Orientational ordering of ethylene on graphite

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Abstract
A neutron diffraction study of the low temperature structure and loss of orientational order was performed for the low density phase of ethylene adsorbed on graphite. At temperatures below 30 K the ethylene molecules form a centered-rectangular herringbone structure with their C=C bond parallel to the surface. At T > 30 K we observe a gradual loss of orientational order and a simultaneous increase in the near-neighbor spacing. This correlates well with the onset of rotational motions about an axis perpendicular to the C=C bond which remains predominately parallel to the surface plane. We speculate that the transition to this rotator phase is accompanied by a structural change to a triangular lattice.

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