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 V_K 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 V_K 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 V_K centre production in pure virgin crystals is very inefficient, but in crystals containing pre-existing F centres, V_K and F^– centres are produced together: 2Cl^– [graphic omitted] Cl^–₂(V_K)+ε^–, ε^–+ F → F^–. Thermal decay of the V_K 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 V_K centre.

At temperatures between 190 and 260 K the disappearance of V_K centres is associated with photoionisation of F^–: the electron released by F^– is captured by V_K in a process analogous to the thermal bleaching. At lower temperatures, down to 140 K, the bleaching of V_K becomes progressively slower than that of F^–(which is temperature-independent). The origin of this difference is not yet clear.