Near-resonance scattering of the para-H2vibrons in (ortho-H2)x(para-H2)1−xmixed crystals

The p-${\mathrm{H}}_2$ vibrons hae been studied by high-resolution Raman scattering in mixed crystals of (o-${\mathrm{H}}_2$ ${)}_{\mathit{x}}$(p-${\mathrm{H}}_2$ ${)}_{1\mathrm{-}\mathit{x}}$, (HD${)}_{\mathit{x}}$(p-${\mathrm{H}}_2$ ${)}_{1\mathrm{-}\mathit{x}}$, and (o-${\mathrm{D}}_2$ ${)}_{0.069}$(p-${\mathrm{H}}_2$ ${)}_{0.931}$. For increasing o-${\mathrm{H}}_2$ concentration, the line shape broadens and becomes asymmetric with a tail towards higher frequencies. This effect is less pronounced in the HD- and the o-${\mathrm{D}}_2$-doped crystals, where the energy difference between the p-${\mathrm{H}}_2$ vibron and the stretching vibration of the impurity is much larger. Isolated impurities still have a symmetrical line shape, showing that the asymmetry is not due to inhomogeneities. Calculated line shapes by the coherent-potential-approximation technique are in very good qualitative agreement with the experimental results. The theory also accounts for previously published experimental data on the variations of the relative positions and intensities of the ortho and para peaks in (o-${\mathrm{H}}_2$ ${)}_{\mathit{x}}$(p-${\mathrm{H}}_2$ ${)}_{1\mathrm{-}\mathit{x}}$.