Isotope shifts for zero-phonon and phonon-assisted transitions in lithium fluoride

Zero-phonon absorption lines at 3909 Å, 4874 Å and 5234 Å associated with radiation induced colour centres have been examined at 4 °K in single crystals of 6 LiF and 7 LiF. Red shifts of all the lines have been observed in 6 LiF relative to their positions in 7 LiF, the largest shift being 6.8 ± 0.4 cm -1 for the 4874 Å line. Large shifts have also been observed in some of the phonon-assisted transitions accompanying the 5234 Å line. No changes in the isotope effects have been observed at 77 °K, although all lines shift to the red and broaden. It is shown that the isotope shift for the zero-phonon lines may be regarded as the sum of two parts: a `static shift' caused by the fractional increase in lattice constant for 6 LiF and a `dynamic shift' resulting from quadratic coupling between the electronic states of the defect and the lattice normal modes. This coupling is also responsible for part of the temperature shifts. The contribution of the static shift may be found from measurements under applied stress, whilst a formula is derived which expresses the dynamic shift in terms of moments of the one-phonon spectrum. Shifts of the correct order of magnitude and direction have been calculated on this basis. Finally, in the light of their isotope shifts, the frequencies of phonon-assisted peaks for the 5234 Å line are compared with dispersion curves calculated by Karo and Hardy in 1963.