Production and decay of the F? centre in KCl studied by pulse radiolysis

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Abstract
The thermal decay of F– centres has been studied by pulse radiolysis in pure KCl crystals. At room temperature 75% of the F– centres decay exponentially with a mean life of 190 µs. The other 25 % decay by a non-first order process (perhaps by reaction with unstable hole centres). As the temperature is lowered, the fraction of non-first order process becomes smaller, and is zero below 270 K. The rate constant for the first order process decreased with temperature; the activation energy was 0.60 ± 0.04 eV. The lifetime of 190 µs was found to be independent fo both the integrated total dose given to the crystal and the dose per pulse. This implies that the F– decay does not involve reaction with any other radiation-produced species, either short- or long-lived. A new mechanism for F– decay is put forward, involving thermal ionisation as the initial step, followed by F2 centre formation.

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