ELECTRONIC ENERGY TRANSFER BETWEEN LIKE POLYATOMIC MOLECULES IN THE VAPOR

Author Institution: Department of Chemistry, Indiana UniversityThe 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