Electronic energy transfer between single vibronic levels of C6H6 and C6D6 vapors

The collisional transfer of electronic energy from the zero point level of S1, C6D6, to specific levels in S1, C6H6, is studied by observation of resolved fluorescence from mixtures of C6H6 and C6D6 vapors. Strong vibronic effects and high efficiencies are observed. The transfer populates predominately the C6H6 zero point level (ΔE=−203 cm−1) in spite of the near resonance (ΔE=+37 cm−1) of the C6H6 fundamental ν′16 with the initial C6D6 level. The respective cross sections for populating the S1 zero point level and the ν16 fundamental are about 0.6 and 0.1–0.2 of gas kinetic values. Energy transfer by the Förster dipole mechanism cannot account for these efficiencies. Among the possible routes to S1 zero point C6H6 formation in collisions between excited C6D6 and thermal ground state C6H6 molecules, the dominant path is the Δν=0 route C6D6(S1;00) +C6H6(S0;00) →C6D6(S0;00)+C6H6(S1;00). The cross section for this particular state-to-state reaction is about equivalent to the gas kinetic value.