Ultrafast Reactivity of IR-Excited Electron in Aqueous Ionic Solutions

Abstract

An ultrafast presolvation reaction involving IR-excited hydrated electrons and bivalent metal cations (Cd2+)aq was investigated at the femtosecond time scale. The initial electron photodetachment is triggered by a two-photon UV excitation of aqueous chloride ions (R = [H2O]/[XCl2] = 110). The photodetached electron reaches a IR p-like state (prehydrated electron) with a time constant of 130 fs at 294 K. In the presence of nonreactive divalent alkaline metal cations (X2+ = Mg2+), this transient IR electronic state exhibits a deactivation process toward the ground state of hydrated electron (s state) with a time constant of 300 ± 20 fs. In aqueous CdCl2 solution, an ultrafast IR electron transfer channel (univalent reduction of Cd2+ by IR prehydrated electrons) competes with the p → s transition of trapped electrons (electron solvation process). This presolvation reaction occurs with a characteristic time of 140 ± 20 fs. Within the electron solvation regime, this elementary redox process is totally achieved in less than 1 × 10-12 s and exhibits a probability 10 times higher than the electron hydration channel. The consequence of an early partition between reactive and nonreactive IR electron dynamics on the subpicosecond formation of fully hydrated electron is discussed. The femtosecond IR spectroscopy of excited p-state electron transfer processes in aqueous electrolyte solutions opens a new area of prethermal reactions in environments relevant to mainstream chemistry and biochemistry.