THE ELECTRONIC SPECTROSCOPY OF JET-COOLED M-DIFLUOROBENZENE

The S-1(B-1(2))-S-0((1)A(1)) electronic transition of m-difluorobenzen e cooled in a supersonic free jet has been investigated in detail by u se of laser induced fluorescence excitation and dispersed fluorescence spectroscopy. By analysis of over 50 vibronic transitions we have ass igned 16 new S-1 vibrational frequencies, and have confirmed seven pre vious assignments. This advances the total number of known S, fundamen tal vibrational frequencies to 23 out of the possible 30. The assignme nts of two SO frequencies have also been somewhat modified. Two differ ent types of symmetry-forbidden transitions were identified: transitio ns involving one-quantum changes in b(2) modes were found; and transit ions involving combinations of b(1) vibrations were observed to be unu sually active in the vibronic spectrum, The observation of these forbi dden transitions in both the fluorescence excitation and dispersed flu orescence spectra are explored in terms of first- and second-order vib ronic coupling. (C) 1995 American Institute of Physics.