Scavenger studies of electron pulse irradiated solutions at hydrated electron half lives in the range 35–0·35 nanoseconds

Abstract

In general hydrated electron rate constants are measured with pulsed electron beams at solute concentrations of less than 10 −4 mol dm −3 when the rate constants is of the order of 10 10 dm 3 mol −1 s −1. We have employed a sensitive technique for determining rate constants at concentrations where the e aq - half-life approaches 0·35 ns during the pulse and its steady-state concentration is about 5 × 10 −7 mol dm −3. A conventional kinetic spectrophotometric apparatus is used with an electronic averaging device which integrates the area under the absorption signal during a train of 3 μs electron pulses. After corrections for Čerenkov emission, the area measured is proportional to Gϵ/ kc where G is the yield of e aq -; ϵ is the molar extinction coefficient of e aq -; k is the rate constant for the reaction e aq -+solute; and c is the solute concentration. In the concentration range 10 −4–10 −1 mol dm −3 HCLO 4, the quantity k/G decreases with increasing concentration. By correcting the known values of k and G respectively for the decreasing effect of ionic strength and increasing effect of H + concentration in the spurs according to the one-radical model of Flanders and Fricke ( J. chem. Phys. 1958, 28, 1126), k/G calculated and experimental are in fairly good agreement. This is equally true for N 2O and CH 3Cl where there is no effect of ionic strength. On the contrary, for (CH 3) 2SO (DMSO) as solute up to 3·5 mol dm −3, where no ionic strength correction is required and corrections of G are negligible (rate constant of e aq - with DMSO = 1·6 × 10 6 dm 3 mol −1 s −1), k/G increases with solute concentration, which corresponds to a decrease in G. This result seems to indicate that unhydrated electrons are scavenged by DMSO before hydration occurs.