In ''outer sphere'' electron transfer reactions, motions of the solvent molecules surrounding the donor and acceptor govern the dynamics of charge flow. Are the relevant solvent motions determined simply by bulk solvent properties such as dielectric constant or viscosity? Or are molecular details, such as the local solvent structure around the donor and acceptor, necessary to understand how solvent motions control charge transfer? In this paper, we address these questions by using ultrafast spectroscopy to study a photoinduced electron transfer reaction with only electronic degrees of freedom: the charge-transfer-to-solvent ͑CTTS͒ reaction of Na Ϫ ͑sodide͒. Photoexcitation of NaϪ places the excited CTTS electron into a solvent-bound excited state; motions of the surrounding solvent molecules in response to this excitation ultimately lead to detachment of the electron. The detached electron can then localize either in an ''immediate'' contact pair ͑in the same cavity as the Na atom͒, which undergoes back electron transfer to regenerate Na Ϫ in ϳ1 ps, or in a ''solvent-separated'' contact pair ͑one solvent shell away from the Na atom͒, which undergoes back electron transfer in tens to hundreds of picoseconds. We present detailed results for the dynamics of each step of this reaction in several solvents: the ethers tetrahydrofuran, diethyl ether and tetrahydropyran and the amine solvent hexamethylphosphoramide ͑HMPA͒. The results are interpreted in terms of a kinetic model that both incorporates spectral shifting of the reaction intermediates due to solvation dynamics and accounts for anisotropic spectral diffusion in polarized transient hole-burning experiments. We find that the rate of CTTS detachment does not correlate simply with any bulk solvent properties, but instead appears to depend on the details of how the solvent packs around the solute. In contrast, the rate for back electron transfer of solvent-separated contact pairs varies inversely with solvent polarity, indicating a barrier to recombination and suggesting that this reaction lies in the Marcus inverted regime. For immediate contact pairs, the rate of recombination varies directly with solvent polarity in the ethers but is slowest in the highly polar solvent HMPA, suggesting that the spatial extent of the solvated electron in each solvent is one of the major factors determining the recombination dynamics. The fact that each step in the reaction varies with solvent in a different way implies that there is not a single set of solvent motions or spectral density that can be used to model all aspects of electron transfer. In addition, all of the results and conclusions in this paper are compared in detail to related work on this system by Ruhman and co-workers; in particular, we assign a fast decay seen in the near-IR to solvation of the CTTS p-to-p excited-state absorption, and polarization differences observed at visible probe wavelengths to anisotropic bleaching of the Na Ϫ CTTS ground state.

Previous analysis of the computer simulation of the relaxation of energetic excess electrons in liquid water (Keszei E.; et al. J. Chem. Phys. 1993, 99, 2004) has led to a detailed molecular level and kinetic picture of this process, including the presence of multiple pathways to the equilibrium ground state. In order to explore the validity of this view, simulation results are directly compared to two available data sets obtained experimentally via ultrafast absorption spectroscopy. The analysis is carried out, first, by convolution of the simulated instantaneous spectral response of the electron with an appropriate instrumental response function. The difference between the resulting data and the reported experimental observations is no larger than the difference between the two experimental data sets. It is further shown by separate analysis that the mechanism of relaxation apparent in the simulation is kinetically consistent with the available experimental data. It is pointed out that a number of available, and apparently different, hypotheses for the sequence of species present during electronic relaxation share key features with this mechanism. Taken together, these considerations support the validity of the microscopic processes evident in simulation and emphasize the limitations inherent in the analysis of the experimentally determined spectral dynamics.

Instantaneous normal mode analysis of hydrated electron solvation dynamicsAqueous solvation dynamics with a quantum mechanical Solute: Computer simulation studies of the photoexcited hydrated electron

%'e present a three-dimensional probabilistic model of particle transport in a medium where the particles sufFer quasielastic collisions. The model accounts for bulk and surface scattering, as well as partial reAections at the boundaries of the medium. %e give analytical and numerical methods for the evaluation of the particle transmission probability in the case of a medium with a planeparallel geometry. The inhuence of the various parameters of the model on this probability is also discussed.

A multiphoton ionization study of neat methanol with subpicosecond 2-eV laser pulses has been previously reported. A hybrid electron solvation mechanism combining both a stepwise transition between two electronsolvent configuration states and a continuous first-order blue shift of the electron absorption spectra was found to closely fit the experimental data. If substantial absorption from free electrons is assumed in this spectral region, we find that another model comprising thermalization prior to a stepwise branching localization without blue-shifting spectra fits equally well at all wavelengths. However, these two models display considerable differences between their respective kinetic parameters, especially the electron localization time. Furthermore, for the nonshifting model, this calculated localization time is considerably longer than that for electron hydration in neat water. We suggest independent studies such as ultrafast electron scavenging experiments before adopting a particular mechanism for electron solvation in methanol.