Microscopic theory of reversible pressure broadening in hole-burning spectra of impurities in glasses

Abstract

Recently, Sesselmann et al. [Phys. Rev. B 36, 7601 (1987)] have examined the effect of pressure changes on hole-burning spectra of dye molecules in polymer glasses, finding that the hole shift and broadening are linear in the pressure change and its magnitude, respectively. We develop a statistical, microscopic theory of this effect, and of the inhomogeneous line shape itself. In the limit that the density of solvent perturbers becomes large, the general theory predicts that both the inhomogeneous line shape and the hole shape after a pressure change will be Gaussian, in qualitative agreement with experiment. By considering a specific model for the solute–solvent interaction, we then provide a quantitative analysis of the experiments that shows that the pressure broadening is due to changes in the local environment of each chromophore. As a further test of the theory, we make a prediction as to the frequency dependence of the pressure-dependent hole shift, which can be easily tested by experiment.