EARLY FORMATION OF ELECTRON RADICAL PAIRS IN A POLAR PROTIC LIQUID - EVIDENCE OF ULTRAFAST CONCERTED ELECTRON PROTON TRANSFERS

Ultrafast reaction dynamics of nonequilibrium electronic states in pure liquid water (D2O vs H2O) have been investigated using ultraviolet femtosecond laser pulses and time-resolved spectroscopy in the visible and near infrared regions. In pure light and heavy water (D2O > 99.95%) at 294 K, ultrashort lived electronic states have been discriminated and assigned to the existence of transient couplings between a subexcitation electron and neoformed prototropic SpeCies (X3O+ and OX, with X = H or D). Contrary to the formation and relaxation of infrared electrons (prehydrated states), the appearance and lifetime of the hybrid electronic states in the near infrared region [X3O+:e-:OX, with X = H or D] is drastically influenced by an H/D isotopic substitution. The formation of electron-radical pairs would be dependent on ultrafast reaction of a molecular cation with surrounding water molecules (X2O+ + X2O --> X3O+, OX). The temporal electron-radical pair formation/ion-molecule reaction ratio is about 2 1/2 times greater in D2O than in H2O. Nonequilibrium electronic configurations are dependent on ultrafast reorganization of water molecules initiated by short-lived prototropic species. These transient structural fluctuations would favor electron localization by self-trapping phenomena. In light water, the present study demonstrates that the deactivation frequency of electron-radical pairs (e-...OH, e-...H3O+)nH2O is similar to the estimate of an H-OH decay channel of excited water molecules (0.29 X 10(13) s-1 vs 0.33 X 10(13) s-1). Additional spectroscopic data obtained in D2O demonstrate that the probability to obtain electron solvation from neutralization of electron-radical pairs is very low at ambient temperature. The existence of a competitive process involving a common precursor of the presolvated electron and electron-radical pairs is unlikely. Transient couplings between hybrid electronic states and a polar solvent would be dependent on the protic character of the water molecules and ultrafast reorganization of the hydrogen-bonded network around short-lived prototropic entities. The photoejection of electrons under the energy band gap of water is discussed considering the existence of concerted electron-proton transfer and the role of dynamical properties of this protic solvent.