Isotope effect on ultrafast charge-transfer-to-solvent reaction from I− to water in aqueous NaI solution

Abstract

A charge-transfer-to-solvent (CTTS) reaction from a photoexcited iodine atomic anion, I⁻ (aq), in bulk water (H₂O and D₂O) was studied by time-resolved photoelectron spectroscopy using a liquid beam (microjet) of aqueous NaI solution. The ²P_3/2 CTTS state of I⁻ (aq) was excited by a 226 nm femtosecond laser pulse and the evolution of the nonstationary electronic state was probed using another ultraviolet femtosecond laser pulse. Global fitting of the observed time-dependent photoelectron kinetic energy distributions provided the time constants of individual reaction steps and the photoelectron spectra from the CTTS state, a contact pair, the solvent-separated state, and a hydrated electron. Most of the elementary reaction steps revealed a strong deuterium isotope effect, indicating coupling of the electron dynamics and the hydrogen atomic motion of solvent water. However, nondiffusive geminate recombination processes from the CTTS state and a contact pair were almost insensitive to deuteration. Consequently, geminate recombination processes from the CTTS state and a contact pair occurs more efficiently in D₂O, because the response of water is decelerated in D₂O. In contrast, the recombination process from the solvent-separated state in the final step of the CTTS reaction is less efficient in D₂O, presumably due to the smaller zero point energy.