Electronic Transition of the l-C₆⁺ Cation at 417 nm

A new electronic transition is reported for the linear C6+ cation with an origin at 416.8 nm. This spectrum can be compared to the matrix isolation spectra at lower energies reported previously by Fulara et al. (J. Chem. Phys. 123, 044305 (2005)), which assigned linear and cyclic isomers, and to the gas phase spectrum reported previously by Campbell and Dunk (Rev. Sci. Instrum. 90, 103101 (2019)), which detected the same cyclic-isomer spectrum reported by Fulara. Comparisons to electronically excited states and vibrations predicted by various forms of theory allow assignment of the spectrum to a new electronic state of linear C6+. The spectrum consists of a strong origin band, two vibronic progression members at higher energy and four hotbands at lower energies. The hotbands provide the first gas phase information on ground state vibrational frequencies. The vibronic structure of this excited state of C6+ provides a severe challenge to computational chemistry.