The homogeneous width of zero-phonon lines in a chlorin-doped para-terphenyl crystal is determined by spectral hole burning at $T=4.2\mathrm{}–14\mathrm{K}$ under various hydrostatic pressures up to 7 kbar. A drastic broadening of spectral holes with pressure occurs on approaching the order-disorder (triclinic-to-monoclinic) phase transition which takes place under $P\approx 5.5\mathrm{kbar}$ at $4.2<~T<~14\mathrm{K}.$ The thermal broadening of spectral holes at fixed pressures is an activated process with the activation energy depending on pressure and having a deep minimum at $P\approx 5.5\mathrm{kbar},$ i.e., in the phase-transition region.

• Received 5 June 1997

DOI:https://doi.org/10.1103/PhysRevB.57.14