Photobleaching of the F– centre in KCl

Abstract

Photobleaching of F^– centres in KCl has been studied between 100 and 200 K. Under conditions of constant incident light intensity the photobleaching efficiency was constant and independent of temperature, indicating that the quantum yield for photodestruction of F^– is constant, and probably equal to unity at all temperatures.

Previous studies of F^– photolysis have shown that at low temperatures (⩽ 80 K) the process F^–→ 2F occurs. The quantum yield for F centre formation falls with increasing temperature, to become virtually zero at 200 K; it has been suggested that the reformation of F^–(F^–→ F^–) is a competing process at higher temperatures.

The present results are inconsistent with the above mechanism; it is necessary to postulate a high temperature process that involves neither F^– nor F centres as an end product. This high temperature process is probably formation of F₂ aggregate centres, since F₂ centres also result from the thermal decay of F^– centres at temperatures above 240 K.