PRESSURE EFFECTS ON HOLE-BURNING SPECTRA IN GLASSES - CALCULATION BEYOND THE GAUSSIAN APPROXIMATION

In a recent publication, Laird and Skinner [J. Chem. Phys. 90, 3274 (1990)] proposed a microscopic statistical theory describing the effects of external hydrostatic pressure on hole-burning spectra of impurity molecules in amorphous solids. Using the so-called Gaussian approximation, which is valid in the limit that the density of the solvent molecules is high, the theory predicts the pressure kernel of a hole spectrum as well as the shape of the inhomogeneous band to be characterized by Gaussian profiles. Whereas the maximum position of the kernel increases from lower to higher solvent shift values in the inhomogeneous distribution, its width is constant. Numerical calculations performed without this approximation, however, show that for the data of poly(ethylene) and poly(styrene) doped with free-base phthalocyanine, not only the pressure shift but also the pressure broadening of hole-burning spectra increases from the blue to the red edge of the absorption band. Moreover, the hole spectra are predicted to become asymmetric when the sample is exposed to hydrostatic pressure. These deviations from the results of the Gaussian approximation are distinctly more pronounced than the deviations of the inhomogeneous band shapes from Gaussian profiles.