Diaelastic pressure-induced broadening of spectral holes in molecular crystals

Broadening and shifts of spectral holes by changing the hydrostatic pressure at 2 K were measured for dimethyl-s-tetrazine probe molecules in durene crystal, both pure and doped with hexachlorobenzene (HCB) at 2 mol%. The HCB molecules act as elastic defects and cause increase in inhomogeneous broadening as well as in the pressure-induced broadening of spectral holes. However, the relative pressure broadening (scaled to the inhomogeneous linewidth) decreases with pressure. The HCB concentration also affects the color effect in pressure broadening, i.e. its dependence on the spectral position. The observations are qualitatively interpreted proceeding from the diaelastic model [J. Chem. Phys. 104 (1996) 5384].