VIBRATIONAL ANALYSIS OF LOW-LYING K-SHELL EXCITED-STATES IN H2O AND D2O CALCULATED BY AN AB-INITIO MULTIREFERENCE CI-METHOD

The potential energy surfaces of low-lying K-shell excited states and the K-shell ionic state of water are calculated along the three vibrational coordinates around the equilibrium geometry of the ground state. The quantum chemical ab initio method employed is of the multireference configuration interaction type and accounts for a large amount of electron correlation. A vibrational analysis is carried out on the potential curves within the Franck-Condon approximation assuming independent modes; the electrons transition moments are calculated for all transitions. The first two K-shell excited states are found to possess considerable valence character, all higher states are predominantly of Rydberg (p and d) type. The vibrational analysis of the various transitions suggests the explanation for the large line widths in the experimental K-shell photoabsorption spectrum: the vibrational substructure with energy levels closer to each other than the natural line width of the resonant K-shell excited state causes the large line width of the first band; the second band obtains its broadening from photodissociation. A vibrationally resolved K-shell photoabsorption spectrum of D2O is predicted.