Excitation-energy dependence of the mechanism for two-photon ionization of liquid H2O and D2O from 8.3to12.4eV

Abstract

Transient absorption measurements monitor the geminate recombination kinetics of solvated electrons following two-photonionization of liquid water at several excitation energies in the range from 8.3to12.4eV. Modeling the kinetics of the electron reveals its average ejection length from the hydronium ion and hydroxyl radical counterparts and thus provides insight into the ionization mechanism. The electron ejection length increases monotonically from roughly 0.9nm at 8.3eV to nearly 4nm at 12.4eV, with the increase taking place most rapidly above 9.5eV. We connect our results with recent advances in the understanding of the electronic structure of liquid water and discuss the nature of the ionization mechanism as a function of excitation energy. The isotope dependence of the electron ejection length provides additional information about the ionization mechanism. The electron ejection length has a similar energy dependence for two-photonionization of liquid D(2)O, but is consistently shorter than in H(2)O by about 0.3nm across the wide range of excitation energies studied.