Electronic absorption spectra of cold organic cations: 2,4-Hexadiyne

The results of a study with an approach to measure electronic spectra of organic cations whereby both the rotational and vibrational motions are relaxed to low temperatures 10–50 K, as relevant for comparison with astronomical observations are presented. Mass-selected ions are constrained in a 22-pole ion trap where the vibrational and rotational degrees of freedom are relaxed by collisions with cryogenically cooled helium gas. This investigation is on a C6H6+ isomer, 2,4-hexadiyne cation, and utilizes 1-photon predissociation to record its A˜Eu2←X˜Eg2 electronic spectrum. The well resolved vibrational pattern and the K-rotational structure on the origin band indicate an internal temperature in the 20–30 K range and leads to the mapping of the excited state vibrational levels.