Picosecond pulse radiolysis. I. Time or concentration dependent rate constants

Rate constants for the reaction of the hydrated electron withreactive solutes at high concentrations have been measured, using our single fine-structure pulse stroboscopic apparatus. The accessible time region is 50 to 3500 psec. A time-dependent rate constant in accord with theory was found for the neutral reactant, nitromethane, whereas acetone reacting one-third as fast showed no time dependence. For e-/sub aq/ + NO3- the rate constant increases with concentration (i.e., at short times), owing both to the ionic strength effect and the time dependence effect. Only time-dependent effects seem to be present in our concentration range for the reactions of the hydrated electron with IO4- and CrO42-. Ionic strength effects seem to be absent, probably because tunneling extends the reaction radii to about 10 A where coulomb forces between the reactants are small. Cd2+ reacts with e-/sub aq/ at nearly the diffusion-limited rate so that no Cd2+ ion atmosphere can form about surviving e-/sub aq/. Thus, reacting Cd2+ has an anion atmosphere but e-/sub aq/ reacts without a cation sheath in pure Cd(ClO4)2 solution, and this decreases the effect of ionic strength on the rate constant. The effect is estimated from experimental data. Cu2+ reacts at half the diffusion-limited rate so a partial Cu2+ atmosphere can build up about e-/sub aq/