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

Abstract

Self-reaction of the hydrated electron Rate constant Yield of H 2 Linear energy transfer (LET) Monte Carlo track chemistry calculationsIt has been a longstanding issue in the radiation chemistry of water that, even though H 2 is a molecular product, its "escape" yield g(H 2 ) increases with increasing temperature. A main source of H 2 is the bimolecular reaction of two hydrated electrons (e − aq ). The temperature dependence of the rate constant of this reaction (k 1 ), 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 hightemperature 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(H 2 ) shows a marked downward discontinuity at ~150°C which is not observed experimentally. The consequences of the presence of this discontinuity in g(H 2 ) for both low and high LET radiation are briefly discussed in this communication. It is concluded that the applicability of the sudden drop in k 1 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.

Monte Carlo track chemistry simulations of the radiolysis of water induced by the recoil ions of the 10B(n,α)7Li nuclear reaction. 1. Calculation of the yields of primary species up to 350 °C

“…3(d)] with temperature. Similar non-monotonic behavior in the temperature dependence of the yields of primary products in low- 29,45,46 and high- 47,48 LET irradiated water has already been predicted, and is due to the fact that the rate constant for the bimolecular self-reaction of the hydrated electron (k 2 ):…”

Source of Molecular Hydrogen in High-Temperature Water Radiolysis

“…However, Jay-Gerin and co-workers have doubted that this bimolecular recombination rate actually decreases. In a more recent note, they complain that for simulations of high LET radiolysis, the simulated maximum in G (H 2 ) at 150 °C reappears, contrary to experiment. The major effect of LET is to change the initial local concentration of transient species in the particle tracks.…”

Generation of Ultrafast Transient Acid Spikes in High-Temperature Water Irradiated With Low Linear Energy Transfer Radiation

“…Although H 2 is a molecular product, g(H 2 ) is observed to continue to increase with temperature, particularly above 200°C. This anomalous increase in g(H 2 ), which is an issue of much debate in the radiation chemistry of high-temperature water, has been discussed at length elsewhere [48][49][50][51][52][53][54][55]. From a theoretical perspective, we have recently performed Monte Carlo track chemistry simulations of the low-LET radiolysis of liquid water over the range 25-350°C [51], incorporating newly measured or re-assessed experimental data.…”