Lennard-Jones Model of Frequency-Selective Barochromism and Thermochromism of Spectral Holes in Glasses,

Abstract

Pressure (P) and temperature (T) effects on (quasi)homogeneous optical spectra of dyes in solvent glasses and polymers were investigated by hole burning. The frequency-dependent P- and T-induced shift and broadening of zero-phonon holes burned in the inhomogeneous spectra are rationalized using two-body Lennard-Jones potentials. The difference between the excited-state potential (U*) and the ground-state potential (Ug) yields the absolute vacuum-to-matrix shift as a function of the intermolecular coordinate. The P- and T-shifts can be described in terms of differences between the derivatives U*‘ and Ug‘ and the ratios of second derivatives U*‘ ‘ and Ug‘ ‘, respectively. The P-shift increases and the T-shift decreases as the optical transition energy increases. The Lennard-Jones model reproduces this opposite frequency dependence very well, by assuming that, in the excited state, the potential well minimum is displaced to shorter distances (σ* < σg). The force constant of a 6−12 potential vanishes at a distance exceeding the equilibrium value only by 11%. This feature explains qualitatively the phonon mode softening in the presence of a free volume in glasses or defect solids.