A LEED study of ethane films adsorbed on graphite in the monolayer range

A48 of residual short-range orientational ordering and perhaps finite size or heterogeneity effects. For increasing coverage at T = 15 K, there is first a transition to a previously unobserved uniaxial incommensurate phase and then a transition to an apparently triangular incommensurate phase. The orientational superlattice spots are clearly present in the uniaxial phase, but are much weaker in the triangular incommensurate phase. At 31 < T < 35 K, an apparently triangular incommensurate phase with no detectable orientational superlattice spots is observed. The lattice constant versus equilibrium vapor pressure curve has been determined in the latter case assuming a continuous transition. The lattice constants of the incommensurate phases are used to place limits on the extent of possible phase-coexistence regions between the commensurate, uniaxial incommensurate, and triangular incommensurate phases. The LEED patterns from the bilayer at T = 15 K indicate a double-period superlattice structure of the triangular incommensurate phase which does not have the glide line symmetries of the commensurate monolayer. Some effects of heterogeneity on these phase transitions are discussed. A phase diagram for 10 < T < 40 K is proposed.

Surface Science 125 (1983) 153-170 North-Holland Publishing Company

153

A LEED S T U D Y OF ETHANE FILMS A D S O R B E D ON GRAPHITE IN THE M O N O L A Y E R RANGE J. S U Z A N N E , J.L. SEGUIN, H. TAUB * and J.P. BIBERIAN Facultb des Sciences de Luminy, Dbpartement de Physique, Case 901, F-13288 Marseille Cbdex. France Received 5 July 1982; accepted for publication 20 August 1982 Ethane monolayers adsorbed on graphite in the monolayer range have been studied by high-resolution LEED in the range 40 < T < 85 K. Three two-dimensional solid phases S I, S2, S3 have been observed at temperatures below 60 K in agreement with previous neutron diffraction experiments. The LEED studies show that S 1 is a 4×Vc3 commensurate rectangular centred cell with a herring-bone packing. It transforms at 64 K into a short range ordered solid I I having the hexagonal symmetry of a 2 x 2 commensurate phase. Then 11 transforms continuously into an isotropic liquid-like phase with increasing temperature. S2 appears to be a 10 × 2~f3 rectangular structure with 12 molecules per unit cell. Finally S3 has a commensurate v/3 x Vc3 unit cell with molecules probably standing on their methyl tripods. This is the first physisorbed system where a complete solution of an orientationally ordered structure (S I) has been obtained by combining neutron diffraction and LEED experiments.

Surface Science 125 (1983) 171-180 North-Holland Publishing Company

171

S Y N C H R O T R O N X-RAY S T U D Y OF S U B - M O N O L A Y E R S OF CF4 O N GRAPHITE K. KJAER, M. N I E L S E N and J. BOHR Ris~ National Laboratory, DK-4000 Roskilde, Denmark