Excited states and electron-lattice coupling of low symmetry F centres: the tetragonal F centres in BaClF

Abstract

Magnetic and stress properties have been studied of the F centres appearing in the tetragonal BaClF crystal: the f(F^-) and the f(Cl^-) centres. The results have been analysed by the method of moments extended to the tetragonal symmetry. The excited state orbital lande factor is found to be close to zero for both centres and the spin-orbit splitting is determined. The introduction of second order spin-orbit mixing allows evidence to be given for the existence of k states associated to the f(F^-) centre. The temperature shift of the F band is shown to be due mostly to the thermal expansion of the lattice. The analysis of higher order moments of absorption, magnetic circular dichroism and stress linear dichroism spectra leads to the determination of the dynamic electron-lattice coupling parameters which are used to explain the shape of the two bands in terms of a Jahn-Teller interaction.