Abstract

Low Energy Electron Diffraction experiments have been performed on an ethane monolayer adsorbed on graphite single crystals for 70 < T < 85 K. The substep previously observed between the first and second layers on volumetric isotherms, at higher temperature, is identified as being the transition between an incommensurate phase I 2 and a √3 × √3 commensurate solid phase S 3. From photometric isotherms, the isosteric heat of adsorption has been determined ( Q st = 7.6 ± 0.5 kcal/mol), in agreement with the previous measurements. From LEED patterns, it is shown that there is not only a first order jump in the lattice parameter at the incommensurate-commensurate transition, but also an abrupt change of 30° in the orientational epitaxy: S 3 is 30° rotated with respect to the graphite axes; I 2 is not rotated even when its parameter differs only by 3% from the S 3 one. This striking behaviour may be due to the nature of I 2 which could be a rather well correlated fluid like phase.

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