Raman scattering in oriented crystals of paradeuterium and orthohydrogen

An extensive rotational-Raman-scattering study on oriented crystals of high-purity paradeuterium and orthohydrogen is presented. The excitations studied include the one- and two-libron bands ($J=1\mathrm{}\to 1$), the $J=1\mathrm{}\to 3$ rotons, and the $J=0\mathrm{}\to 2$ transitions of the impurity $J=0\mathrm{}$ molecules. The use of oriented crystals has permitted a determination of the single-libron mode symmetries which, together with very good frequency agreement with the most recent anharmonic libron theory, provides a firm experimental basis for the $\mathrm{Pa}3\mathrm{}$ structure. When corrected for virtual ($J=3\mathrm{}$) processes, the single-libron frequencies yield values for ${\Gamma }_{\mathrm{eff}}$ of 0.747 ± 0.019 ${\mathrm{cm}}^{-1\mathrm{}}$ (${\mathrm{D}}_2$) and 0.582 ± 0.017 ${\mathrm{cm}}^{-1\mathrm{}}$ (${\mathrm{H}}_2$), which are in generally good agreement with other experimental values. New results on the concentration and temperature dependence of libron frequencies and line shapes are also presented. An analysis of the $J=0\mathrm{}\to 2$ and $J=1\mathrm{}\to 3$ spectra shows that both excitations are strongly coupled to the librons. This results in substantial modifications of the frequencies expected from simple theories, and in the appearance of extra features, interpreted as libron sidebands. In the case of the $J=1\mathrm{}\to 3$ band, linear combinations of polarized spectra obtained from oriented crystals has allowed the separation of previously unresolved lines. Spectra of the various rotational transitions in the disordered phase of both ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ are also presented.