FRANCK-CONDON FACTORS AND THE EFFECT OF NORMAL MODE MIXING OH BAND INTENSITIES IN $^{1}B_{2u} \rightarrow ^{1}A_{1g}$ FROM BENZENE VAPOR SVL
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Date
1974
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Ohio State University
Abstract
fluorescence spectra from $^{1}B_{2u}$ benzene vapor have provided quantitative band Intensities within 13 $\nu_{1}(a_{1g}$) progressions from seven emitting levels. The emitting states with $\nu_{1} = 0, 1$, or 2 yield progressions of three types: $1^{0}_{n}$, $1_{n}^{1}$, and $1^{2}_{n}$. The intensity contours of progressions within each type match closely, but progressions of different types are distinctive. The Intensity envelopes of all progressions are consistently replicated by simple overlap calculations using $\Delta (C-C) = 0-034$ {\AA} upon $B_{2}U$ excitation. Bond length changes of 0.032, 0.036, or 0.038 {\AA} yield poorer fits. The intensity ratios of corresponding members of the v1bronically induced $\Delta\nu_{6} = \pm 1$ transitions (for example, 61/2 $1_{n}$ vs 61/0 $1_{n}$ with $n = 0, 1, 2\ldots)$ show a marked and curious dependence upon the $\nu_{1}$ quantum number. No such dependence is predicted by first or second order Herzberg-Teller formulations of vibronic intensity acquisition (including B-O breakdown) In the absence of normal mode mixing. Fisher and Sharf have suggested that the specific level interactions $6_{n}1_{m} \stackrel{\leftrightarrow } 6_{n} + _{2}{1}_{m}-1$ may be significant contributors to the vi ratio dependence, even though the levels are widely separated $(\sim 225 cm^{-1})$. Indeed such interactions with matrix elements of about $15 cm^{-1}$ prove surprisingly successful In accounting for the observed effect, even though they have only a minor effect in setting the intensity contours within a. given progression envelope.
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Author Institution: Department of Chemistry, Indiana University; Department of Chemistry, University of Western Ontario London