Stress spectra of colour centres in lithium fluoride

Sharp zero-phonon absorption lines in lithium fluoride have been studied under uniaxial stresses of up to 16 kg mm^-2 applied along crystalline symmetry directions to investigate the electronic properties of the colour centres involved. The centres have been produced by electron irradiation at dose rates of 10⁶ rad min^-1 for one hour and, in some cases, subsequent optical bleaching procedures. High resolution optical absorption measurements have been made at 4 °K on eight of the most prominent lines produced, which have air wavelengths of 3599, 3909, 3932, 4747, 4874, 5234, 5612 and 6009 Å. All the lines show splittings into several components on the application of the stress, the maximum observed splitting being about 40 cm^-1. The spectra have been investigated using polarized light and the results are interpreted in terms of electronic and/or orientational degeneracy of the centres.