Magnetic circular dichroism and optically detected electron paramagnetic resonance of trapped hole centres in KBr and KI

In KBr and KI the magnetic circular dichroism of the optical absorption (MCDA) of V K and H centres created by X-irradiation at 4.2 K in situ was investigated as well as the MCDA-detected electron paramagnetic resonance (EPR) spectra. MCDA extrema were found at the spectral positions of the known absorption band peaks for VK centres including a second band in the ultraviolet (UV), which had been reported previously for KI but not for KBr. Only one MCDA band was found for H centres, corresponding to the infrared (IR) transition. No MCDA-detected EPR spectra were observed for VK and H centres with their molecular axes parallel to the magnetic field. Using perturbation theory to describe the effect of spin-orbit coupling and magnetic field mixing of the X 2 - molecular orbitals used to describe V K and H centres previously, the experimental results could be explained except for the new UV MCDA band, which cannot be explained assuming inversion symmetry of the V K centres. It is suggested that this band arises because of the low-frequency (10 12 s -1 ) axial displacement mode of V K centres, breaking the inversion symmetry, which is a quasi-static distortion for the optical transition, but fast with respect to EPR, and therefore not seen there.