Optical absorption and photochromism in iron-doped rutile

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Abstract
Photochromism has been observed in single-crystal iron-doped rutile. Colouring occurs on irradiation with light of photon energy greater than 2.5 eV. The ratio of the activation energies for optical bleaching (2.5 eV) and thermal bleaching (0.5 eV) agrees with approximate theory. The colouring process is interpreted as the optical transfer of an electron from the valence band to an Fe3+ Square Operator centre where Square Operator signifies an iron-adjacent anion vacancy: the majority of the resultant holes are trapped by Fe3+ Square Operator ions on sites without an adjacent vacancy. Bleaching is a combination of optical and thermal release of electrons from Fe2+ Square Operator centres direct and thermal release of electrons from Fe2+ Square Operator centres into the conduction band followed by recombination with the trapped holes, and direct recombination between the Fe2+ Square Operator centres and non- trapped holes.

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