THE S-1((1)A(1))S-0((1)A(1)) ELECTRONIC-TRANSITION OF JET-COOLED O-DIFLUOROBENZENE

A detailed study of the S-1((1)A(1))-S-0((1)A(1)) transition of jet-co oled o-difluorobenzene has been completed using the two techniques of laser-induced fluorescence excitation and dispersed, single vibronic l evel fluorescence spectroscopy. Analysis of over 60 dispersed fluoresc ence spectra resulted in both the assignment of 22 excited state vibra tional frequencies and the confirmation of 23 ground state frequencies . The spectrum is dominated by Franck-Condon activity in totally symme tric vibrations with long progressions in the ring-breathing mode, nu( 9). By analogy with benzene and the para- and meta-substituted isomers , two vibronic coupling mechanisms are postulated to be responsible fo r the wealth of weaker symmetry-forbidden structure that has been obse rved. Single quantum changes in b(2) vibrations are postulated to appe ar due to first order vibronic coupling to a higher lying B-2 electron ic state. Combinations of b(1) and a(2) modes are postulated to appear from second order vibronic coupling to an A, electronic state. This s econd order coupling causes a pronounced Duschinsky mixing among excit ed state b(1) and a(2) modes with respect to their ground state counte rparts. Franck-Condon factors are calculated for the a, progression-fo rming modes, anharmonic contributions are evaluated, one strong Fermi resonance is identified and analyzed, and the Duschinsky rotation matr ix elements are evaluated for the most strongly affected modes, nu(17) and nu(18). Several transitions in the oDFB-oDFB van der Waals dimer and oDFB-Ar complex are also assigned in the spectrum, (C) 1998 Academ ic Press.