Electronic Spectroscopy of PAHs

Abstract

Abstract. Polycyclic aromatic hydrocarbons are a class of molecules of broad interest that has long been explored by various spectroscopic techniques. The electronic spectroscopy of these species is of particular interest since it provides a framework for the understanding of the electronic structure of large polyatomic molecules. Such studies also allow the systematic investigation of electronic relaxation mechanisms in large molecules. In this review, we focus on the gas-phase experimental work on such systems and present the latest progress. We also underline the challenges that remain to be tackled. A focus on the understanding of the electronic relaxation pathways at work in gas-phase PAHs will also be presented, as well as their possible manifestation in space.

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

“…However, for coronene, mean fluorescence and phosphorescence yields are of only 0.2 and 0.1, respectively (Dawson & Kropp 1969). Because of smaller energy gaps, fluorescence and phosphorescence for large species is unimportant (Pino et al 2011). For ions, internal conversion will be even more important as the energy gaps involved are smaller (cf.…”

“…For ions, internal conversion will be even more important as the energy gaps involved are smaller (cf. Pino et al 2011). Delayed fluorescence -also sometimes called Poincaré fluorescence, where the excited species revisits the S 1 (or T 1 ) state (Léger et al 1988) -is measured to be unimportant for coronene as it only accounts for a small fraction of the total radiationless de-activation of triplet coronene (Kropp & Dawson 1967).…”

Diffuse Interstellar Bands: The Way Forward

“…Some relevant experimental data is summarized in Figure 3 (Pino et al 2011), illustrating the energy gap law (derived from measured linewidths) but similar results also apply to hydrocarbon chains (Ding et al 2003;Boguslavskiy et al 2005;Boguslavskiy & Maier The Way Forward 405 2006). Thus, for visible transitions where the energy gap is 0.5 − 1 eV (eg., typically one or more lower lying, excited electronic states), the line width will be some 20Å.…”

“…A number of research groups have made a dedicated effort to study relevant DIB carriers in the laboratory: notably John Maier and his group in Basel have focused on the spectroscopic characteristics of hydrocarbon chains, while Farid Salama and his group at NASA Ames, Huiskens and his group at Jena, and Thomas Pino and Philippe Brechignac at the IAS have put the emphasis on the PAH molecules (Maier et al 2004Salama et al 2011;Gredel et al 2011;Pino et al 2011). These efforts are reviewed elsewhere in this volume.…”

The Diffuse Interstellar Bands: an Elderly Astro-Puzzle Rejuvenated

“…Recent studies have focused on obtaining laboratory spectroscopic data of low temperature gas-phase neutral PAHs to compare with astronomical spectra of diffuse and translucent clouds (e.g. Pino et al (2011), Dietsche et al (2011). Ten different PAHs (containing 2 to 13 benzene rings / 11 to 42 carbon atoms) were searched for but none were detected in the ISM by both and Gredel et al (2011) independently.…”