Polycyclic Aromatic Hydrocarbon Molecules and the 2175 Angstrom Interstellar Extinction Bump

TL;DR

This study uses quantum chemical calculations to evaluate if polycyclic aromatic hydrocarbons (PAHs) can explain the 2175 Angstrom interstellar extinction bump, finding that PAH mixtures can produce a similar broad absorption feature.

Contribution

It provides a quantitative analysis of PAHs as carriers of the 2175 Angstrom bump, highlighting the importance of specific PAH sizes and structures for matching observed width.

Findings

PAH mixtures produce a broad absorption band similar to the bump

Bump position shifts with PAH size, matching observations

Uniformly weighted PAH mixtures have broader width than observed

Abstract

The exact nature of the 2175 Angstrom extinction bump, the strongest spectroscopic absorption feature superimposed on the interstellar extinction curve, remains unknown ever since its discovery in 1965. Popular candidate carriers for the extinction bump include nano-sized graphitic grains and polycyclic aromatic hydrocarbon (PAH) molecules. To quantitatively evaluate PAHs as a possible carrier, we perform quantum chemical computations for the electronic transitions of 30 compact, pericondensed PAH molecules and their cations as well as anions with a wide range of sizes from 16 to 96 C atoms and a mean size of 43 C atoms. It is found that a mixture of such PAHs, which individually exhibit sharp absorption features, show a smooth and broad absorption band that resembles the 2175 Angstrom interstellar extinction bump. Arising from \pi^{*} --\pi, the width and intensity of the absorption bump for otherwise randomly-selected and uniformly-weighted PAH mixtures, do not vary much with PAH sizes and charge states, whereas the position shifts to longer wavelengths as PAH size increases. While the computed bump position, with the computational uncertainty taken into account, appears to agree with that of the interstellar extinction bump, the computed width is considerably broader than the interstellar bump if the molecules are uniformly weighted. It appears that, to account for the observed bump width, one has to resort to PAH species of specific sizes and structures.