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.