which has carbon two pentagons connected with five hexagons. In this paper, we tried to test the simplest model, that is, one pentagon connected with one hexagon, which is indene like molecule (C9H7) n+ (n=0 to 4). DFT based harmonic frequency analysis resulted that observed spectrum could be almost reproduced by a suitable sum of ionized C9H7n+ molecules. Typical example is C9H7++. Calculated peaks were 3.2, 7.4, 7.6, 8.4, and 12.7 micron, whereas observed one 3.3, 7.6, 7.8, 8.6 and 12.7 micron. By a combination of different degree of ionized molecules, we can expect to reproduce total spectrum. For a comparison, hexagon-hexagon molecule naphthalene (C10H8) n+ was studied. Unfortunately, ionized naphthalene shows little coincidence with observed one. Carbon pentagon- hexagon molecules may play an important role as interstellar molecular dust.