Polycyclic Aromatic Hydrocarbons with armchair edges and the 12.7 {\mu}m band

TL;DR

This study uses density functional theory to analyze medium-sized armchair-edge PAHs, linking their vibrational modes, especially at 12.7 μm, to observed infrared bands in HII regions, with implications for PAH lifecycle understanding.

Contribution

It is the first to connect specific vibrational modes of armchair PAHs with the 12.7 μm infrared band observed in astrophysical environments.

Findings

Armchair PAHs with NC >65 likely produce the 12.7 μm band.

Strong C-H stretching and bending modes are identified around 3 μm and 10-15 μm.

Ring deformation modes at 12.7 μm are common in PAHs.

Abstract

In this Letter we report the results of density functional theory calculations on medium-sized neutral Polycyclic Aromatic Hydrocarbon (PAH) molecules with armchair edges. These PAH molecules possess strong C-H stretching and bending modes around 3 {\mu}m and in the fingerprint region (10-15 {\mu}m), and also strong ring deformation modes around 12.7 {\mu}m. Perusal of the entries in the NASA Ames PAHs Database shows that ring deformation modes of PAHs are common - although generally weak. We then propose that armchair PAHs with NC >65 are responsible for the 12.7 {\mu}m Aromatic Infrared Band in HII regions and discuss astrophysical implications in the context of the PAH life-cycle.