Extended spectroscopic studies of Ar(H-2)(2) at high pressure and low temperature

Infrared absorption and Raman scattering studies on the stoichiometric van der Waals compound Ar(H-2)(2) have been performed at pressures from 5 to 40 GPa and at temperatures from 30 to 300 K. On the basis of the infrared spectroscopic features obtained by the use of polarised radiation, and on the results of group theory, a precise assignment of the components of band that correspond to the fundamental vibration of the hydrogen molecule is presented. A sharp polarised infrared peak, present in the spectra at all pressures, is assigned to the allowed infrared mode. When its frequency is compared with the frequency of the Raman mode at various pressures, an increasing frequency difference, of the same sign of that measured for pure H-2, is observed. A quantitative derivation of the vibrational coupling parameter between molecules has then been possible. A very good agreement with values derived from measurements on pure H-2 is obtained. Moreover, from this analysis structural information concerning the positions of the hydrogen molecules in the unit cell has been derived. (C) 1999 Elsevier Science B.V. All rights reserved.