The S1(1A1)-S0(1A1) Electronic Transition of Jet-Cooled o-Difluorobenzene.

A detailed study of the S1(1A1)-S0(1A1) transition of jet-cooled o-difluorobenzene has been completed using the two techniques of laser-induced fluorescence excitation and dispersed, single vibronic level fluorescence spectroscopy. Analysis of over 60 dispersed fluorescence spectra resulted in both the assignment of 22 excited state vibrational frequencies and the confirmation of 23 ground state frequencies. The spectrum is dominated by Franck-Condon activity in totally symmetric vibrations with long progressions in the ring-breathing mode, nu9. By analogy with benzene and the para- and meta-substituted isomers, two vibronic coupling mechanisms are postulated to be responsible for the wealth of weaker symmetry-forbidden structure that has been observed. Single quantum changes in b2 vibrations are postulated to appear due to first order vibronic coupling to a higher lying B2 electronic state. Combinations of b1 and a2 modes are postulated to appear from second order vibronic coupling to an A1 electronic state. This second order coupling causes a pronounced Duschinsky mixing among excited state b1 and a2 modes with respect to their ground state counterparts. Franck-Condon factors are calculated for the a1 progression-forming modes, anharmonic contributions are evaluated, one strong Fermi resonance is identified and analyzed, and the Duschinsky rotation matrix elements are evaluated for the most strongly affected modes, nu17 and nu18. Several transitions in the oDFB-oDFB van der Waals dimer and oDFB-Ar complex are also assigned in the spectrum. Copyright 1998 Academic Press.