Perturbative-polarization-propagator study of the photoionization cross section of the water molecule

Perturbative-polarization-propagator methods of different orders employing large Gaussian basis sets are used to calculate discrete pseudostate representations of the oscillator strength distribution of the water molecule, from which continuous total and partial photoionization cross sections are then determined by moment-theoretical methods. The cross sections obtained in the first-order and second-order polarization-propagator approximation are in very good agreement with recent photoionization measurements. In particular, the present results agree much better with experiment than previous calculations that use the static-exchange approximation. The main reason for this is that first- and higher-order polarization-propagator approximations obey certain sum rules for the oscillator strength distribution, whereas the static-exchange approximation does not. This makes propagator methods particularly well suited for use in connection with moment theory. Disagreement of previously reported cross sections of the water molecule with experiment, such as the overestimation of the 1$b_2$ partial cross section, have been partially attributed to the neglect of channel coupling in these calculations. Therefore, in the present work, the separated-channel approximation is avoided and all possible couplings between ionizations from all valence-shell molecular orbitals are allowed for.