The effects of local modes and Morse oscillator potentials on the nonradiative ‘‘communicating states’’ rate calculations in benzene

By means of ‘‘communicating states’’ model calculations it is shown that the sharp increase of nonradiative transitions with excess vibrational energy in the first excited electronic state S1(B2u) of benzene has to be interpreted as an internal conversion process. Intersystem crossing S1uT1 cannot explain the disappearance of fluorescence quantum yields. This remains true even if a Morse potential for the ν2(a1g) vibration or for all C–H stretching modes in a local mode description is assumed. The sharp increase of the rate in the ‘‘channel-three’’ excess energy region is still governed by the ν4(b2g) out-of-plane mode. In addition, the excess energy dependence of the nonradiative rates is found to be extremely nonexponential.