Back to the roots of ‘‘channel three’’: Rotationally resolved spectra of the 6113 band of C6H6

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Abstract
Rotationally resolved fluorescence excitation and resonance enhanced multiphoton ionization (MPI) spectra of the 610130 one-photon band of benzene at the onset of ‘‘channel three’’ are reported. The fluorescence decay is monitored after rotationally selected excitation and a large variation of the nonradiative decay time (<1 to 6.8 ns) is found for the different rotational states at the vibrational excess energy of 3287 cm−1 in S1. The rotational structure of the fluorescence excitation spectrum and the MPI spectrum measured with delayed laser pulses differ considerably. All observed lines of the MPI spectrum were assigned and the rotational line structure can only be understood with a model which incorporates interference between different decay channels. Due to this interference, particular rotational states decay fairly slowly and give rise to lines in the spectrum while states with neighboring rotational quantum numbers decay rapidly and are therefore not found in the spectrum. The previously reported drastic increase of the electronic, nonradiative decay of benzene in this region of excess energy, which led to the postulation of ‘‘channel three,’’ cannot be confirmed. Instead, the optically excited rovibronic states are thought to be coupled to background states within S1 which are themselves broadened due to strong coupling to the highly excited S0 electronic state rather than due to an unknown (‘‘channel three’’) or isomerization process.