Magneto optical properties of F centres in alkaline earth fluorides

Abstract

The position of the F band peak is determined in additively coloured CaF 2 , SrF 2 and BaF 2 by the Faraday rotation method. However, in additively coloured crystals, the F band is normally overlaid by absorption due to higher F aggregate centres and a determination of the spin orbit coupling constant in the excited 2 P state of the F centre is not feasible since a precise knowledge of the F band optical density is required. It is found that trapped electron centres are produced by X-irradiation at 77 °K of undoped SrF 2 and BaF 2 crystals and hydrogen doped CaF 2 , SrF 2 and BaF 2 crystals and the spin orbit coupling is determined for these centres. Paramagnetic resonance measurements show (Bessent, Hayes, Hodby & Smith 1968) that the trapped electron centres are closely related to the normal F centres found in additively coloured crystals. Wavefunctions obtained from a point ion model (Bennett & Lidiard 1965) are used to calculate the spin orbit coupling for the 2 P state of the F centre in CaF 2 , SrF 2 and BaF 2 . For comparison with theory it is assumed that the spin orbit coupling for the irradiation induced trapped electron centres is not appreciably different from that for normal F centres and we obtain good agreement between theory and experiment.