Spectroscopic studies of the $\mathrm{Ar}({\mathrm{H}}_2{)}_2$ compound crystal at high pressure and low temperatures

Fourier-transform infrared and Raman studies are performed on $\mathrm{Ar}({\mathrm{H}}_2{)}_2$ compound single crystals over a large pressure (5–30 GPa) and temperature (30–300 K) range in a diamond-anvil cell. The effect of polarization on the infrared absorption is investigated. On the basis of the spectroscopic features and comparing to the cases of pure solid ${\mathrm{H}}_2$ and of matrix isolated ${\mathrm{H}}_2$ in argon crystal, we assign the quite complex infrared spectra to the pure vibron and vibron plus phonons and vibron plus rotons combination bands. The difference of the infrared and Raman vibron frequency, as a function of pressure, is interpreted quantitatively with reference to vibrational coupling between hydrogen molecules, as in the hydrogen crystal case. Information on the structural properties can thus be derived. Finally, this study highlights the similarity in the molecular problems for the $\mathrm{Ar}({\mathrm{H}}_2{)}_2$ compound and for solid hydrogen.