  We use transient absorption spectroscopy to monitor the ionization and
dissociation products following two-photon excitation of pure liquid water. The
two decay mechanisms occur with similar yield for an excitation energy of 9.3
eV, whereas the major channel at 8.3 eV is dissociation. The geminate
recombination kinetics of the H and OH fragments, which can be followed in the
transient absorption probed at 267 nm, provide a window on the dissociation
dynamics at the lower excitation energy. Modeling the OH geminate recombination
indicates that the dissociating H atoms have enough kinetic energy to escape
the solvent cage and one or two additional solvent shells. The average initial
separation of H and OH fragments is 0.7+-0.2 nm. Our observation suggests that
the hydrogen bonding environment does not prevent direct dissociation of an O-H
bond in the excited state. We discuss the implications of our measurement for
the excited state dynamics of liquid water and explore the role of those
dynamics in the ionization mechanism at low excitation energies.
