Spectroscopic study on deuterated benzenes. III. Vibronic structure and dynamics in the S(1) state

We observed the fluorescence excitation spectra and mass-selected resonance enhanced multiphoton ionization (REMPI) excitation spectra for the 6(0)(1), 6(0)(1)1(0)(1), and 6(0)(1)1(0)(2) bands of the S1←S0 transition of jet-cooled deuterated benzene and assigned the vibronic bands of C6D6 and C6HD5. The 6(0)(1)1(0)(n) (n = 0, 1, 2) and 0(0)(0) transition energies were found to be dependent only on the number of D atoms (ND), which was reflected by the zero-point energy of each H/D isotopomer. In some isotopomers some bands, such as those of out-of-plane vibrations mixed with 6(1)1(n), make the spectra complex. These included the 6(1)10(2)1(n) level or combination bands with ν12 which are allowed because of reduced molecular symmetry. From the lifetime measurements of each vibronic band, some enhancement of the nonradiative intramolecular vibrational redistribution (IVR) process was observed. It was also found that the threshold excess energy of "channel three" was higher than the 6(1)1(2) levels, which were similar for all the H/D isotopomers. We suggest that the channel three nonradiative process could be caused mainly by in-plane processes such as IVR and internal conversion at the high vibrational levels in the S1 state of benzene, although the out-of-plane vibrations might contribute to some degree.

Intramolecular Processes Revealed Using UV-Laser-Induced IR-Fluorescence: A New Perspective on the "Channel Three" of Benzene

Radiative relaxation in the infrared (IR) is common following nonradiative electronic relaxation processes, but it is rarely measured. We present ultraviolet laser-induced infrared fluorescence (UV-LIIRF) excitation spectroscopy and dispersed UV-LIIRF spectroscopy of gas phase benzene vibronically excited around the onset of channel 3, using a homemade spectrometer. We found that the vibrational IR fluorescence yield is clearly higher when benzene is excited above the onset than when it is excited below. Significant changes in dispersed IR emission profiles resulting from excitations below and above the onset of channel 3 were also observed. These results suggest that isomerization of benzene toward fulvene occurs efficiently below the opening of channel 3 and confirm that channel 3 involves a photophysical relaxation pathway that efficiently competes with isomerization.