AN APPRAISAL OF THE MOLECULAR MODEL FOR THE Vk CENTRE

M&a&--The V, centre in halide crystals is often described in terms of an X2molecule ion, Recent moteeuiaf calculations for the haiogen molecule negative ions have enabled a detailed comparison to be made between the theoretical anion and the resuhs from optical and spin resonance studies on the crystals. The optical absorption line widths are naturally dependant on the host lattice, but the excitation energies and spin resonance constants may be readily interpreted in terms of the molecule. Certain anomalies in the spin resonance data for the heteronuclear defects, in particular CRand Brf-, show that the simple wavefunction constructed from ‘Is” and “p” atomic orbitals does not adequately describe the polarisation of the large halogen atoms, and a more flexible wavefunction will be needed to calculate the spin resonance constants for these ions.