Mechanisms of the ultrafast production and recombination of solvated electrons in weakly polar fluids: Comparison of multiphoton ionization and detachment via the charge-transfer-to-solvent transition of Na− in THF

Abstract

The processes by which solvated electrons are generated and undergo recombination are of great interest in condensed phase physical chemistry because of their relevance to both electron transfer reactions and radiation chemistry. Although most of the work in this area has focused on aqueous systems, many outstanding questions remain, especially concerning the nature of these processes in low polarity solvents where the solvated electron has a fundamentally different structure. In this paper, we use femtosecond spectroscopic techniques to explore the dynamics of solvated electrons in tetrahydrofuran (THF) that are produced in two different ways: ejection by multiphoton ionization of the neat solvent, and detachment via the charge-transfer-to-solvent (CTTS) transition of sodide (Na−). Following multiphoton ionization of the solvent, the recombination of solvated electrons can be well described by a simple model that assumes electrons are first ejected to a given thermalization distance and then move diffusively in the presence of the Coulombic attraction with their geminate cation. The short-time transient absorption dynamics of the THF radical cation in the visible region of the spectrum do not match the kinetics of the solvated electron probed at ∼2 μm, indicating that caution is warranted when drawing conclusions about recombination based only on the dynamics of the solvent cation absorption. With ∼4 eV of excess energy, geminate recombination takes place on the hundreds of picoseconds time scale, corresponding to thermalization distances ⩾40 Å. The recombination of solvated electrons ejected via CTTS detachment of Na−, on the other hand, takes place on two distinct time scales of ⩽2 and ∼200 ps with kinetics that cannot be adequately fit by simple diffusive models. The fraction of electrons that undergo the fast recombination process decreases with increasing excitation energy or intensity. These facts lead us to conclude that electrons localize in the vicinity of their geminate Na atom partners, producing either directly overlapping or solvent-separated contact pairs. The distinct recombination kinetics for the two separate electron generation processes serve to emphasize the differences between them: multiphoton ionization produces a delocalized electron whose wave function samples the structure of the equilibrium fluid before undergoing localization, while CTTS is an electron transfer reaction with dynamics controlled by the motions of solvent molecules adjacent to the parent ion. All the results are compared to recent experiments on the photodetachment of electrons in aqueous systems where contact pairs are also thought to be important, allowing us to develop a qualitative picture for the mechanisms of electron generation and recombination in different solvent environments.