Anisotropic interactions of hydrogen molecules from the pressure dependence of the rotational spectrum in the Ar(H-2)(2) compound

We report the pressure evolution, up to 70 GPa, of the fine structure of the S-0(0) rotational excitation in the high-pressure Ar(H-2)(2) compound (with almost 100% para-H-2) at about 30 K. A perturbative theoretical analysis is developed to calculate intensities and frequency shifts of the active Raman rotational components, on the basis of the intermolecular anisotropic interaction. The comparison between experimental results up to 35 GPa and calculation allows a reliable determination of the anisotropic intermolecular potential in the solid, both for H-2-H-2 and H-2-Ar at short range. Such results are important for the interpretation of the high-pressure orientational properties of solid hydrogen.