High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 {\mu}m region: Role of periphery

TL;DR

This study uses high-resolution IR spectroscopy and advanced calculations to explore how the peripheral structure of PAHs influences their 3 μm absorption features, with implications for astrophysical observations.

Contribution

It provides detailed experimental and theoretical analysis of PAH spectra, highlighting anharmonic effects and their role in shaping the 3 μm IR band in space.

Findings

Anharmonicity dominates the 3 μm region, producing more bands than harmonic models predict.

SPECTRO's anharmonic calculations align well with experimental spectra.

The dataset covers diverse PAH structures, aiding astrophysical interpretation.

Abstract

In this work we report on high-resolution IR absorption studies that provide a detailed view on how the peripheral structure of irregular polycyclic aromatic hydrocarbons (PAHs) affects the shape and position of their 3 {\mu}m absorption band. To this purpose we present mass-selected, high-resolution absorption spectra of cold and isolated phenanthrene, pyrene, benz[a]antracene, chrysene, triphenylene, and perylene molecules in the 2950-3150 cm-1 range. The experimental spectra are compared with standard harmonic calculations, and anharmonic calculations using a modified version of the SPECTRO program that incorporates a Fermi resonance treatment utilizing intensity redistribution. We show that the 3 {\mu}m region is dominated by the effects of anharmonicity, resulting in many more bands than would have been expected in a purely harmonic approximation. Importantly, we find that anharmonic spectra as calculated by SPECTRO are in good agreement with the experimental spectra. Together with previously reported high-resolution spectra of linear acenes, the present spectra provide us with an extensive dataset of spectra of PAHs with a varying number of aromatic rings, with geometries that range from open to highly-condensed structures, and featuring CH groups in all possible edge configurations. We discuss the astrophysical implications of the comparison of these spectra on the interpretation of the appearance of the aromatic infrared 3 {\mu}m band, and on features such as the two-component emission character of this band and the 3 {\mu}m emission plateau.