Intermolecular repulsive-dispersive potentials explain properties of impurity spectra in soft solids

Behavior of optical impurity spectra in van der Waals glasses is rationalized with the aid of a two-particle Lennard-Jones model of intermolecular interactions. Simple mathematical manipulations with the 6-12 potential yield inhomogeneous distribution functions (IDFs) of zero phonon lines (ZPLs) at different compressions, and the expressions for wavelength dependent pressure shift coefficients of ZPLs (or holes), local phonon frequencies, and linear and quadratic coupling constants. Experimentally, the ZPL to phonon wing intensity ratios (Huang-Rhys or Debye-Waller factors) are measured for bacteriochlorophyll a in glass-forming triethylamine 5 K. Enhancement of coupling strength with increasing transition wavelength is observed, in qualitative agreement with the model.