Vibronic coupling mechanism in the à 2A2–B̃ 2B2 excited states of benzyl radical

Abstract

We report two-color resonant two-photon ionization spectra of internally cold benzyl-h7, benzyl-αd2, and benzyl-d7 radicals in the region of the vibronically mixed à 2A2–B̃ 2B2 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 à 2A2–B̃ 2B2 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 b1-symmetry modes ν28 (an in-plane skeletal deformation) and ν21 (an in-plane skeletal plus CCH bending motion) couple the à and B̃ states most strongly. In contrast to earlier interpretations, we find that the b1 combination ν17+ν36 plays a prominent role, while the b1 in-plane–CH2 rock ν29 is unimportant. The dispersed fluorescence work of Selco and Carrick and of Fukushima and Obi shows clear evidence of substantial coupling of the à and C̃ states through the a1 mode ν13, in accord with the semiempirical vibronic coupling calculations of Negri et al. In contrast with those calculations, our model seemingly demands no ÖB̃ vibronic coupling matrix elements larger than 100–200 cm−1. Thus the dramatic effects of ÖB̃ vibronic coupling result primarily from the near-degeneracy of the two excited states rather than unusually strong vibronic coupling matrix elements. Some fluorescence and PFl band intensities involving ν28 and ν21 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.