Vibronic coupling mechanism in the (A)over-tilde(2)A(2)-(B)over-tilde(2)B(2) excited states of benzyl radical

We report two-color resonant two-photon ionization spectra of internally cold benzyl-h(7), benzyl-alpha d(2), and benzyl-d(7) radicals in the region of the vibronically mixed (A) over tilde(2)A(2)-(B) over tilde(2)B(2) excited states near 450 nm. Spectra of the corresponding 1:1 van der Waals complexes benzyl . Ar are reported as well. Band intensities of threshold photoionization spectra using a variety of mixed (A) over tilde(2)A(2)-(B) over tilde(2)B(2) vibronic states as intermediates provide additional new information about the mechanism of vibronic coupling. A semiquantitative coupling model based on crude adiabatic states attempts to interpret all available data from absorption, dispersed fluorescence, and pulsed field ionization (ZEKE-PFI) spectra. The two b(1)-symmetry modes nu(28) (an in-plane skeletal deformation) and nu(21) (an in-plane skeletal plus CCH bending motion) couple the (A) over tilde and (B) over tilde states most strongly. In contrast to earlier interpretations, we find that the b(1) combination nu(17) + nu(36) plays a prominent role, while the b(1) in-plane-CH2 rock nu(29) is unimportant. The dispersed fluorescence work of Selco and Carrick and of Fukushima and Obi shows clear evidence of substantial coupling of the (A) over tilde and (C) over tilde states through the a(1) and nu(13), in accord with the semiempirical vibronic coupling calculations of Negri et al. In contrast with those calculations, our model seemingly demands no (A) over tilde-(B) over tilde vibronic coupling matrix elements larger than 100-200 cm(-1). Thus the dramatic effects of (A) over tilde-(B) over tilde vibronic coupling result primarily from the near-degeneracy of the two excited states rather than unusually strong vibronic coupling matrix elements. Some fluorescence and PF1 band intensities involving nu(28) and nu(21) deviate substantially from simple predictions based on products of squared mixing coefficients times Franck-Condon factors. A complete understanding of the spectra will require a quantitative account of Duschinsky mixing, which in turn requires accurate excited state vibrational modes. (C) 1996 American Institute of Physics.