Dynamics of Electron Solvation in Molecular Clusters

Abstract

Solvated electrons, and hydrated electrons in particular, are important species in condensed phase chemistry, physics, and biology. Many studies have examined the formation mechanism, reactivity, spectroscopy, and dynamics of electrons in aqueous solution and other solvents, leading to a fundamental understanding of the electron-solvent interaction. However, key aspects of solvated electrons remain controversial, and the interaction between hydrated electrons and water is of central interest. For example, although researchers generally accept that hydrated electrons, eaq-, occupy solvent cavities, another picture suggests that the electron resides in a diffuse orbital localized on a H3O radical. In addition, researchers have proposed two physically distinct models for the relaxation mechanism when the electron is excited. These models, formulated to interpret condensed phase experiments, have markedly different timescales for the internal conversion from the excited p state to the ground s state.Studies of negatively charged clusters, such as (H2O)n- and I-(H2O)n, offer a complementary perspective for studying aqueous electron solvation. In this Account, we use time-resolved photoelectron spectroscopy (TRPES), a femtosecond pump-probe technique in which mass-selected anions are electronically excited and then photodetached at a series of delay times, to focus on time-resolved dynamics in these and similar species. In (H2O)n-,TRPES gives evidence for ultrafast internal conversion in clusters up to n=100. Extrapolation of these results yields a p-state lifetime of 50 fs in the bulk limit. This is in good agreement with the nonadiabatic solvation model, one of the models proposed for relaxation of eaq-. Similarly, experiments on (MeOH)n- up to n=450 give an extrapolated p-state lifetime of 150fs. TRPES investigations of I-(H2O)n and I-(CH3CN)n probe a different aspect of electron solvation dynamics. In these experiments,an ultraviolet pump pulse excites the cluster analog of the charge-transfer-to-solvent (CTTS) band, ejecting an electron from the iodide into the solvent network. The probe pulse then monitors the solvent response to this excess electron,specifically its stabilization via solvent rearrangement. In I-(H2O)n, the iodide sits outside the solvent network, as does the excess electron initially formed by CTTS excitation. However, the iodide in I-(CH3CN)n is internally solvated, and both experimental and theoretical evidence indicate that electrons in (CH3CN)n- are internally solvated. Hence, these experiments reflect the complex dynamics that ensue when the electron is photo detached within a highly confined solvent cavity.