Laser-induced fluorescence of cyclohexadienyl (c-C6H7) radical in the gas phase

A laser-induced fluorescence spectrum was observed in the 500-560 nm region when a mixture of 1,4-cyclohexadiene and oxalyl chloride was photolyzed at 193 nm. The observed excitation spectrum was assigned to the A-tilde 2A2<-X-tilde 2B1 transition of the cyclohexadienyl radical c-C6H7, produced by abstraction of a hydrogen atom from 1,4-cyclohexadiene by Cl atoms. The origin of the A-tilde<-X-tilde transition of c-C6H7 was at 18 207 cm-1. From measurements of the dispersed fluorescence spectra and ab initio calculations, the frequencies of several vibrational modes in both the ground and excited states of c-C6H7 were determined: ν5(C-H in-plane bend)=1571, ν8(C-Hin-plane bend)=1174, ν10(C-C-C in-plane bend)=981, ν12(C-C-C in-plane bend)=559, ν16(C-C-C out-of-plane bend)=375, and ν33(C-C-C in-plane bend)=600 cm-1 for the ground state and ν8=1118, ν10=967, ν12=502, ν16=172, and ν33=536 cm-1 for the excited states