ELECTRONIC ENERGY TRANSFER BETWEEN LIKE POLYATOMIC MOLECULES IN THE VAPOR

Abstract

The switching of electronic energy between (almost) like molecules in collision has been studied under conditions where the transfer (or non-transfer) of vibrational quanta can also be followed. A low pressure of $C_{6}D_{6}$, can be pumped to its $S_{1}(0^{0})$ level in the presence of a low pressure of $C_{6}H_{6}$ without light absorption by the latter. If this is done, fluorescence is observed from the $S_{1}$ states of both species as a result of collisional E-E transfer. The transfer is efficient; the cross section is about half of that for a hard sphere collision. The fluorescence spectra are sufficiently open so that the relative populations of specific vibrational levels in the newly excited $C_{6}H_{6}$ molecules can be followed. Excitation in two levels of $C_{6}H_{6}$ can be seen clearly as a result of the energy transfers $$ \begin{array}{l@{\qquad}l} C_{6}D_{6}^{*}(0^{0}) + C_{6}H_{6} \rightarrow C_{6}D_{6} + C_{6}H_{6}\ast(0^{0}) &\Delta E = -203 { cm}^{-1}\\ C_{6}D_{6}^{*}(0^{0}) + C_{6}H_{6} \rightarrow C_{6}D_{6} + C_{6}H_{6}\ast(16^{1}) &\Delta E = 34 { cm}^{-1} \end{array}$$ The cross section for the first channel exceeds that for the other by more than a factor of five, even though not favored by energy.