New High-Pressure Excitations in Parahydrogen

Raman and infrared spectroscopy of para- ${\mathrm{H}}_2$ to pressures in excess of 200 GPa and to 8 K using new ultrapure synthetic diamond anvils reveals numerous new vibrational excitations in the three high-pressure phases. Highly resolved Raman-active librons indicate differences in orientational ordering between phases II and III, including evidence for changes within phase II. The librons in phase III are strongly pressure dependent and reflect a substantial increase in ordering with pressure. Multiple vibrons in all three phases (I, II, and III) are observed. The results place new bounds on predicted crystal structures and dynamics of the dense molecular solid.