A new class of structures is predicted to exist for monolayer films on solid surfaces. These structures involve two incommensurate lattices-the monolayer lattice and the surface lattice-which have a preferred relative orientation. The precise orientation depends upon the lattice constant and symmetry of each lattice. We believe this orientationally ordered incommensurate phase to be present in many physisorbed films. In particular, the existence of this phase appears to explain recent low-energy electron diffraction data for rare gases adsorbed on homogeneous substrates.

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