✍ Scribed by David C. Walker
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In the preceding paper I , Bewick et al. show that it is necessary to produce hydrated electrons (eaq) at greater than ~ 10 -12 mol cm -2 near a silver electrode in order to observe optical absorption by eaq when using a single-reflection experiment. They show that absorbances smaller than 10 -s would be swamped by electroreflectance effects. These findings are very similar to those previously reported by Postl and Schindewolt a . However, these authors 1,2 have simply extended their conclusions about the relative importance of electroreflectance to results obtained by a continuous specular reflection method 3 ,4, without taking into account (i) the fact that the optical absorption path is larger by several orders of magnitude in the latter method due to the multiple reflections and the grazing angle of incidence, and (ii) the electroreflectance effect changes with angle of incidence.
It has already been indicated s that 10 -12 mol cm-2 is about the maximum number of hydrated electrons, per unit area of electrode surface, that can possibly exist in solution during electrolysis (at readily accessible current densities). This limitation is imposed by the high reactivity Ofeaq towards each other, and other decomposition reactions. In fact, simply from reaction rate constants and diffusion constants one can calculate an upper limit to the absorbance of ~ 10 -s for a single-reflection experiment. It was for this reason 4 that a continuous specular reflection method (c.s.r.) was used in the experiments 3 ,4 with which the whole discussion by Bewick et al. 1 is concerned. A similar upper limit was obtained by Postl and Schindewolf 2 who calculated a value of 2 x 10 -12 tool cm -2 for the maximum possible concentration per cm-2 electrode surface under their high current conditions when they considered only decay through the bimolecular combination reaction. In the c.s.r, experiments 3 , the corresponding figure was calculated to be ~ 10 -13 mol cm -2 Thus, 10 -12 mol cm -2 is seen to be the approximate theoretical upper limit of hydrated electrons which can be obtained near an electrode for current densities up to 50 mA cm-2. Therefore, the observation of Bewick et al. 1 is entirely in accord with expectations. In other words, one cannot expect to observe eaq by a single-reflection method if one requires absorbances of at least 10 -s in order to make the measurements.
In the c.s.r, method, a narrow parallel beam of light from a laser was directed
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