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Island formation and condensation of a chemisorbed overlayer
A49 Surface Science 92 (1980) 119-132 © North-Holland Publishing Company
INTERACTION O F C E S I U M A N D O X Y G E N O N W ( 1 1 0 ) t II. C o d e p o s i t i o n : c e s i u m o x i d e f o r m a t i o n C.A. P A P A G E O R G O P O U L O S * a n d J . - L D E S P L A T **
Stanford/NASA Ames Joint Institute for Surface and Microstructural Research, NASA-Ames, Moffett Field, California 94035, USA Received 18 June 1979; accepted for publication 14 September 1979 The codeposition of cesium and oxygen on W ( l l 0 ) has been studied by work function measurements, Auger electron and energy loss spectroscopies and thermal desorption. Thick layers of cesium oxide are grown on the substrate and theix properties depend on which of the two constituents is in excess in the incident fluxes. With an excess of cesium, work functions between 0.9 and 1 eV are obtained for thicknesses around 30 A, and the main cesiated desorp° tion product is Cs20 which has a desorption energy of 1.2 eV. We propose that Cs20 is grown on the surface and that it does not include a large amount of Cs doping. With an excess of oxygen, it is suggested that part of it is imbedded in the Cs20 matrix. A reaction with the substrate occurs below 900 K, leading to the formation of a ternary compound, possibly a cesium tungsten bronze. Surface Science 92 (1980) 133-144 © North-Holland Publishing Company ISLAND FORMATION AND CONDENSATION OF A CHEMISORBED OVERLAYER t T.-M. LU, G.-C. W A N G *, a n d M.G. L A G A L L Y **
Department of Metallurgical and Mineral Engineering and Materials Science Center, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA Received 25 June 1979; accepted for publication 21 September 1979 The condensation of a chemisorbed overlayer from a lattice gas into a particular ordered structure in discussed in terms of the lattice-gas theory of Lee and Yang. The formation of islands of ordered structure is identified with the condensation phenomenon predicted by the theory. The phase diagram (transition temperature versus coverage) based on the theory of a two-dimensional Ising ferromagnet in zero magnetic field is constructed for the condensation of a lattice gas system with net attractive interactions between the particles. It is demonstrated that critical points at coverages other than 0 = 0.5 are achieved for overlayer systems with unit meshes larger than (1 X 1). Low-energy electron diffraction results of the thermal disordering (island dissolution) for oxygen chemisorbed on W(110) are compared with the theory, and the effect of substrate surface heterogeneity on the phase diagram is discussed. Surface Science 92 (1980) 145:-152 © North-Holland Publishing Company RAMAN SCATTERING FROM ADSORBATES ON SILVER A. O T T O
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Physikalisches Institut 111der Universit,~'tDfisseldorf, D-4000 Diisseldorf 1, Fed. Rep. Germany Received 2 May 1979 Raman spectra of chemisorbed cyanide on a silver (110) surface exposed to air show an
INTERACTION O F C E S I U M A N D O X Y G E N O N W ( 1 1 0 ) t II. C o d e p o s i t i o n : c e s i u m o x i d e f o r m a t i o n C.A. P A P A G E O R G O P O U L O S * a n d J . - L D E S P L A T **
Stanford/NASA Ames Joint Institute for Surface and Microstructural Research, NASA-Ames, Moffett Field, California 94035, USA Received 18 June 1979; accepted for publication 14 September 1979 The codeposition of cesium and oxygen on W ( l l 0 ) has been studied by work function measurements, Auger electron and energy loss spectroscopies and thermal desorption. Thick layers of cesium oxide are grown on the substrate and theix properties depend on which of the two constituents is in excess in the incident fluxes. With an excess of cesium, work functions between 0.9 and 1 eV are obtained for thicknesses around 30 A, and the main cesiated desorp° tion product is Cs20 which has a desorption energy of 1.2 eV. We propose that Cs20 is grown on the surface and that it does not include a large amount of Cs doping. With an excess of oxygen, it is suggested that part of it is imbedded in the Cs20 matrix. A reaction with the substrate occurs below 900 K, leading to the formation of a ternary compound, possibly a cesium tungsten bronze. Surface Science 92 (1980) 133-144 © North-Holland Publishing Company ISLAND FORMATION AND CONDENSATION OF A CHEMISORBED OVERLAYER t T.-M. LU, G.-C. W A N G *, a n d M.G. L A G A L L Y **
Department of Metallurgical and Mineral Engineering and Materials Science Center, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA Received 25 June 1979; accepted for publication 21 September 1979 The condensation of a chemisorbed overlayer from a lattice gas into a particular ordered structure in discussed in terms of the lattice-gas theory of Lee and Yang. The formation of islands of ordered structure is identified with the condensation phenomenon predicted by the theory. The phase diagram (transition temperature versus coverage) based on the theory of a two-dimensional Ising ferromagnet in zero magnetic field is constructed for the condensation of a lattice gas system with net attractive interactions between the particles. It is demonstrated that critical points at coverages other than 0 = 0.5 are achieved for overlayer systems with unit meshes larger than (1 X 1). Low-energy electron diffraction results of the thermal disordering (island dissolution) for oxygen chemisorbed on W(110) are compared with the theory, and the effect of substrate surface heterogeneity on the phase diagram is discussed. Surface Science 92 (1980) 145:-152 © North-Holland Publishing Company RAMAN SCATTERING FROM ADSORBATES ON SILVER A. O T T O
*
Physikalisches Institut 111der Universit,~'tDfisseldorf, D-4000 Diisseldorf 1, Fed. Rep. Germany Received 2 May 1979 Raman spectra of chemisorbed cyanide on a silver (110) surface exposed to air show an