Knowledge Bank

The $S1,T1\leftarrow S0$ phosphorescence excitation spectrum for benzaldehyde was observed in several environments at $4.2\circ K$ and nearly complete vibronic analyses were made. The $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}n,\Pi \ast$ 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, $T1\leftarrow S0$ and $T1\to S01$ behave differently showing strong dependences upon the environment. The vibronic interactions between close lying $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi ,\Pi \ast$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi ,\Pi \ast$ 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. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ M. Koyanagi and L. Goodman, J. Chem. Phys., 55, 2959 (1971).