Orientation ordering in solid H2 and D2 at intermediate J=1 concentrations

Abstract

At temperatures below 4K for zero pressure the previously free rotation of the J=1 excitations in solid H2 or D2 becomes hindered. The hindered rotor is then orientationally ordered. This orientational ordering is discussed for concentrations of J=1 molecules x<0.55. The largest number of experiments that have probed the orientational ordering have used NMR techniques. These experiments as well as approaches using Monte Carlo simulations, x‐ray diffraction, and specific heat measurements will be discussed. The bulk of experimental evidence suggests that the freezing of the orientational degrees of freedom proceeds continuously with no definite transition temperature but the evidence for a transition of a cooperative nature with a well defined transition temperature will also be presented.