Production and decay of hole centres in KCl. A pulse radiolysis study
Abstract
A pulse radiolysis study has been carried out on the two best-established hole centres in KCl—the H and VK centres.
The decay of the H centre was studied between 124 and 263 K. In this temperature range it undergoes first-order thermal decay with an activation energy of 0.24 eV. The decay product of the H centre was not the VK centre, as has been previously suggested, nor is the first-order behaviour apparently consistent with recent theoretical calculations which indicate that the H centre dimerises.
The VK centre production in pure virgin crystals is very inefficient, but in crystals containing pre-existing F centres, VK and F– centres are produced together: 2Cl– [graphic omitted] Cl–2(VK)+ε–, ε–+ F → F–. Thermal decay of the VK centre was studied between 270 and 320 K. It was established that the rate at all temperatures was the same as that of the F– centre studied earlier. The mechanism of the reaction is thermal ionisation of the F– centre, followed by capture of the electron by a VK centre.
At temperatures between 190 and 260 K the disappearance of VK centres is associated with photoionisation of F–: the electron released by F– is captured by VK in a process analogous to the thermal bleaching. At lower temperatures, down to 140 K, the bleaching of VK becomes progressively slower than that of F–(which is temperature-independent). The origin of this difference is not yet clear.