Short-Time Electron Transfer Processes in Ionic Aqueous Solution: Counterion and H/D Isotope Effects on Electron−Atom Pairs Relaxation

Abstract

Counterion and H/D isotope substitution effects on early electron transfer steps in aqueous sodium chloride solution (X2O/NaCl = 55, X = H,D) have been investigated by femtosecond absorption spectroscopy of short-lived electron−chlorine atom pairs in the 18500−8000 cm-1 spectral range. The frequency dependence of the overall signal rise time and early signal decay (geminate recombination process) from 1.77 to 2.29 eV has been analyzed in the framework of inhomogeneous populations of hydrated electron for which the absorption bands overlap in the visible spectral region. By use of a kinetic model, two well-defined electron hydration channels have been discriminated. The spectral signature of a delayed hydration channel (e-hyd‘) is fully developed 4.5 ps after the initial energy deposition in the sample against 1.4 ps for the fastest electron hydration channel (e-hyd). Compared to the absorption band of the hydrated electron ground state (e-hyd), which peaks around 1.7 eV, the e-Hyd‘ population exhibits a spectral blue shift of about 0.15 eV. This electronic ground state is assigned to a polaron-like state, i.e., an electron localized in the vicinity of sodium ion: {e-···Na+}hyd. The time-resolved spectroscopic experiments demonstrate that this electron localization process can be triggered by short-range counterion effects on transient electron−chlorine atom pairs ({Cl:e-}pair:Na+). The existence of selective H/D isotope substitution effects on short-lived electronic configurations of aqueous Cl- (electron−Cl atom)pairs provides direct evidence that solvent cage dynamics around Cl- can differently interfere with multiple electron transfer channels. Contrary to infrared presolvated electron relaxation (p-like excited hydrated electron), early electron−chlorine atom pair relaxation (Cl:e- → Cl-) or 1D geminate recombination of fully hydrated electron with chlorine atom (Cl + e-hyd → Cl-) for which no significant isotope effect is observed, the electron transfer process involving the deactivation of {Cl:e-}pair:(Na+) population is drastically influenced by a change of intramolecular OH/OD vibrational mode frequency. In aqueous NaCl solution, the electron photodetachment from these transient electronic configurations [{Cl:e-}pair:(Na+)] → {Cl}, {e-···Na+}hyd] takes place with a characteristic time of 750 fs in H2O and 980 fs in D2O. The complete buildup of a polaron-like state (second electron hydration channel) is delayed by a factor of 1.6 (4.5 ps in H2O and 7.4 ps in D2O) when the energetic vibrational modes of water molecules (OH vs OD) are decreased by √2. The conjugate effects of very fast responses and slower cooperative motions of water molecules during short-range (electron−atom)pairs−Na+ couplings and a second electron hydration channel are discussed.