Geminate recombination of hydrated electrons in liquid-to-supercritical water studied by ultrafast time-resolved spectroscopy

Abstract

Hydrated electrons were prepared by multi-photon ionization of neat water with 266 nm light. Using femtosecond pump-probe spectroscopy the dynamics of geminate recombination of the solvated electrons were studied over a wide temperature (296 K ≤T≤ 660 K) and density (0.18 g cm(-3)≤ρ≤ 1.00 g cm(-3)) range extending from the liquid well into the supercritical phase of water. The probability that hydrated electrons escape an initial recombination was found to strongly decrease with increasing temperature. In contrast, the isothermal density-dependence of this survival probability above the critical temperature was surprisingly weak. The peculiar dependence of the initial electron annihilation process on the thermodynamic state variables is discussed in terms of the Onsager model for initial recombination of ion pairs and an effective shielding of the electrostatic interactions of the recombining partners. A finite escape probability for a dielectric constant approaching unity can be interpreted by the existence of a minor fraction of highly mobile electrons created via autoionization.

Direct observation of the collapse of the delocalized excess electron in water

“…Femtosecond pump-probe experiments have been used to unravel details of the equilibration processes following the trapping by the solvent [5][6][7][8][9][10][11][12]. In addition, picosecond measurements on the recombination kinetics of e − aq have described the ejected electron's migration lengths from its geminate partners [13][14][15][16][17][18][19][20][21][22]. These studies evoke a picture of an excess electron that is at first delocalized in the conduction band and then localizes to the hydrated electron e − aq within about a picosecond.…”

Femtosecond two-photon ionization of fluid NH3at 9.3 eV

“…The model simulations can be brought into agreement with the experimental data when a thermalization distance of 6.6 nm is used. This rather large value compared to that obtained for photolytically generated hydrated electrons at the same ionization energy of 9.3 eV [7,8] gives fairly convincing evidence that the ammoniated electrons are initially created as highly mobile charge carriers in the conduction band of the solvent. More detailed experiments targeted at studying the ionization and recombination dynamics as a function of excitation energy are currently in progress in our laboratories.…”

Ultrafast primary processes of the stable neutral organic radical, 1,3,5-triphenylverdazyl, in liquid solution

“…[56][57][58][59] Yet, the pump-probe signals scale linearly with the pump intensity as was discussed in the context of Fig. Since acetonitrile is transparent at the pump wavelength, it must be concluded that solvated electrons can only be generated from the solvent through a multi-photon ionization process.…”