Ion Spectroscopy\nin the Context of the Diffuse Interstellar\nBands: A Case Study with the Phenylacetylene Cation

Identification of the molecular carriers of diffuse interstellar\nbands (DIBs) requires gas phase electronic spectra of suitable candidate\nstructures. Recording the spectra of these in the laboratory is challenging\nbecause they include large, carbon-rich molecules, many of which are\nlikely to be ionic. The electronic spectra of ions are often obtained\nusing action spectroscopy methods, which can induce small perturbations\nto the absorption characteristics and hinder comparison with astronomical\nobservations. In this contribution, the appropriateness of helium-tagging\nand two-color resonant-enhanced photodissociation spectroscopy as\nsuitable techniques to obtain the requisite laboratory data for comparison\nto DIBs is explored. As a proof-of-concept, the C̃ <sup>2</sup>B<sub>1</sub> ← X̃ <sup>2</sup>B<sub>1</sub> electronic transition of the phenylacetylene cation\n(PA<sup>+</sup>, C<sub>8</sub>H<sub>6</sub><sup>+</sup>), obtained by helium-tagging and two-color\nphotodissociation, is compared to the direct absorption spectrum recorded\nusing cavity ring-down spectroscopy. The results indicate that for\nDIBs with typical widths of a few ångströms, the wavelengths,\nbandwidths, and relative intensities from action spectroscopy are\nobtained with sufficient precision to facilitate accurate comparisons\nto catalogued DIBs.