Orientational disorder and melting of CF2Cl2 monolayers physisorbed on graphite

We present specific heat experiments on CF2Cl2 monolayers physisorbed on gr aphite and determine the entropy changes at the structural and melting phas e transitions. The results confirm previously reported x-ray studies where a phase sequence from a low-temperature commensurate hexagonal phase to an incommensurate triangular mesophase and finally the liquid phase was observ ed. The temperature dependence of the excess entropy reveals that the low t emperature phase is orientationally ordered. Angular disorder with respect to reorientation around the surface normal evolves in the mesophase, and is fully developed just below the melting transition where the molecule start s to rotate around an axis parallel to the surface. We observe strong evide nce for two-dimensional random exchange or random field behavior at the mel ting transition of the mesophase due to the bond; disorder produced by the rotation about an axis parallel to the surface.