Abstract
The position of theFband peak is determined in additively coloured CaF2, SrF2and BaF2by the Faraday rotation method. However, in additively coloured crystals, theFband is normally overlaid by absorption due to higherFaggregate centres and a determination of the spin orbit coupling constant in the excited2Pstate of theFcentre is not feasible since a precise knowledge of theFband optical density is required. It is found that trapped electron centres are produced by X-irradiation at 77 °K of undoped SrF2and BaF2crystals and hydrogen doped CaF2, SrF2and BaF2crystals 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 normalFcentres found in additively coloured crystals. Wavefunctions obtained from a point ion model (Bennett & Lidiard 1965) are used to calculate the spin orbit coupling for the2Pstate of theFcentre in CaF2, SrF2and 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 normalFcentres and we obtain good agreement between theory and experiment.
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