LATTICE MODEL OF INHOMOGENEOUS BROADENING IN CRYSTALS - CORRELATION OF FREQUENCY-DISTRIBUTIONS FOR DIFFERENT TRANSITIONS

Inhomogeneous broadening of a dilute chromophore's optical transition can provide detailed information about disorder in crystalline systems . Certain nonlinear experiments such as fluorescence and phosphorescen ce line narrowing and spectral hole burning, which probe the correlati on between the inhomogeneous frequency distributions of two different transitions, are capable of yielding even more information. In this pa per we present a microscopic lattice theory of inhomogeneous broadenin g by point defects, focusing specifically on the correlation between a pair of transitions. We develop a series of approximations for the jo int frequency distribution for the two transitions that are valid for all possible defect densities, and which are shown to be accurate in c omparison with exact numerical calculation. The model is capable of de scribing frequency distributions with any amount of correlation. We di scuss several experiments within the context of this theory.