SINGLE VIBRONIC LEVEL FLUORESCENCE SPECTRA FROM THE $S_{1} (^{1}B_{2u})$ STATE OF DEUTERATED p-DIFLUOROBENZENE $[p-DFB(d_{4})]$ VAPOR.

Abstract

$^{1}B_{2u} \to ^{1}A_{1g}$ fluorescence spectra have been obtained after pumping each of 6 absorption bands in pDFB(d). Transitions in these spectra can be assigned based on assignments of ground state vibrational frequencies of $pDFB(d_{4})$ [1], previous work on SVLF of $pDFB(h_{4}) [2]$, and a simple qualitative picture of normal mode vibrations. Assignments of the low energy and of $^{1}B_{2u} \to ^{1}A_{1g}$ absorption spectrum are secured by SVL fluorescence spectroscopy. Two low frequency $S_{1}$ modes undergo large frequency changes upon deuteration. The out-of-plane ring distortion $\nu_{8}$ drops from $175 cm^{-1}$ in $-h_{4}$ to $151 cm^{-1}$ in $-d_{4}$. As a result the level of the lowest frequency $S_{1}$ mode $\nu_{30}^{\prime} - 118 cm^{-1}$ in $-d_{4}$ is very near that of $\nu_{8}$. The mode $\nu_{4}^{\prime}$ drops from 1016 to $819 cm^{-1}$ where it almost matches the frequency of the most prominent progression forming mode $\nu_{5}^{\prime}$. As a consequence the $-d_{4}$ spectra in absorption and (as Dunn et al. [1] showed) in emission as well contain and extra set of progressions not present in the $-h_{4}$ molecule. These changes open new opportunities for $S_{1}$ energy transfer studies. $^{1.}$ A.F. Childs, T.M. Dunn, A.H. Francis, J. Mol. Spectros., 102, 56 (1983). $^{2.}$ R.A. Coveleskie and C.S. Parmenter, J. Mol. Spectros., 86. (1981).