Knowledge Bank

The transfer and partitioning of electronic excitation energy is a continuing topic of interest to photochemists. However, only recently have there been sufficient studies done in the gas phase where the partitioning of electronic excitation energy of a donor into vibration, rotation, and translation of the collision complex can be observed. From a theoretical point of view little attention has been given to this problem. Data from our laboratory and others on the physical quenching of $O2(1\Sigma g+)$ suggested that an ab initio theory of such quenching processes might be developed within the framework of quantum scattering theory along lines similar to those used in the development of theories for the transfer of vibrational and rotational energy. The theory, which we have developed involves the expression of the short-range, non-adiabatic collision interaction potential in terms of exponentials of the separated variables, i.e., the collision coordinate and the internal rovibronic coordinates of the collision partners. From these, matrix elements expressing the probabilities of the individual transitions, electronic, vibrational; rotational and translational, in each of the collision partners are constructed using, where possible, reasonable wave functions. The resulting expression for the quenching efficiency is consistent with the form of the empirical equation based on experimental data. Furthermore, the calculated quenching efficiencies are in substantial agreement with experimental values for the quenching of $O2(1\Sigma g+)$. The theory should prove useful in the treatment of the quenching of small electronically excited molecules, particularly where the overall transition probability is small, and the energy transfer takes place in the vicinity of hard sphere collisions. In these circumstances, the use of a long-range multipole-multipole perturbation to express intermolecular interactions is no longer valid.