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

The thermal decay of F- centres has been studied by pulse radiolysis in pure KCl crystals. At room temperature approximately 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 of both the integrated total dose given to the crystal and the dose per pulse. This implies that the F- decay does no 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. (author)