The metastable triplet state of the M centre in CaF 2

Optical excitation of additively coloured CaF 2 crystals gives rise to population of a metastable excited 3 Sigma u -type state of the M centre. The optical excitation spectrum of this state has been investigated. Measurement of the epr spectrum of the 3 Sigma u state confirms a model proposed earlier by Beaumont and Hayes) for the M centre in CaF 2 . The lifetime of the 3 Sigma u state at 77K is 1.7 s. The temperature dependence of creation and of decay of the 3 Sigma u state population has been investigated by optical and epr methods; the mechanisms of creation and decay are discussed.