Centres in Inorganic Compounds

The primary process involved in the x-ray production of colour centres is presumed to be20 

The ultra-violet and visible absorption spectra of irradiated alkali halides were first studied by Pohl21, who observed an intense band between 400 and 800 m/x. The centres responsible for this absorption were called F-centres and the band F-band. The fact that only one band was observed in the absorption range indicated that only one electron was involved in the transition. 

So far, no equation which is rigorously valid for correlating the concentration of F-centres to the intensity of the band is proposed. But the equation given by Rauch and Heer28 is useful 

During irradiation in the F-band at low temperatures, extinction gradually decreases and the additively coloured salts show a broad band at low temperatures towards the long wavelength side. On warming, this broad F -band is bleached accompanied by the intensification of the F-band. The quantum yields for the two processes are the same. 

The presence of impurities or foreign ions in KC1 has a marked effect on the coagulation of F-eentres. Watson and Scott83 have shown that the introduction of alkaline earth ions into additively coloured KC1 enhances the stability of F-centres. Markham and Hersh84 have found that the optical and thermal bleaching properties of potassium halides are radically affected by doping with thallium ions. 

---