SHORT-TIME ELECTRON-TRANSFER PROCESSES IN IONIC AQUEOUS-SOLUTION - COUNTERION AND H D ISOTOPE EFFECTS ON ELECTRON-ATOM PAIRS RELAXATION/

Counterion and H/D isotope substitution effects on early electron tran sfer steps in aqueous sodium chloride solution (X(2)O/NaCl = 55, X = H ,D) have been investigated by femtosecond absorption spectroscopy of s hort-lived electron-chlorine atom pairs in the 18500-8000 cm(-1) spect ral range. The frequency dependence of the overall signal rise time an d early signal decay (geminate recombination process) from 1.77 to 2.2 9 eV has been analyzed in the framework of inhomogeneous populations o f hydrated electron for which the absorption bands overlap in the visi ble spectral region. By use of a kinetic model, two well-defined elect ron 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 ab sorption band of the hydrated electron ground state (e(hyd)(-)), which peaks around 1.7 eV, the e(Hyd)(-)' population exhibits a spectral bl ue 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 experi ments demonstrate that this electron localization process can be trigg ered by short-range counterion effects on transient electron-chlorine atom pairs ({Cl:e(-)}(pair):Na+). The existence of selective H/D isoto pe substitution effects on short-lived electronic configurations of aq ueous Cl- (electron-Cl atom)(pairs) provides direct evidence that solv ent cage dynamics around Cl- can differently interfere with multiple e lectron transfer channels. Contrary to infrared presolvated electron r elaxation (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-) f or which no significant isotope effect is observed, the electron trans fer process involving the deactivation of {Cl:e(-)}(pair):(Na+) popula tion is drastically influenced by a change of intramolecular OH/OD vib rational mode frequency. In aqueous NaCl solution, the electron photod etachment from these transient electronic configurations [{Cl:e(-)}(pa ir):(Na+)] --> {Cl}, {e(-)... Na+}(hyd)] takes place with a characteri stic 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 root 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.