The laser photoelectron spectrum of gas phase benzene

Benzene vapor in both effusive and supersonic molecular beams has been irradiated with the frequency doubled output from a tunable dye laser. Photoelectrons ejected as a result of two-step ionization have been kinetic energy analyzed by a time-of-flight method. Electron energy resolution is between 3 and 10 meV depending on beam conditions. Spectra resulting from the excitation of ten different vibronic bands of C6H6 and three of C6D6 are presented. Ions are found to be generated in their 2E1g ground electronic state in vibrational state distributions that are strongly wavelength dependent. Analysis of these distributions leads to an improved understanding of Jahn–Teller effects in benzene cation and observation for the first time of Jahn–Teller splitting in C6H+6 and C6D+6. Normal mode ν16 is conclusively assigned as 39.5 meV (319 cm−1) and 34.5 meV (278 cm−1) in C6H+6 and C6D+6. ν4 is tentatively assigned as 51.5 meV (415 cm−1) and 43.5 meV (351 cm−1) in these species, respectively. An explanation for the anomalously large frequency of ν6 previously measured with conventional photoelectron spectroscopy is provided. It is also demonstrated that information contained in laser photoelectron spectra can be of assistance in assigning vibronic features in near ultraviolet molecular absorption spectra.