Response theory calculations of the vibronically induced ¹A_1g-¹B_2u two-photon spectrum of benzene

We perform multiconfiguration quadratic response theory calculations of vibronically induced symmetry and parity forbidden ¹A_1g-¹B_2u two-photon transitions of benzene. The following order of vibronic activity is found: ν₁₄ > ν₁₅ > ν₁₈ > ν₁₉ > ν₁₇ > ν₁₆ > ν₂₀. The two-photon activity is crucially dependent on the description of the carbon skeletal motion in the ν₁₄ (b_2u) 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 ν₁₉ mode should appear in a high-resolution experiment. Two-photon polarization ratios are predicted and discussed.