Spectroscopic Studies of p-H2 to Above 200 GPa

Raman and infrared vibrational spectra of H ₂ have been measured to pressures in excess of 200 GPa and at liquid helium temperatures using new high sensitivity techniques. Detailed study of the pressure dependence of o-p conversion rate reveals an initial increase followed by a decrease above 1 GPa. The conversion rate then increases dramatically with pressure, and this continues to above 50 GPa. New sets of vibron, phonon, roton, and libron excitations in converted para samples are documented as a function of pressure through phases I, II, and III. The results provide important information on the crystal structures, molecular orientational state, and vibrational dynamics of the high-pressure phases.