The diffuse interstellar bands (DIBs) are the longest unsolved mystery in molecular spectroscopy. They are a series of over 500 absorption features in near-IR, visible, and near-UV light, observed through all lines of sight within the interstellar medium (ISM). Despite over a century of research, only one molecule has been conclusively identified as carrying any DIBs: ionised buckminsterfullerene (C60+)1.
These unknown molecules are generally assumed to be carbonaceous radicals and cations present in molecular clouds where they account for a large percentage of the cosmic carbon budget. Their identification will provide further insight into the chemical and physical environments of the ISM and the cosmic lifecycle of carbon.
Small polycyclic aromatic hydrocarbons (PAHs), and the molecules they grow into, are of interest in these experiments, as their discovery in the ISM may reveal information on the chemical pathways which form these, and eventually larger, molecules. Interstellar conditions conducive to PAH processing were replicated in this experiment with the use of a jet-cooled molecular beam within a vacuum chamber and a high voltage plasma discharge, with these molecules being analysed with laser induced fluorescence (LIF), dispersed fluorescence (DF), and resonance-enhanced multiphoton ionization (REMPI) spectroscopy.
This research reveals that the electronic transition of a PAH growth product, a neutral radical of a substituted bicyclic hydrocarbon, is coincident with a weak DIB, λ4689, and hence may be present in the ISM. This may mark the second identified DIB carrier discovered from over a century of systematic research. This presentation will include a detailed methodology of this experiment, the experimental data used for structure determination, comparisons to interstellar data, and astrochemical implications.