Photodissociation dynamics of H2O and D2O in the first absorption band: A complete <i>a</i> <i>b</i> <i>i</i> <i>n</i> <i>i</i> <i>t</i> <i>i</i> <i>o</i> treatment

Abstract

We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the à state and a calculated SCF dipole moment function for the X̃→à transition. The dynamical calculations are performed within the infinite-order-sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of ‘‘vibrational’’ structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the à state potential energy surface and details of the dissociation dynamics. Vibrational excitation of the OH and OD products is significantly wavelength dependent. The distribution of the three lowest vibrational states at 157 nm is in good accord with recent LIF measurements. Particular attention is paid to the sensitivity of the final results with respect to the coordinate dependence of the transition dipole function, the parent nuclear wave function and the excited state potential energy surface.