Formation of hydrated electrons upon dissolution of crystals containingF centres

Abstract

The hydrated electron (symbol e~q) joined definitively itself to the family of chemical reagents during the last decade. Its discovery will be remembered without any doubt among the most brilliant results achieved in the chemical field ia these years. The hydrated electron is formed by an electron surrounded by four oriented molecules of water, in other words c~ is an electron trapped in a cage of water molecules. I ts fundamental chemical characteristic is an unusually high electron donor capacity which makes its reactivity very high and therefore its mean life very short in all media. Hydrated electrons can be produced in several manners (1), for instance by metal dissolution in water, by electrochemical reduction, by photolysis, etc., but the principal method for producing e~Q is the radiolysis of water solutions. Indeed the most important contributions to the discovery and chemical characterization of hydrated electron came and are coming from the radiation chemistry, particularly from pulsed radiolysis. Sometimes the individuation of the chemical reactivity of e~ towards certain substances is made difficult by the fact that many other radicals are produced together with e~q by radiolysis. Therefore a method capable to produce only, or almost only, e~q without any serious chemical complications, as for instance those connected with the method of alkali-metal dissolution in water, should be interesting. The dissolution in water of crystals containing E centres seemed to us a potential source of hydrated electrons having the properties mentioned above. I t is well known that an _~ centre results when an electron is trapped at the site of a missing anion. When crystals conta in ing/~ centres are dissolved in water the cation cage which traps the electron is destroyed and the electron should be freed into the solution giving rise to e~. As the concentration of F centres in alkali halides can at tain values as high as 101 F centres/em 3 (that is 5.1017 _F centres/g) and the solubility for instance of NaC1 in water is ~ 330 g/l, the potential amount of e:~ which can be generated upon crystal dissolution reaches values of the order of a few 10-dg equiv/l. The F centres in alkali halides can be produced by Xor y-ray irradiation or by incorporating an excess of alkali metal in the crystal. The alkali excess occupies normal