COLOUR CENTRE

defect in the regular spacing of atoms within a solid that absorbs visible light of a particular colour or infrared or ultraviolet radiation. Each colour centre involves the absence of an atom from the place it would normally occupy in the solid and the relation of an electron with such an empty place, or vacancy (q.v.). Solids without colour centres may still have colour if impurity atoms or other structures that absorb light are present. There are many types of colour centres. The best understood one, called an F-centre (German Farbe, colour), results from the absence of a negatively charged ion from a particular point in an ionic solid. This vacancy attracts and traps an electron, which combination constitutes an F-centre. The electron so trapped can absorb only certain colours of light. F-centres in sodium chloride absorb only blue light, giving the solid a yellow-orange tinge. Sodium chloride is usually colourless, however, because its electrons are not free to move to vacancies formed by removal of negative chloride ions from the solid. X-rays striking the solid, for example, may produce colour centres by freeing electrons. Removal of an electron from an atom leaves an empty site, or hole, that may behave as a positively charged particle within the solid. A second type of colour centre, called a V-centre, corresponds to the entrapment of such a hole by a positive ion vacancy or by a combination of two negative ions. V-centres, which are less well understood than F-centres, disappear after a short time in solids at room temperature.