LOW-FREQUENCY RAMAN-SPECTROSCOPY OF DEUTERIUM TO MEGABAR PRESSURES AT77-295-K

Rotational and lattice phonon excitations in the Raman spectrum of sol id molecular deuterium have been measured from 1.8 GPa to approximatel y 200 GPa at 77-295 K to study pressure-induced changes in structural and dynamical properties of the dense solid. Continuous and discontinu ous changes in three distinct pressure ranges are observed. At lower p ressures (< 30 GPa), there is a gradual increase in the linewidth of t he S0(0) band, together with gradually decreasing.resolution of the hi gher-energy S0(1) and S0(2) features. At intermediate pressures (60-10 0 GPa), a change in the pressure dependence and linewidth of S0(0), an d a linewidth and intensity decrease in the E2g phonon band occur. Thi s is interpreted as evidence for a phase transformation in the molecul ar solid beginning at approximately 65 GPa (77 K). At the highest pres sures (approximately 160 GPa), abrupt changes in the low-frequency exc itations suggest either an expansion of the molecular bond or a change in ordering at the vibron discontinuity. Evidence from the low-freque ncy spectra for interaction between the deuterium sample and diamond a nvil is also examined.