The theory of the strain broadened line shapes of spin resonance and optical zero phonon lines

A number of spin resonance and zero phonon optical absorption lines are broadened by microscopic strains in the lattice containing the centres observed. In MgO, for example, the electron spin resonance line of Fe 2+ , the ENDOR line of Co 2+ and the zero phonon lines of colour centres all show strain broadening. The strains appear to be due to dislocations and other crystal imperfections. It is shown that these line shapes can be calculated in terms of the statistical distribution of the imperfections and their individual strain fields. The approach can be generalized to cases where other broadening mechanisms are important, such as spin-spin interactions. The method may also be used to obtain information about the statistical distribution of the strain sources from the observed line shape. To illustrate the method the line shape of MgO: Fe 2+ is calculated on the simple, if rather unrealistic, assumption of an isotropic homogeneous distribution of dislocations. The results are in fair agreement with experiment.