Multiphoton Ionization of Liquid Water with 3.0−5.0 eV Photons

We report a picosecond laser study of the transient absorption of hydrated electrons generated by the 3−5 eV multiphoton ionization of liquid water. The geminate kinetics indicate that eaq - is produced by at least three different mechanisms over this energy range. Power dependence of the signal amplitude shows a two-photon threshold for 4.0 eV excitation and a three-photon threshold absorption at 3.47 eV, consistent with two- or three-photon excitation of the Ã(1B1) lowest excited state. For (three-photon) excitation in the range 3.02−3.47 eV very little (≤15%) geminate recombination is observed while for the (two-photon) excitation at shorter wavelengths significant recombination (≥55%) is observed. In the region of 3.85−4.54 eV, photon-energy-independent kinetics indicate that eaq - is produced via two-photon excitation of the à state followed by an ionization process in which the electrons do not obtain any excess kinetic energy. For photon energies in the range of 4.75−5.05 eV, the escape fraction increases slightly, consistent with two-photon excitation of higher energy states. Simulation with a diffusion model shows that the electron is ejected at least 25 Å farther into the bulk for the 3.02−3.47 eV photon energies relative to two-photon ionization in the 3.67−5.0 eV range. We conclude that the larger distances result from a (3 + 1)-photon resonance-enhanced multiphoton ionization (REMPI) process, made possible by visible/near-UV absorption of the water excited states. Possible mechanisms of the water ionization are discussed. A new mechanism is proposed to explain the production of solvated electrons from excitation of the Ã(1B1) state of liquid water, well below the Born−Oppenheimer ionization threshold. On the basis of the gas phase properties of this state, we assume a direct dissociation to give OH radical and H atoms, with the excess energy almost entirely transferred to kinetic energy of the H atoms: H2O* → OH + H(hot). It is proposed that the hot H atoms immediately react with an adjacent water molecule to form hydronium ion and a solvated electron, in a process analogous to the thermal reaction of H atoms with water at elevated temperatures: H(hot) + H2O → H3O+ + eaq -.