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In this study, we use Monte Carlo track chemistry simulations to show that 'dry' secondary electrons, precursors of the 'hydrated' electron ([e.sup.-.sub.aq]), can be scavenged on the sub-picosecond time scale prior to hydration, by a high concentration (>0.1-1M) of azide ions ([N.sub.3.sup.-])in water irradiated with [.sup.60]Co [gamma]-rays and tritium [beta]-electrons at 25 [degrees]C. This is a striking result, as [N.sub.3.sup.-] is known to react very slowly with [e.sup.-.sub.aq]. These processes tend to significantly reduce the yields of [H.sub.2] as observed experimentally. For both energetic Compton electrons ('linear energy transfer', LET ~ 0.3 keV/[micro]m), which are generated by the cobalt-60 [gamma]-rays, and [.sup.3]H [beta]-electrons (LET ~ 6keV/[micro]m), our [H.sub.2] yield results confirm previous Monte Carlo simulations, which indicated the necessity of including the capture of the precursors to [e.sup.-.sub.aq]. Interestingly, our calculations show no significant changes in the scavenging of 'dry' electrons at high azide concentrations in passing from [gamma]-radiolysis to tritium [beta]-radiolysis (i.e., with LET). This led us to the conclusion that the higher [H.sub.2] yield observed experimentally for [.sup.3]H [beta]-electrons compared with [.sup.60]Co [gamma]-rays is mainly explained by the difference in the radiation track structures during the chemical stage (>1 ps). The higher LET of tritium [beta]-electrons leads to more molecular products ([H.sub.2] in this case) in tritium radiolysis than in [gamma]-radiolysis. Finally, a value of ~0.5 nm was derived for the reaction distance between [N.sub.3.sup.-] and the 'dry' electron from the [H.sub.2] yields observed in [.sup.60]Co [gamma]-radiolysis at high [N.sub.3.sup.-] concentrations. Key words: liquid water, azide ion ([N.sub.3.sup.-]) solutions, radiolysis, linear energy transfer (LET), cobalt [gamma]-rays, tritium [beta]-electrons, secondary electrons, 'dry' electron scavenger, mobility, [H.sub.2] yield, Monte Carlo track chemistry simulations. Dans le cadre de la presente etude, nous avons utilise des simulations Monte Carlo de la trajectoire des particules pour montrer que les electrons secondaires dits << secs >>, precurseurs des electrons << hydrates >> ([e.sup.-.sub.aq]), peuvent etre pieges, a une echelle de tempssousla barriere de la picoseconde avant l'hydratation, par des ions azoture ([N.sub.3.sup.-]) a forte concentration (>0,1 a 1 M) dans l'eau soumise a l'irradiation par des rayons [gamma] de [.sup.60]Co et des electrons [beta] de tritium a 25 [degrees]C. Ce resultat est etonnant, puisque l'on sait que [N.sub.3.sup.-] reagit tres lentement avec les [e.sup.-.sub.aq]. Ces processus ont tendance a reduire de facon considerable les rendements en [H.sub.2], comme cela est observe experimentalement. Pour les electrons Compton de haute energie (transfert d'energie lineaire [TEL] ~ 0,3 keV/[micro]m), qui sont generes par les rayons [gamma] du cobalt 60, et les electrons [beta] du [.sup.3]H (TEL ~ 6keV/[micro]m), nos resultats du rendement en [H.sub.2] confirment les simulations Monte Carlo precedentes, qui ont indique la necessite d'inclure le piegeage des precurseurs des [e.sup.-.sub.aq]. Fait interessant, lorsque l'on passe de la radiolyse [gamma] a la radiolyse [beta] par le tritium, nos calculs ne montrent aucun changement substantiel quant au piegeage des electrons << secs >> a des concentrations elevees d'azoture. Ces resultats nous amenent a conclure que le rendement plus eleve en [H.sub.2] observe experimentalement pour les electrons [beta] du [.sup.3]H par rapport aux rayons [gamma] du [.sup.60]Co s'explique principalement par la difference entre la structure des trajectoires de rayonnement pendant le stade chimique (>1 ps). Le TEL plus eleve des electrons [beta] du tritium donne lieu a la formation d'un plus grand nombre de produits moleculaires ([H.sub.2] dans ce cas) lors de la radiolyse par le tritium que lors de la radiolyse [gamma]. Enfin, nous avons calcule une valeur d'environ 0,5 nm pour ce qui est de la distance de reaction entre [N.sub.3.sup.