Ab initio characterization of possible dissociation pathways for multiphoton ionization of the water dimer in supersonic free jets

Abstract

Unrestricted Hartree-Fock and second-order Møller-Plesset ab initio calculations using several split-valence basis sets complemented with polarization and diffuse functions were done on H 2O + and H 2O · H 2O +. We studied the performance of the level of calculation and the basis sets used in predicting the structure and energy ordering of the ground and excited states of the water radical cation. Very good agreement was found for the three states (ground 2B 1 and excited 2A 1 and 2B 1); deviations were less than 0.3 eV in all cases. The calculated optimized structures were in close agreement with experiment. We also made calculations on the two lowest-lying A' and A″ states of the water dimer radical cation at the optimized geometry of neutral (H 2O) 2 and at its own intermolecular optimized structure within each basis set. The results predict a charge transfer complex for the A' state of the water dimer radical cation and that if no proton transfer is allowed both states require more or less the same excitation energy. This suggests the possibility of two potential energy hypersurfaces being close enough for a large coupling to take place. Comparison with previous theoretical studies shows that inclusion of dynamic correlation through perturbation theory is sufficient to obtain as good an agreement with experiment as that obtained with the best configuration interaction calculations done so far.