Photodissociation of water. I. Electronic structure calculations for the excited states

Results of ab initio calculations for the four lowest excited states of both A′ and A″ have been discussed. In the multireference configuration interaction calculations, a large Rydberg basis set has been used. Three-dimensional potential energy surfaces, and matrix elements of the transition dipole moment between the excited states and the ground X̃ state, and the electronic angular momentum operator between the à state and the B̃ and X̃ states have been presented. The calculations show that above about 124 nm the photodissociation can be well described by the three lowest electronic states, X̃, Ã, and B̃. The ab initio results of matrix elements of the electronic angular momentum operator allow a realistic nonadiabatic treatment of the photodissociation in the B̃ band. At wavelengths smaller than about 124 nm, the dynamics will be more complicated because of the coupling between various electronic states.