QUANTUM AND CLASSICAL ORIENTATIONAL ORDERING IN SOLID HYDROGEN

We present a unified view of orientational ordering in phases I, II, a nd III of solid hydrogen. Phases II and III are orientationally ordere d, but the ordering objects in phase II are angular moments of rotatin g molecules, whereas in phase III the molecules order themselves. This concept provides a quantitative explanation for the vibron softening, libron and roton spectra, and the increase of the vibron effective ch arge in phase III, as well as a framework for understanding the topolo gy of the phase diagram and ortho-pam state at high pressure. The effe ctive charge and the infrared and Raman vibron frequency shifts are al l Linear in the order parameter in phase III.