ANALYTICAL DESCRIPTION OF EXTERNAL-FIELD EFFECTS ON PERSISTENT-HOLE SPECTRA IN DISORDERED MATERIALS

External-field effects on persistent-hole spectra in disordered solids can be described with a stochastic theory of dye-matrix interaction. For obtaining analytical results, usually two approximations are perfo rmed in the calculation, the continuum approximation which is valid fo r sufficiently low matrix densities, and the Gaussian approximation wh ich is valid for high densities. Hence, the validity range of the resu lts is not clear a priori. The paper shows for the special case of pre ssure tuning that both approximations must be relaxed in a proper desc ription of the physical situation. Especially the continuum approximat ion which consists in neglecting any correlations between the matrix m olecules, leads to quantitatively incorrect results.