Vibronic coupling mechanism in the {tilde {ital A}}{sup 2}{ital A}{sub 2}{endash}{tilde {ital B}}{sup 2}{ital B}{sub 2} excited states of benzyl radical
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706-1396 (United States)
We report two-color resonant two-photon ionization spectra of internally cold benzyl-{ital h}{sub 7}, benzyl-{alpha}{ital d}{sub 2}, and benzyl-{ital d}{sub 7} radicals in the region of the vibronically mixed {tilde {ital A}}{sup 2}{ital A}{sub 2}{endash}{tilde {ital B}}{sup 2}{ital B}{sub 2} excited states near 450 nm. Spectra of the corresponding 1:1 van der Waals complexes benzyl{center_dot}Ar are also reported. Band intensities of threshold photoionization spectra using a variety of mixed {tilde {ital A}}{sup 2}{ital A}{sub 2}-{tilde {ital B}}{sup 2}{ital B}{sub 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 {ital b}{sub 1}-symmetry modes {nu}{sub 28} (an in-plane skeletal deformation) and {nu}{sub 21} (an in-plane skeletal plus CCH bending motion) couple the {tilde {ital A}} and {tilde {ital B}} states most strongly. In contrast to earlier studies, we find that the {ital b}{sub 1} combination {nu}{sub 17}+{nu}{sub 36} plays a prominent role, while the {ital b}{sub 1} in-plane-CH{sub 2} rock {nu}{sub 29} is unimportant. The dispersed fluorescence work of Selco and Carrick and of Fukushima and Obi shows clear evidence of substantial coupling of the {tilde {ital A}} and {tilde {ital C}} states through the {ital a}{sub 1} mode {nu}{sub 13}, in accord with the semiempirical vibronic coupling calculations of Negri {ital et al}. In contrast, our model seemingly demands no {tilde {ital A}}-{tilde {ital B}} vibronic coupling matrix elements larger than 100-200 cm{sup -1}. Thus the dramatic effects of {tilde {ital A}}-{tilde {ital B}} vibronic coupling result primarily from the near-degeneracy of the two excited states rather than unusually strong vibronic coupling matrix elements.
- DOE Contract Number:
- FG02-92ER14306
- OSTI ID:
- 286170
- Journal Information:
- Journal of Chemical Physics, Vol. 104, Issue 22; Other Information: PBD: Jun 1996
- Country of Publication:
- United States
- Language:
- English
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