Time evolution of individual rotational states after pulsed Doppler-free two-photon excitation: Influence of perturbations in S1 benzene

Single rovibronic level excitation has been demonstrated for a molecule representing the statisticallimit of a nonradiative electronic relaxation process. Resolution of the complex 14Ö two-photon band of benzene was possible after elimination of the Doppler broadening in a Doppler-free two-photon absorption process with narrow bandwidth pulsed laser light and allowed for selective excitation ofindividual rotational states. Fluorescence decay measurements ofthese states under low pressure conditions display a pure exponential behavior and reveal that the nonradiative rate is independent of the rotational quantum numbers J, K for an electronie nonradiative relaxation process in the statistieallimit. The decrease in lifetime found for perturbed states is attributed to the mixing with a background state containing quanta of an accepting mode.