High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 micron region: role of hydrogenation and alkylation

TL;DR

This study uses high-resolution IR spectroscopy combined with theoretical calculations to analyze how hydrogenation and alkylation alter the 3 micron spectral features of polycyclic aromatic hydrocarbons, providing insights into their astrophysical emission signatures.

Contribution

It presents the first detailed high-resolution spectra of H- and Me-PAHs in the 3 micron region, highlighting the importance of anharmonic effects and their relevance to astrophysical observations.

Findings

Anharmonicity significantly influences CH-stretch vibrations.

Variation in PAH substitution explains spectral diversity in space.

Hydrogenated PAHs may be key carriers of IR emission in certain regions.

Abstract

Aims. We aim to elucidate the spectral changes in the 3 micron region that result from chemical changes in the molecular periphery of polycyclic aromatic hydrocarbons (PAHs) with extra hydrogens (H-PAHs) and methyl groups (Me-PAHs). Methods. Advanced laser spectroscopic techniques combined with mass spectrometry were applied on supersonically cooled 1,2,3,4-tetrahydronaphthalene, 9,10-dihydroanthracene, 9,10-dihydrophenathrene, 1,2,3,6,7,8-hexahydropyrene, 9-methylanthracene, and 9,10-dimethylanthracene, allowing us to record mass-selective and conformationally selective absorption spectra of the aromatic, aliphatic, and alkyl CH-stretches in the 3.175-3.636 micron region with laser-limited resolution. We compared the experimental absorption spectra with standard harmonic calculations and with second-order vibrational perturbation theory anharmonic calculations that use the SPECTRO program for treating resonances. Results. We show that anharmonicity plays an important if not dominant role, affecting not only aromatic, but also aliphatic and alkyl CH-stretch vibrations. The experimental high-resolution data lead to the conclusion that the variation in Me- and H-PAHs composition might well account for the observed variations in the 3 micron emission spectra of carbon-rich and star-forming regions. Our laboratory studies also suggest that heavily hydrogenated PAHs form a significant fraction of the carriers of IR emission in regions in which an anomalously strong 3 micron plateau is observed.