Roton Excitations of the Hydrogen Molecule in the Ar(H2)2 Compound

In the Ar(H 2 ) 2 stoichiometric compound, stable only at high pressure,the S(0) rotational excitation shows a complicated fine structure when the ortho-H2 concentration is sufficiently low in the sample. In this work experimental results for the rotational frequencies are reported up to 70 GPa. Moreover, we present here the details of the theoretical analysis, by which we calculate the number of active Raman rotational components, their symmetries, relative intensities, and shifts from the unperturbed rotational frequency, on the basis of the known models for the H2-H2 and H2-Ar anisotropic interaction. The calculated frequencies are extremely sensitive to the shape of the anisotropic interaction potential components, and agree with the measured ones, up to 35 GPa, only if some adjustments are done on the literature models, especially for small intermolecular distances. All the newly determined anisotropic H2–H2 and H2–Ar interaction potential components are reported and compared with literature values.