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

Results of ab initio calculations for the four lowest excited states of both $A^{\prime }$ 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 $\mathrm{X̃}$ state, and the electronic angular momentum operator between the $Ã$ state and the $\mathrm{B̃}$ and $\mathrm{X̃}$ states have been presented. The calculations show that above about 124 nm the photodissociation can be well described by the three lowest electronic states, $\mathrm{X̃,}$ $\mathrm{Ã,}$ and $\mathrm{B̃.}$ The ab initio results of matrix elements of the electronic angular momentum operator allow a realistic nonadiabatic treatment of the photodissociation in the $\mathrm{B̃}$ band. At wavelengths smaller than about 124 nm, the dynamics will be more complicated because of the coupling between various electronic states.