On the relevance of polyynyl-substituted PAHs to astrophysics

TL;DR

This study investigates the spectral properties of polyynyl-substituted PAHs, revealing their potential as carriers of diffuse interstellar bands due to their distinctive infrared and ultraviolet features.

Contribution

It provides the first detailed spectral analysis of polyynyl-substituted PAHs, linking their features to astrophysical phenomena and suggesting their role in interstellar chemistry.

Findings

Polyynyl-substituted PAHs produce collective infrared features similar to non-substituted PAHs.

Addition of ethynyl groups causes redshift and broadening of ultraviolet absorption bands.

Longer butadiynyl chains enhance pi-pi* transitions, making these molecules strong candidates for interstellar band carriers.

Abstract

We report on the absorption spectra of the polycyclic aromatic hydrocarbon (PAH) molecules anthracene, phenanthrene, and pyrene carrying either an ethynyl (-C2H) or a butadiynyl (-C4H) group. Measurements were carried out in the mid infrared at room temperature on grains embedded in CsI pellets and in the near ultraviolet at cryogenic temperature on molecules isolated in Ne matrices. The infrared measurements show that interstellar populations of polyynyl-substituted PAHs would give rise to collective features in the same way non-substituted PAHs give rise to the aromatic infrared bands. The main features characteristic of the substituted molecules correspond to the acetylenic CH stretching mode near 3.05 mum and to the almost isoenergetic acetylenic CCH in- and out-of-plane bending modes near 15.9 mum. Sub-populations defined by the length of the polyynyl side group cause collective features which correspond to the various acetylenic CC stretching modes. The ultraviolet spectra reveal that the addition of an ethynyl group to a non-substituted PAH molecule results in all its electronic transitions being redshifted. Due to fast internal energy conversion, the bands at shorter wavelengths are significantly broadened. Those at longer wavelengths are only barely affected in this respect. As a consequence, their relative peak absorption increases. The substitution with the longer butadiynyl chain causes the same effects with a larger magnitude, resulting in the spectra to show a prominent if not dominating pi-pi* transition at long wavelength. After discussing the relevance of polyynyl-substituted PAHs to astrophysics, we conclude that this class of highly conjugated, unsaturated molecules are valid candidates for the carriers of the diffuse interstellar bands.