A Theoretical Study on the Vibrational Spectra of PAH Molecules with Aliphatic Sidegroups

TL;DR

This study uses density functional theory to analyze how aliphatic side groups influence the vibrational spectra of PAH molecules, shedding light on their role in astronomical infrared emission features.

Contribution

It provides a detailed theoretical analysis of the impact of aliphatic groups on PAH vibrational spectra and their potential role in astronomical UIE features.

Findings

Aliphatic groups cause merging of vibrational bands, forming broad emission features.

Many UIE bands are coupled vibrational modes, not purely aromatic.

Aliphatic structures significantly influence the 8 μm emission plateau.

Abstract

The role of aliphatic side groups on the formation of astronomical unidentified infrared emission (UIE) features is investigated by applying the density functional theory (DFT) to a series of molecules with mixed aliphatic-aromatic structures. The effects of introducing various aliphatic groups to a fixed polycyclic aromatic hydrocarbon (PAH) core (ovalene) are studied. Simulated spectra for each molecule are produced by applying a Drude profile at $T$=500 K while the molecule is kept at its electronic ground state. The vibrational normal modes are classified using a semi-quantitative method. This allows us to separate the aromatic and aliphatic vibrations and therefore provide clues to what types of vibrations are responsible for the emissions bands at different wavelengths. We find that many of the UIE bands are not pure aromatic vibrational bands but may represent coupled vibrational modes. The effects of aliphatic groups on the formation of the 8 $\mu$m plateau are qua ntitatively determined. The vibrational motions of methyl ($-$CH$_3$) and methyl ene ($-$CH$_2-$) groups can cause the merging of the vibrational bands of the pa rent PAH and the forming of broad features. These results suggest that aliphatic structures can play an important role in th e UIE phenomenon.