Nanosecond Pulse Radiolysis and Nonhomogeneous Kinetics Involving Hydrated Electrons

Nanosecond pulse‐radiolysis studies on the behavior of hydrated electrons at concentrations of about 10⁻⁴M showed that the decay during approximately the first half‐life (∼110 nsec) was characterized by a rate constant significantly different from that during the second half‐life. The latter rate appeared to follow second‐order kinetics. The initial decay is attributed to nonuniform distributions of species and to intratrack reactions involving $e_{\mathrm{aq}}^{-}$ before the system became truly homogeneous. Qualitative calculations on the period of nonhomogeneity were in agreement with the experimental transition time. It is apparent that in some pulse‐radiolysis studies one cannot justifiably calculate second‐order rate constants from concentrations evaluated from optical‐density measurements. The experiments were performed on pure deaerated water subjected to an intense 3‐nsec pulse of 0.5‐MeV electrons from a Febetron accelerator. Spectrophotometric analyses were conducted over very small reaction volumes by using the 632.8‐nm light from a He/Ne laser collimated to 0.07‐mm² cross‐sectional area. One possible implication of the fast initial decay is that ${\mathrm{G(e}}_{\mathrm{aq}}^{-}$ is much larger than 2.5.