$S_{1}, T_{1} {\leftarrow} S_{0}$ PHOSPHORESCENCE EXCITATION SPECTRUM OF BENZALDEHYDE

Abstract

The $S_{1}, T_{1} {\leftarrow} S_{0}$ phosphorescence excitation spectrum for benzaldehyde was observed in several environments at $4.2^{\circ} K$ and nearly complete vibronic analyses were made. The $^{1}n, {\Pi}^{*}$ state couples strongly with several ring out-of-plane modes such as ${\nu_{11}}, {\nu_{17b}}, {\nu_{10a}},{\cdots}$ The absence of any two or more quanta of non-totally symmetric modes and any combination bands among them support a planar CHO-group --- benzene ring geometry. On the other hand, $T_{1} {\leftarrow} S_{0}$ and $T_{1} \rightarrow S_{0}^{1}$ behave differently showing strong dependences upon the environment. The vibronic interactions between close lying $^{3}{\Pi}, {\Pi^{*}}$ and $^{3}{\Pi},{\Pi^{*}}$ states can be discussed in terms of the energy gap showing the importance of the dynamical pseudo Jahn-Teller problem in the T --- S spectrum of benzaldehyde. This research was supported by National Science Foundation Grant GP 6301X. $^{1}$ M. Koyanagi and L. Goodman, J. Chem. Phys., 55, 2959 (1971).