Photodissociation of water in the first absorption band: A prototype for dissociation on a repulsive potential energy surface

The photodissociation of water in the first absorption band, H₂O(X̃) + (Latin small letter h with stroke)ω → H₂O(ùB₁) → H(²S) + OH(²Π), is a prototype of fast and direct bond rupture in an excited electronic state. It has been investigated from several perspectives - absorption spectrum, final state distributions of the products, dissociation of vibrationally excited states, isotope effects, and emission spectroscopy. The availability of a calculated potential energy surface for the à state, including all three internal degrees of freedom, allows comparison of all experimental data with the results of rigorous quantum mechanical calculations without any fitting parameters or simplifying model assumptions. As the result of the confluence of ab initio electronic structure theory, dynamical theory, and experiment, water is probably the best studied and best understood polyatomic photodissociation system. In this article we review the joint experimental and theoretical advances which make water a unique system for studying molecular dynamics in excited electronic states. We focus our attention especially on the interrelation between the various perspectives and the correlation with the characteristic features of the upper-state potential energy surface.