He-tagging spectroscopy of tetracene cations*

Abstract

Polyacenes among other polycyclic aromatic hydrocarbons (PAHs) were suggested to be abundant in the dense regions of the interstellar medium (ISM) and to be responsible for some of the diffuse interstellar bands (DIBs). The identification of DIB carriers requires the laboratory spectra of cold cations for comparison with astrophysical observations. We employed He-tagging spectroscopy to record the spectrum of tetracene cations. In contrast to previous measurements of other polyacene cations, we found very narrow absorption bands of tetracene +. This makes the detection of tetracene+ in the ISM much easier compared to other studied polyacene cations. The four absorption bands of tetracene+ that are the best suited for the detection in observational spectra appeared at 859, 850.9, 838.8, and 827.3 nm. We did not find any DIBs at these positions. However, the observational spectra in this region from ground-based telescopes are heavily contaminated with telluric absorption bands, which greatly complicates the detection of DIBs. Comparison with observations performed by space telescopes is expected to significantly increase the sensitivity of the detection of the DIBs that could potentially be assigned to tetracene+.