Ion Spectroscopy in the Context of the Diffuse Interstellar Bands: A Case Study with the Phenylacetylene Cation

Identification of the molecular carriers of diffuse interstellar bands (DIBs) requires gas phase electronic spectra of suitable candidate structures. Recording the spectra of these in the laboratory is challenging because they include large, carbon-rich molecules, many of which are likely to be ionic. The electronic spectra of ions are often obtained using action spectroscopy methods, which can induce small perturbations to the absorption characteristics and hinder comparison with astronomical observations. In this contribution, the appropriateness of helium-tagging and two-color resonant-enhanced photodissociation spectroscopy as suitable techniques to obtain the requisite laboratory data for comparison to DIBs is explored. As a proof-of-concept, the C̃ ²B₁ ← X̃ ²B₁ electronic transition of the phenylacetylene cation (PA⁺, C₈H₆ ⁺), obtained by helium-tagging and two-color photodissociation, is compared to the direct absorption spectrum recorded using cavity ring-down spectroscopy. The results indicate that for DIBs with typical widths of a few ångströms, the wavelengths, bandwidths, and relative intensities from action spectroscopy are obtained with sufficient precision to facilitate accurate comparisons to catalogued DIBs.