Communicating states model for intersystems crossing of isolated molecules

The excess energy dependence of the intersystems crossing rate constant for large isolated molecules is studied assuming rapid redistribution of vibrational energy after excitation. Exponential increase of the rate constant is predicted for large separation between the initial singlet and the final triplet states. For small energy separation the rate stays almost constant provided the transition goes directly via spin-orbit coupling. It increases linearly if spin-orbit vibronic coupling is involved.