MULTIPHOTON IONIZATION OF LIQUID WATER WITH 3.0-5.0 EV PHOTONS

We report a picosecond laser study of the transient absorption of hydr ated electrons generated by the 3-5 eV multiphoton ionization of liqui d water, The geminate kinetics indicate that e(aq)(-) is produced by a t least three different mechanisms over this energy range. Power depen dence of the signal amplitude shows a two-photon threshold for 4.0 eV excitation and a three-photon threshold absorption at 3.37 eV, consist ent with two- or three-photon excitation of the (A) over tilde(B-1(1)) lowest excited state, For (three-photon) excitation in the range 3.02 -3.47 eV very little (less than or equal to 15%) geminate recombinatio n is observed while for the (two-photon) excitation at shorter wavelen gths significant recombination (greater than or equal to 55%) is obser ved. In the region of 3.85-4.54 eV, photon-energy-independent kinetics indicate that e(aq)(-) is produced via two-photon excitation of the ( A) over tilde state followed by an ionization process in which the ele ctrons 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. Simulat ion with a diffusion model shows that the electron is ejected at least 25 Angstrom farther into the bulk for the 3.02-3.47 eV photon energie s relative to two-photon ionization in the 3.67-5.0 eV range. We concl ude that the larger distances result from a (3 + 1)-photon resonance-e nhanced multiphoton ionization (REMPT) process, made possible by visib le/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 ( A) over tilde(B-1(1)) state of liquid water, well below the Born-Oppen heimer 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 t hermal reaction of H atoms with water at elevated temperatures: H(hot) + H2O --> H3O+ + e(aq)(-).