Hydrated electron relaxation dynamics

Madsen D, Thomsen CL, Thogersen J, Keiding SR (2000) Temperature dependent relaxation and recombinaison dynamics of the hydrated electron. J Chem Phys 113 1126-1134. 

This narrative echoes the themes addressed in our recent review on the properties of uncommon solvent anions. We do not pretend to be comprehensive or inclusive, as the literature on electron solvation is vast and rapidly expanding. This increase is cnrrently driven by ultrafast laser spectroscopy studies of electron injection and relaxation dynamics (see Chap. 2), and by gas phase studies of anion clusters by photoelectron and IR spectroscopy. Despite the great importance of the solvated/ hydrated electron for radiation chemistry (as this species is a common reducing agent in radiolysis of liquids and solids), pulse radiolysis studies of solvated electrons are becoming less frequent perhaps due to the insufficient time resolution of the method (picoseconds) as compared to state-of-the-art laser studies (time resolution to 5 fs ). The welcome exceptions are the recent spectroscopic and kinetic studies of hydrated electrons in supercriticaF and supercooled water. As the theoretical models for high-temperature hydrated electrons and the reaction mechanisms for these species are still rmder debate, we will exclude such extreme conditions from this review. 

Madsen D., Thomsen C.L., Thogersen J., Keiding S.R., Temperature dependent relaxation and recombination dynamics ofthe hydrated electron, J. Chem. Phys., 2000,113,1126-1134. 

Figure 8. Energy-level diagram of ultrafast electron-transfer processes in aqueous sodium chloride solution. Transitions (eV) correspond to experimental spectroscopic data obtained for different test wavelengths. The abscissa represents the appearance and relaxation dynamics of nonequilibrium electronic populations (CTTS ", CTTS, (e hyd) fCl e pairs). The two channels involved in the formation of an s-like ground hydrated electron state (e hyd, c hyd ) (dso reported in the figure. From these data, it is clear that the high excited CTTS state (CTTS ) corresponds to an ultrashort-lived excited state of aqueous chloride ions preceding an electron photodetachment process.

One technique, Overhauser Dynamic Nuclear Polarization (ODNP), is based on the well-known chemical shift of water in NMR spectra. Ordinarily, the liquid water signal intensity is low however, intensity can be magnified 1000-fold by addition of a nitroxide spin label such as TEMPO. Precession of the unpaired electron in TEMPO at the Larmor frequency results in Nuclear Overhauser-mediated polarization of the protons in water. These get polarized within 15 A of the spin labels and then relax with a relaxation time determined by the local diffusivity, i.e. in bulk water, the diffusivity is high and so relaxation is rapid by contrast, in hydration layers, relaxation takes 10-fold longer than in bulk water. Next, the trick is to covalently tether spin labels to surfaces of interest and measure how relaxation rates in hydration layers change as adhesive proteins approach and locally dehydrate the surfaces. 

With respect to the dynamical properties of the hydrated electron in cluster systems, the first principle dynamics using ab initio molecular dynamics and so on have been extensively applied. [135, 180, 371, 408, 446] They revealed information about the structure and relative stabilities of the isomer clusters. Nonadiabatic dynamics of a solvated electron in various photochemical processes has also been studied experimentally. [62, 123, 294, 329] Rossky and co-workers [327, 468] also studied the relaxation dynamics of excess electrons using quantum molecular dynamics simulation techniques. Here the nonadiabatic interactions were taken into account basically within the scheme of surface hopping technique. [444]... 

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