Rotational state distributions in the photolysis of water: Influence of the potential anisotropy

We report a detailed investigation of rotationally inelastic effects in the photodissociation of water in the first absorption band using a recently calculated ab initio potential energy surface of the à 1B1 state. Although the excited state potential anisotropy is large it has only very weak influence on the rotational state distributions which thus simply reflect the angular behavior of the ground state wave function of the parent molecule. The reason is that both potential energy surfaces have roughly the same equilibrium angle. Strong inelastic effects are observed, however, for dissociation out of excited bent states because the corresponding ground state wave function extends over a considerably wider angular range and thus the more anisotropic regions of the excited state potential are probed. Calculations are performed on three levels of accuracy for the continuum wave function: close-coupling, coupled-states (CS), and infinite-order-sudden (IOS) approximation. The ground state wave function is treated numerically exactly. The CS approximation is found to be extremely reliable for those cases studied in this work. The accuracy of the IOS approximation depends very much on the region of orientation angle probed by the ground state and therefore a general conclusion is not possible. Finally, OH rotational state distributions obtained from the dissociation of water and from H–OH scattering at equivalent translational energies are compared and found to be extremely different.