ORDERING AND PHASE-DIAGRAMS OF XENON ADSORBED ON THIN EPITAXIAL NACL(100) FILMS AND ON GE(100)

An adsorption study of Xe on both the bare Ge(100) surface, as well as on thin NaCl films grown pseudomorphically on Ge(100), was carried ou t using high-resolution low-energy electron diffraction. Ordered phase s and phase diagrams were investigated in quasiequilibrium with room-t emperature gas. Starting from low pressures, a transition from a disor dered lattice gas to a distorted hexagonal phase was found for Xe on N aCl(100) in addition to the hexagonal bulk phase, which grows as a mul tilayer phase. The distorted hexagonal phase was found to be commensur ate with the substrate in only one direction. At lower temperatures an d/or higher pressures, the onset of three-dimensional growth is observ ed in the form of a hexagonal nanocrystalline phase (average grain siz e 80 angstrom) with Xe bulk lattice constant which is still oriented a long the [110] direction with one of its axes. Both phase transitions are of first order, with pronounced hysteresis effects for the second. On Ge(100), no additional superstructure spots are induced by Xe befo re the bulk phase grows. The (2x1) reconstruction is maintained, but c haracteristic jumps in intensity of integral order as well as of half- order beams are observed as a function of temperature and pressure so that Xe seems to lock on defined sites in (2x1) registry. Phase diagra ms for Xe adsorbed on both surfaces were derived and heats of transiti on at the phase boundaries were determined.