Helium compressional effect onH2molecules surrounded by denseH2-He mixtures

H${\mathrm{-}}^4$He mixtures have been studied in a diamond-anvil cell up to 8 GPa at 300 K. Phase separation has been observed and Raman spectra of the $Q_1$ mode of ${\mathrm{H}}_2$ have been measured at several concentrations. For dilute ${\mathrm{H}}_2$ in a ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ matrix, this mode shifts under pressure over three times faster than in pure ${\mathrm{H}}_2$ with respect to the room-pressure frequency of the ${\mathrm{H}}_2$ molecule. This unexpectedly large effect can be semiquantitatively understood in terms of a collective compressional response of the He matrix on the perturbing ${\mathrm{H}}_2$ molecule.