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

Abstract

Charge-transfer-to-solvent (CTTS) transitions have been the subject of a great 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 freedom. Most of the work in this area, both experimental and theoretical, has focused on aqueous halides. Experimentally, however, halides make a challenging choice for studying the CTTS phenomenon because the relevant spectroscopic transitions are deep in the UV and because the charge-transfer dynamics 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 anions, in particular, the near-IR CTTS band of sodide (Na−) in tetrahydrofuran (THF). Using femtosecond pump–probe techniques, we have been able to spectroscopically separate and identify transient absorption contributions not only from the solvated electron, but also from the bleaching dynamics of the Na− ground state and from the absorption of the neutral sodium atom. Perhaps most importantly, we also have been able to directly observe the decay of the Na−* excited CTTS state, providing the first direct measure of the electron transfer rate for any CTTS system. Taken together, the data at a variety of pump and probe wavelengths provide a direct test for several kinetic 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 ∼700 fs. Once ejected, a fraction of the electrons, which remain localized in the vicinity of the neutral sodium parent atom, recombine on a ∼1.5-ps time scale. The fraction of electrons that recombine depends sensitively on the choice of excitation wavelength, suggesting multiple pathways for charge transfer. The spectrum of the neutral sodium atom, which appears on the ∼700-fs charge-transfer time scale, matches well with a species of stochiometry (Na+, e−) that has been identified in the radiation chemistry literature. All the results are compared to previous studies of both CTTS dynamics and alkali metal solutions, and the implications for charge transfer are discussed.