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

The processes by which solvated electrons are generated and undergo recombi nation are of great interest in condensed phase physical chemistry because of their relevance to both electron transfer reactions and radiation chemis try. Although most of the work in this area has focused on aqueous systems, many outstanding questions remain, especially concerning the nature of the se processes in low polarity solvents where the solvated electron has a fun damentally different structure. In this paper, we use femtosecond spectrosc opic techniques to explore the dynamics of solvated electrons in tetrahydro furan (THF) that are produced in two different ways: ejection by multiphoto n ionization of the neat solvent, and detachment via the charge-transfer-to -solvent (CTTS) transition of sodide (Na-). Following multiphoton ionizatio n of the solvent, the recombination of solvated electrons can be well descr ibed by a simple model that assumes electrons are first ejected to a given thermalization distance and then move diffusively in the presence of the Co ulombic attraction with their geminate cation. The short-time transient abs orption dynamics of the THF radical cation in the visible region of the spe ctrum do not match the kinetics of the solvated electron probed at similar to2 mum, indicating that caution is warranted when drawing conclusions abou t recombination based only on the dynamics of the solvent cation absorption . With similar to4 eV of excess energy, geminate recombination takes place on the hundreds of picoseconds time scale, corresponding to thermalization distances greater than or equal to 40 Angstrom. The recombination of solvat ed electrons ejected via CTTS detachment of Na-, on the other hand, takes p lace on two distinct time scales of less than or equal to2 and similar to 2 00 ps with kinetics that cannot be adequately fit by simple diffusive model s. The fraction of electrons that undergo the fast recombination process de creases with increasing excitation energy or intensity. These facts lead us to conclude that electrons localize in the vicinity of their geminate Na a tom partners, producing either directly overlapping or solvent-separated co ntact pairs. The distinct recombination kinetics for the two separate elect ron generation processes serve to emphasize the differences between them: m ultiphoton ionization produces a delocalized electron whose wave function s amples the structure of the equilibrium fluid before undergoing localizatio n, while CTTS is an electron transfer reaction with dynamics controlled by the motions of solvent molecules adjacent to the parent ion. All the result s are compared to recent experiments on the photodetachment of electrons in aqueous systems where contact pairs are also thought to be important, allo wing us to develop a qualitative picture for the mechanisms of electron gen eration and recombination in different solvent environments. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)52048-0].