Chemical Effects associated with ‘Colour Centres’ in Alkali Halides

Abstract

MEASUREMENTS of the chemical effects of colour centres have previously been made on additively coloured crystals^1,2 and on crystals irradiated with X-rays^3,4. The usual method of measuring electron-excess centres has been to follow the change of pH which occurs on dissolving the crystals in water, due to the reaction e + H₂O = ½H₂ + OH⁻. This method is subject to errors due to the variable carbon dioxide content of solutions, and large effects have been reported with untreated material². We have found that measurement of the gas evolved on dissolution of a treated crystal in carefully degassed water can be used successfully. Analysis of the gas in a Blacet–Leighton micro-apparatus always revealed 95–99 per cent hydrogen present. This method entails no uncertainties due to solution effects, and blank results were always obtained with untreated crystals. It can be applied most easily to concentrations of centres in the region of 10¹⁸ per cm.³ produced either by additive coloration or by ionizing radiation. Direct comparison of this chemical effect with the optical absorption of the F-band in additively coloured crystals is difficult at these concentrations, since very thin sections are required, and an appreciable concentration of other centres is usually present. However, work is in progress to correlate the optical and chemical effects of additively coloured crystals in carefully chosen specimens, so that further comparison may be made with irradiated crystals.