Vibronic mechanisms in the two-photon spectrum of benzene

Intensity measurements have been made on the single quantum bands of the active modes in the two-photon 1B2u←1A1g spectra of C6H6 and its isotopic homologs C6H5D, o-C6H4D2, C6HD5, p-C6H4D2, p-C6H2D4, s-C6H3D3, and C6D6. ν14 (b2u) and ν18 (e1u) (D6h symmetry) are found to be active in all the isotopic benzenes, and ν12 (b1u) in all except C6H6, s-C6H3D3, and C6D6 where it is identity forbidden, and p-C6H4D2 where it is weak. The results show that the band strength of 1410 is insensitive to deuterium substitution (confirming Duschinsky rotation of this mode), while 1210 and 1810 are strongly sensitive to the number and orientation of the deuterium atoms. INDO/S calculations of transition tensors at displaced nuclear coordinates, with and without doubly excited configurations, indicate that the ground state vibronic mechanisms usually neglected in one-photon spectra are important in the benzene two-photon spectra. The activity of ν14 can be attributed to vibronic perturbation of the final B2u state by the ground state, and that of ν18 by vibronic perturbation of the ground state by an excited state. The favored mechanism for the activity of ν12 in the spectra of the deuterium benzenes involves intensity borrowing from ν18 triggered by the kinematic perturbation of the deuterium atoms. The coupling states for ν12 are then E1u and E2g (D6h symmetry). Evidence is presented for Duschinsky rotation of the e1u modes.