Some evidence of ultrafast H 2O +-water molecule reaction in femtosecond photoionization of pure liquid water: Influence on geminate pair recombination dynamics

Abstract

The elucidation of detailed mechanisms of primary events in molecular dynamics, charge transfer or reaction dynamics have been made possible by advances in spectroscopic techniques using ultrashort laser pulse generation. In this paper we will center on femtosecond investigations of the very primary processes occurring in pure liquid water following a photoionization of solvent molecules by ultraviolet femtosecond pulses. We have observed in the near ultraviolet region (460, 410 nm), an instantaneous transient absorption with ultrashort lifetime which appears during the initial energy deposition. This induced absorption rises within the pulse, i.e. in less than 100 fs and faster than the precursor of the fully hydrated electron. Its relaxation can be described by a monoexponential law. This ultrashort transient absorption is tentatively assigned to the water cation H 2O +. The relaxation would then correspond to the ion-molecule reaction H 2O + + H 2O → H 3O + + OH for which the cleavage rate constant is measured to be 10 13 s −1 at 294 K. An H/D isotope effect on the dynamics of the ion-water molecule reaction has been observed. These results are analyzed considering the dynamical properties of the protic solvent. The influence of the local dynamical molecular structure of the fluid on the primary reactions involving an ultrafast geminate recombination (e − hyd…X 3O +; e − hyd…OX with X = H or D) will be discussed.