Frequency shifts of vibrational and rotational states of diluteH2,D2, and HD impurities in solid Ar under pressure

The frequency shifts of the vibrational and rotational transitions of ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD molecules trapped in solid Ar are calculated at zero temperature and at pressures 0≤P≤373 kbar. It is found that the pure vibrational and rotational-vibrational transition frequencies are strongly red-shifted in the solid at P=0, compared to gas-phase values, and the agreement with Raman scattering measurements is generally good. The calculated pure rotational transitions also show a small red shift at P=0 in the solid and are in generally good agreement with the measurements of Jodl and Bier, but less so with those of Prochaska and Andrews, who, except for ${\mathrm{D}}_2$(Ar), measure small blue shifts. The calculated local-mode frequencies of the impurity molecules in the solid at P=0 are also in good agreement with experiment, especially when thermal corrections are considered. With increasing pressure all transition frequencies and the local-mode frequencies are strongly blue-shifted with respect to P=0 solid values.