RESPONSE THEORY CALCULATIONS OF THE VIBRONICALLY INDUCED (1)A(1G)-(1)B(2U) 2-PHOTON SPECTRUM OF BENZENE

We perform multiconfiguration quadratic response theory calculations of vibronically induced symmetry and parity forbidden 1A1g-B-1(2u) two-photon transitions of benzene. The following order of vibronic activity is found: nu14 > nu15 > nu18 > nu19 > nu17 > nu16 > nu20. The two-photon activity is crucially dependent on the description of the carbon skeletal motion in the nu14 (b2u) mode, which gives a good illustration of the fact that two-photon spectroscopy can be used to derive information on vibronic motions and force fields. It is suggested that the so far unobserved nu19 mode should appear in a high-resolution experiment. Two-photon polarization ratios are predicted and discussed.