An experimental and theoretical investigation of the far-infrared spectra of HCl and HBr crystals under pressure at 4.2 K

Abstract

By slowly cooling a gaseous mixture of Ar and HX hydrogen halides (X=Cl, Br) a rare gas solid containing crystallites of either HCl or HBr is formed at 4.2 K. The far infrared spectrum of this solid is recorded under a pressure range of 0 to 5 kbar. Pressure frequency shifts of external modes are observed. Dynamical calculations of external modes are performed using an intermolecular potential including multipolar expansion up to quadrupolar term as well as short and long range contributions represented by a Buckingham type potential. The pressure frequency shifts are evaluated using the model of Pawley and Mika which provides a simple method of estimating the effects of pressure constraint on the intermolecular potential. The P–V relation is numerically derived and the Grüneisen parameters are calculated.