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Orientational ordering of hydrogen molecules adsorbed on graphite Kubik, Philip Roman


NMR has been employed to measure the orientational behaviour of sub-monolayers of H₂ and D₂ adsorbed on graphite from .3 K to 12 K. For the temperature and coverages used, the hydrogen forms a lattice commensurate with that of the graphite (√3 x √3 structure) and can be modelled by a hexagonal lattice of interacting quantum quadrupoles. Each molecule also experiences a crystal field arising from Van der Waals interactions with the substrate and with the other hydrogen molecules. For such a system, mean field theory predicts a variety of orientationally ordered phases, depending on the relative strength of the crystal field and the molecular field, which are always in opposition. A key question to be answered by experiment was whether the strong fluctuations associated with quantum effects and the two dimensional nature of the system would in fact allow a finite transition temperature. We have observed that for 90% ortho-H₂ (J=l), the splitting of the high temperature NMR doublet increases rapidly with decreasing temperature near .6 K and additional structure appears. In addition, the .3 K spectrum agrees well with the expected T=0 spectrum for the orientationally ordered pinwheel phase of mean field theory. Consequently, the rapid increase in the splitting is interpreted as an orientational ordering transition. In contrast to the results for H₂, the NMR spectrum of 90% para-D₂ (J=l) shows no rapid changes, evolving smoothly from a doublet to a very broad and weak structure. There is no evidence of orientational ordering, at least down to .3 K. The different behaviour of H₂ and D₂ is particularly puzzling in light of the fact that the bulk solids both order orientation-ally into the same structure at similar temperatures. From the temperature dependence of the splitting of the high temperature NMR doublets, we have extracted the values of the crystal fields and effective quadrupole coupling constants of H₂ and D₂. The quadrupole coupling constant of D₂ is somewhat less than the rigid lattice value as one would expect in the presence of translational zero point motion. However, that of H₂ is much too small for this mechanism to be responsible.

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