Ro-vibronic resolved two-photon excitation spectra of benzene, C6H6, and C6D6 have been measured in the region of the S1 ←S0 transition which is parity and symmetry forbidden in two-photon absorption but can be induced by suitable  ungerade  vibrations. Rotational envelopes for the vibronic transitions are calculated for circular and linear polarized light excitation and are in good agreement with experiment. The polarization behavior is shown to change strongly across even a single band contour of a totally symmetric vibronic two-photon transition. The polarization analysis even for a randomly rotating gas phase molecule provides a severe constraint on the possible assignments and hence is an important tool for the assignment of new transitions. About 85% of the observed two-photon excitation spectrum of benzene can thus be assigned. The appearance of combination bands shows that anharmonic mixing plays an important role in the excited state.

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Gas phase benzene is excited by two-photon absorption at various pressures between 4 and 80 torr. Fluorescence is observed even from states located at excess energies of 3000 cm -1 -7000 cm -1 , i.e. well above the onset of "channel three". The nonradiative decay rates as estimated from this fluorescence intensity continue the well known shortening of lifetime with excess energy in a monotone fashion and do not show a drastic cut-off as often concluded from previous linewidth measurements.

We present here a detailed theoretical development of two-photon absorption (TPA) in the gas phase including rotational effects. A general formula for the transition probabilities between rovibronic states is derived in the rigid rotor approximation. Similarly to Raman theory the intensity of the rotational lines can be attributed to three independent contributions, which derive from the isotropic, antisymmetric and anisotropic part of the TPA-tensor. Each of these contributions carries a different geometrical factor so that they may be distinguished by polarization experiments. It follows directly from the theory that different rotational branches of one and the same band may differ in their polarization behaviour. Until recently, the latter effect, although central to symmetry assignments, has been overlooked. Application to the asymmetric rotor is discussed in detail and two-photon rotational band contour calculations for naphthalene- h 8 are presented.