Rotationally-resolved excitation spectrum of the jet-cooled cyclohexadienyl radical

A laser-induced fluorescence excitation spectrum of the cyclohexadienyl (c-C6H7) radical was observed in the region between 550 and 530 nm under jet-cooled conditions. A partly rotationally resolved spectrum of the vibronic origin band was obtained with a high-resolution laser scan. The observed rotational structure of the origin band is that of a b-type transition, being not discrepant with the A ∼ 2 A 2 – X ∼ 2 B 1 electronic transition which has been indicated by a few theoretical calculations. The origin band position and the rotational constants in the upper and lower states were determined from a rotational analysis of the spectrum. The determined molecular constants agreed very well with results of our ab initio calculations. The theoretical vibrational frequencies for the upper and lower electronic states were also calculated for comparison with the experimental results.