FEMTOSECOND SPECTROSCOPY OF AN ENCOUNTER PAIR RADICAL (H3O+ ... E-)HYD IN CONCENTRATED AQUEOUS-SOLUTION

The spectral properties and the lifetime of an encounter pair (H3O+:e-)hydproduced by femtosecond ultraviolet photolysis of concentrated hydrochloric acid aqueous solutions have been investigated at 294 K. For 11 M HC1 with R = [H2O]/[HCl] = 5, the transient absorption spectrum obtained following the electron detachment from chloride ion consists of three bands (λ(1)max1 eV, λ(2)max= 1.35 eV, λ(3)max= 1.72 eV) that are respectively assigned to a prehydrated electron (e-prehyd), an encounter electron-ion pair (H3O+:e-)hyd, and a fully relaxed electron (e-hyd). The encounter pair involving the hydronium ion and an epithermal electron (H3O+:e-)hydis characterized by a near-infrared absorption band that peaks around 920 nm. This transient species appears following a pseudo-first-order kinetics with a time constant of 250 fs. The following single exponential relaxation of this induced absorption corresponds to the deactivation of the transient encounter pair. The cleavage rate constant [(H3O+:e-)nH2O → H2O + H] is measured to be 1.17 X 1012s-1at ambient temperature. These data bring direct evidence that, in concentrated hydrochloric acid aqueous solution, a fast step of single electron transfer reaction is involved in the formation of a short-lived reactive configuration (encounter pair). This electron transfer occurs in a time (250 fs) that is short as compared to the electron solvation dynamics (550 fs). It is interesting to notice (I) that the encounter pair deactivation occurs through a monoexponential process and (II) the relaxation dynamics is faster than the average lifetime of the hydrated hydronium ion but is comparable to the H-bond time scale. The mechanism of the encounter pair relaxation will be discussed taking into account the interactions between the hydronium ion and water molecules. © 1990, American Chemical Society. All rights reserved.