On the Temperature Dependence of the Rate Constant of the Bimolecular Reaction of Two Hydrated Electrons

Abstract

It has been a longstanding issue in the radiation chemistry of water that, even though H₂ is a molecular product, its “escape” yield g(H₂) increases with increasing temperature. A main source of H₂ is the bimolecular reaction of two hydrated electrons (e^-_aq). The temperature dependence of the rate constant of this reaction (k₁), measured under alkaline conditions, reveals that the rate constant drops abruptly above ~150°C. Recently, it has been suggested that this temperature dependence should be regarded as being independent of pH and used in high-temperature modeling of near-neutral water radiolysis. However, when this drop in the e^-_aq self-reaction rate constant is included in low (isolated spurs) and high (cylindrical tracks) linear energy transfer (LET) modeling calculations, g(H₂) shows a marked downward discontinuity at ~150°C which is not observed experimentally. The consequences of the presence of this discontinuity in g(H₂) for both low and high LET radiation are briefly discussed in this communication. It is concluded that the applicability of the sudden drop in k₁ observed at ~150°C in alkaline water to near-neutral water is questionable and that further measurements of the rate constant in pure water are highly desirable.

Received:13 June 2013; Revised: 27 August 2013; Accepted: 28 August 2013