Magneto optical properties of F centres in alkaline earth fluorides

The position of the F band peak is determined in additively coloured CaF2, SrF2 and BaF2 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 2P 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 SrF2 and BaF2 crystals and hydrogen doped CaF2, SrF2 and BaF2 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 2P state of the F centre in CaF2, SrF2 and BaF2. 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.