Colour, symmetry and imperfect crystals

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Abstract
This review is a discussion of colour introduced into normally transparent and colourless inorganic crystals by lattice defects and impurity ions. First (deliberately starting from the most elementary ideas) we outline the simple theory of processes which cause absorption and emission of light by isolated atoms. This is extended to the case of optically active agents in solids where both the static and dynamic effects of the crystalline electric field must be taken into account. There then follows a description of the optical properties of defects which have electronic configurations analogous to H and He atoms as well as H2 + and H2 molecules. It is shown that many aspects of the spectroscopy of such defects may be understood in terms of relatively unsophisticated models, based upon simple extension of the ideas of atomic and molecular spectroscopy. Some applications of colouration are reviewed briefly.

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