Abstract
The effects of optical illumination and impurities on colloid formation are studied in additively coloured KCl and in NaBr coloured by ultra-violet light. It is shown that at elevated temperatures additively coloured KCl behaves as an auto-sensitized photographic system. A theory of the colloid band is presented which takes account of ion core polarization and harmonic mean effective mass for arbitrary ellipsoidal particles. The colloid band is identified as a bounded plasma resonance, shifted into the visible region by the boundary conditions. Plasma frequencies computed from the positions of the colloid bands are in good agreement with other estimates. The spectral dependence of photoconductivity associated with the colloid band is related to the band structure of the host medium.