Low-temperature structures of xenon monolayers adsorbed on graphite

Transmission high-energy electron diffraction (THEED) has been used to study the incommensurate-commensurate ($I-C$) transition of Xe adsorbed on graphite for a pressure of (3.6 ± 0.5) × ${10}^{-7\mathrm{}}$ Torr and a temperature range of 50-70 K, in a transmission electron microscope converted to ultrahigh-vacuum operation. New misfit measurements confirm the existence of the $I-C$ transition at 60 ± 0.5 K. The diffraction patterns do not show any evidence of the striped-domain structure reported from x-ray results. We show that double diffraction causes spot splitting which is different in detail from that expected from striped domains. We have observed double-diffraction effects in the incommensurate region, and used them to obtain the misfit to 0.1% accuracy. Possible reasons for the differences between the THEED and x-ray results are discussed.