Direct observation of charge-transfer-to-solvent (CTTS) reactions: Ultrafast dynamics of the photoexcited alkali metal anion sodide (Na-)

Charge-transfer-to-solvent (CTTS) transitions have been the subject of a gr eat deal of interest recently because they represent the simplest possible charge transfer reaction: The CTTS electron transfer from an atomic ion to a cavity in the surrounding solvent involves only electronic degrees of fre edom. Most of the work in this area, both experimental and theoretical, has focused on aqueous halides. Experimentally, however, halides make a challe nging choice for studying the CTTS phenomenon because the relevant spectros copic transitions are deep in the UV and because the charge-transfer dynami cs can be monitored only indirectly through the appearance of the solvated electron. In this paper, we show that these difficulties can be overcome by taking advantage of the CTTS transitions in solutions of alkali metal anio ns, in particular, the near-IR CTTS band of sodide (Na-) in tetrahydrofuran (THF). Using femtosecond pump-probe techniques, we have been able to spect roscopically separate and identify transient absorption contributions not o nly from the solvated electron, but also from the bleaching dynamics of the Na- ground state and from the absorption of the neutral sodium atom. Perha ps most importantly, we also have been able to directly observe the decay o f the Na-* excited CTTS state, providing the first direct measure of the el ectron transfer rate for any CTTS system. Taken together, the data at a var iety of pump and probe wavelengths provide a direct test for several kineti c models of the CTTS process. The model which best fits the data assumes a delayed ejection of the electron from the CTTS excited state in similar to 700 fs. Once ejected, a fraction of the electrons, which remain localized i n the vicinity of the neutral sodium parent atom, recombine on a similar to 1.5-ps time scale. The fraction of electrons that recombine depends sensit ively on the choice of excitation wavelength, suggesting multiple pathways for charge transfer. The spectrum of the neutral sodium atom, which appears on the similar to 700-fs charge-transfer time scale, matches well with a s pecies of stochiometry (Na+, e(-)) that has been identified in the radiatio n chemistry literature. All the results are compared to previous studies of both CTTS dynamics and alkali metal solutions, and the implications for ch arge transfer are discussed. (C) 2000 American Institute of Physics. [S0021 -9606(00)50221-9].