Low temperature pulse radiolysis of concentrated aqueous solutions. Evidence for trap-to-trap tunnelling in 50/50 v/v ethylene glycol + water and 10 mol dm–3 OH– aqueous solutions

Abstract

Reactions of et with several solutes have been investigated over a wide range of temperature in 50/50 v/v ethylene glycol+water (EG+HzO) and 10 mol dm-3 OH-aqueous solutions.In the fluid media the reactions are generally diffusion controlled, and values of the encounter radius, r, between et and solute have been estimated by applying the theory of time-dependent rate constants to the kinetics.In the rigid media the reaction rate of et with solutes is independent of temperature and proportional to log (time). The rate varies from one solute to another, and is faster in 10 mol dm-3 OHthan in EG+HzO. The kinetics are consistent with el tunnelling from trap to trap in the matrix and reacting with the solute when it becomes trapped within the encounter radius Y. Independent assessments of Y from tunnelling kinetics agree well with those obtained from diffusion kinetics.The eiry capture efficiency of each solute has been estimated from its effect on G(et). The efficiencies of H+, Cuz+ and Fe(CN),NOZare negligibly small, which is in marked contrast to their high reactivities with et.Evidence for long lived encounter pairs of ei and Hf, et and Cu2+, and el and SeOi-at low temperatures is presented.* The beginning of the pulse is taken as the zero of time.

Pulse radiolysis of deuterated aqueous LiCl glasses. Dependence of the yields and rates of reaction of visible and infrared absorbing trapped electrons on LiCl concentration

“…S~O ,~-is a very efficient acceptor of e,,,-in the illore dilute LiCl glasses (see also Fig. 4) although it is a very poor acceptor in other aqueous glasses (8,12). On the face of it these results seem to be the opposite of what one would expect to observe if the direct tunnelling model outlitled above was operative.…”

“…[6] with [S] replaced by [trap], and P, is the probability that the electron will be scavenged and is given by eq. [6] acceptor it has a b o~l t tlie same value as the reaction distance for solvated electrons obtained from diffusion kinetics measurements in the corresponding aqueous liquid (12). In general these values of r d o not change very much from one aqueous liquid to another (6,12).…”

“…[6] acceptor it has a b o~l t tlie same value as the reaction distance for solvated electrons obtained from diffusion kinetics measurements in the corresponding aqueous liquid (12). In general these values of r d o not change very much from one aqueous liquid to another (6,12). 111 this case the variation in r is, presumably, also a reflection of Franck-Condon restrictions to the electron transfer process.…”

“…Before discussing the reactions observed in this study it will be helpful to summarise the pertinent features of the two types of tunnelling mechanism which have been postulated, viz. direct tunnelling (I I) and trap-to-trap tunnelling (12). The basis of tlie direct tunnelling model is that the electron tunnels from its trap to the acceptor so that the reaction rate depends upon the concentration of tlie acceptor but not on the presence of other (empty) traps.…”

Partial deconvolution of the absorption spectrum of trapped electrons in 15 M LiCl aqueous glasses at 77 K

“…The observed recovery of the 'red' band in 7.5 M and 9.5 M LiCl glasses (10,15) may have the same origin. If this is true, then this reversed photoshuffling provides a direct observation of a trap-to-trap tunnelling proposed previously by Buxton and Kemsley (27) (although they excluded 10 M LiCl glasses in their conclusions).…”