-] et l'electron << sec >> a partir des rendements en [H.sub.2] observes lors de la radiolyse [gamma] par le [.sup.60]Co a des concentrations elevees de [N.sub.3.sup.-]. [Traduit par la Redaction] Mots-cles : eau liquide, solutions d'ions azoture ([N.sub.3.sup.-]), radiolyse, transfert d'energie lineaire (TEL), rayons [gamma] de cobalt, electrons [beta] de tritium, electrons secondaires, piegeur d'electrons << secs >>, mobilite, rendement en [H.sub.2], simulations Monte Carlo de la trajectoire des particules., 1. Introduction The radiation chemistry of liquid water (1-3) is of fundamental importance for a variety of practical applications, particularly in the nuclear power industry, where radiolysis products cause corrosion [...]

A re-examination of our Monte-Carlo modeling of the radiolysis of liquid water by low linear-energy-transfer (LET ~ 0.3 keV [micro][m.sup.-1]) radiation is undertaken herein in an attempt to reconcile the results of our simulation code with recently revised experimental hydrated electron ([e.sup.-.sub.aq]) yield data at early times. The thermalization distance of subexcitation electrons, the recombination cross section of the electrons with their water parent cations prior to thermalization, and the branching ratios of the different competing mechanisms in the dissociative decay of vibrationally excited states of water molecules were taken as adjustable parameters in our simulations. Using a global-fit procedure, we have been unable to find a set of values for those parameters to simultaneously reproduce (i) the revised [e.sup.-.sub.aq] yield of 4.0 [+ or -] 0.2 molecules per 100 eV at "time zero" (that is, a reduction of ~20% over the hitherto accepted value of 4.8 molecules per 100 eV), (ii) the newly measured [e.sup.-.sub.aq] decay kinetic profile from 100 ps to 10 ns, and (iii) the time-dependent yields of the other radiolytic species H*, *OH, [H.sub.2], and [H.sub.2][O.sub.2] (up to ~1 [micro]s). The lowest possible limiting "time-zero" yield of [e.sup.-.sub.aq] that we could in fact obtain, while ensuring an acceptable agreement between all computed and experimental yields, was ~4.4 to 4.5 molecules per 100 eV. Under these conditions, the mean values of the electron thermalization distance and of the geminate electron-cation recombination probability, averaged over the subexcitation electron "entry spectrum," are found to be equal to ~139 [Angstrom] and ~18%, respectively. These values are to be compared with those obtained in our previous simulations of liquid water radiolysis, namely ~88 [Angstrom] and ~5.5%, respectively. Our average electron thermalization distance is also to be compared with the typical size (~64-80 [Angstrom]) of the initial hydrated electron distributions estimated in current deterministic models of "spur" chemistry. Finally, our average probability for geminate electron-cation recombination agrees well with an estimated value of ~15% recently reported in the literature. In conclusion, this work shows that an adaptation of our calculations to a lower hydrated electron yield at early times is possible, but also suggests that the topic is not closed. Further measurements of the [e.sup.-.sub.aq] yields at very short times are needed. Key words: liquid water, radiolysis, electron-cation geminate recombination, electron thermalization distance, hydrated electron ([e.sup.-.sub.aq]), [e.sup.-.sub.aq] decay kinetics, time-dependent molecular and radical yields, Monte-Carlo simulations.

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Presents a correction to the study "Radiolysis of Liquid Water: An Attempt to Reconcile Monte-Carlo Calculations With New Experimental Hydrated Electron Yield Data at Early Times."