- Email: [email protected]
Surface melting and superheating
A666 Surface Science 294 (1993) 265-272 North-Holland
The coadso
tion of Ne and H 2 on graphite
Feng-Chuan Liu 1, Yuan-Ming Liu and Oscar E. Vilches Department of Physics, University of Washington, Seattle, WA 98195, USA Received 8 March 1993; accepted for publication 21 May 1993 We have measured the heat capacity of H 2 films adsorbed on graphite (Gr) plated with a monolayer of Ne, of H 2 / G r films at monolayer completion and in the initial stages of bilayer film formation, and of monolayer, 2.5 layer, and 4.5 layer films of N e / G r , From the H 2 / N e / G r measurements we find evidence of the formation of bilayer H 2 on bare graphite, at the expense of displacing up to 80% of the Ne preplating. This is supported by an energy calculation based on a model proposed by Asada et al. A comparison between the Ne heat capacity extracted from the H 2 / N e / G r measurements and the measured N e / G r heat capacity indicates that the Ne taking the remaining graphite surface area is likely to be in the form of an ~ 5 layers thick film.
Surface Science 294 (1993) 273-283 North-Holland
Surface melting and superheating Y. Teraoka Department of Physics, University of Osaka Prefecture, Sakai 593, Japan Received 17 February 1993; accepted for publication 25 May 1993 In order to obtain microscopic understanding of surface melting, in particular the role in bulk melting transitions, a semi-infinite system with a surface is investigated by use of a simple model describing gas, liquid and solid phases on the same ground. Surface melting appears and the number of quasi-liquid layers increases with increasing temperature towards the melting point T M. At relatively low temperatures, the boundary between quasi-liquid and solid layers continuously moves into the crystal with increasing temperature. While approaching T M, however, it starts to move discontinuously. Such a step-like movement with temperature is found to be due to the existence of many stable states with different numbers of quasi-liquid layers. Each state corresponds to a local minimum of the free energy. Even above T M, many such states can be found. This leads to the appearance of superheating under some experimental conditions. However, superheating cannot be found, if the system is in the thermodynamical equilibrium state.
Surface Science 294 (1993) 284-296 North-Holland
The bimetallic Pt/Mo(ll0) surface: structural and CO chemisoq tion studies Amy Linsebigler, Guangquan Lu and John T. Yates, Jr. Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA Received 26 February 1993; accepted for publication 2 June 1993 Auger electron spectrosco~ (AES), low energy electron diffraction (LEED), and temperature-programmed desorption (TPD) measurements were performed to monitor the growth, thermal stability, and CO adsorption characteristics of Pt layers on Mo(ll0). Pt grows in a layer-by-layer or Frank-van der Merwe fashion on the molybdenum surface below 600 K. The first monolayer of Pt is stable and no signifmant structural change is observed at higher substrate temperatures, A monolayer of Pt eliminates the dissociative adsorption of CO observed on the Mo(110) surface. The thermal desorption of CO from this Pt overla~-r occurs at lower temperatures (in comparison to CO desorption from the bulk Pt(111) and Mo(110) surfaces), indicating a withdrawal of electron density from the Pt monolayer due to interactions with the Mo(110) surface. As the Pt coverage is increased above a monolayer, LEED measurements indicate the growth of Pt(111) layers at 300 K. The muitilayers of Pt are thermally unstable and agglomerate into three-dimensional (3D) clusters upon heating to above ~ 600 K. The 3D clusters contr~ute little to the AES signal. The CO desorption spectra from the annealed layers are very similar to the CO desorptioa spectra from one monolayer of Pt on the Mo(110) surface, suggesting that Pt clustering occurs on top of a single Pt monolayer on Mo(110). The spatial distribution of Pt clusters is estimated from Auger intensity changes following annealing of a 6 monolayer Pt film.
The coadso
tion of Ne and H 2 on graphite
Feng-Chuan Liu 1, Yuan-Ming Liu and Oscar E. Vilches Department of Physics, University of Washington, Seattle, WA 98195, USA Received 8 March 1993; accepted for publication 21 May 1993 We have measured the heat capacity of H 2 films adsorbed on graphite (Gr) plated with a monolayer of Ne, of H 2 / G r films at monolayer completion and in the initial stages of bilayer film formation, and of monolayer, 2.5 layer, and 4.5 layer films of N e / G r , From the H 2 / N e / G r measurements we find evidence of the formation of bilayer H 2 on bare graphite, at the expense of displacing up to 80% of the Ne preplating. This is supported by an energy calculation based on a model proposed by Asada et al. A comparison between the Ne heat capacity extracted from the H 2 / N e / G r measurements and the measured N e / G r heat capacity indicates that the Ne taking the remaining graphite surface area is likely to be in the form of an ~ 5 layers thick film.
Surface Science 294 (1993) 273-283 North-Holland
Surface melting and superheating Y. Teraoka Department of Physics, University of Osaka Prefecture, Sakai 593, Japan Received 17 February 1993; accepted for publication 25 May 1993 In order to obtain microscopic understanding of surface melting, in particular the role in bulk melting transitions, a semi-infinite system with a surface is investigated by use of a simple model describing gas, liquid and solid phases on the same ground. Surface melting appears and the number of quasi-liquid layers increases with increasing temperature towards the melting point T M. At relatively low temperatures, the boundary between quasi-liquid and solid layers continuously moves into the crystal with increasing temperature. While approaching T M, however, it starts to move discontinuously. Such a step-like movement with temperature is found to be due to the existence of many stable states with different numbers of quasi-liquid layers. Each state corresponds to a local minimum of the free energy. Even above T M, many such states can be found. This leads to the appearance of superheating under some experimental conditions. However, superheating cannot be found, if the system is in the thermodynamical equilibrium state.
Surface Science 294 (1993) 284-296 North-Holland
The bimetallic Pt/Mo(ll0) surface: structural and CO chemisoq tion studies Amy Linsebigler, Guangquan Lu and John T. Yates, Jr. Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA Received 26 February 1993; accepted for publication 2 June 1993 Auger electron spectrosco~ (AES), low energy electron diffraction (LEED), and temperature-programmed desorption (TPD) measurements were performed to monitor the growth, thermal stability, and CO adsorption characteristics of Pt layers on Mo(ll0). Pt grows in a layer-by-layer or Frank-van der Merwe fashion on the molybdenum surface below 600 K. The first monolayer of Pt is stable and no signifmant structural change is observed at higher substrate temperatures, A monolayer of Pt eliminates the dissociative adsorption of CO observed on the Mo(110) surface. The thermal desorption of CO from this Pt overla~-r occurs at lower temperatures (in comparison to CO desorption from the bulk Pt(111) and Mo(110) surfaces), indicating a withdrawal of electron density from the Pt monolayer due to interactions with the Mo(110) surface. As the Pt coverage is increased above a monolayer, LEED measurements indicate the growth of Pt(111) layers at 300 K. The muitilayers of Pt are thermally unstable and agglomerate into three-dimensional (3D) clusters upon heating to above ~ 600 K. The 3D clusters contr~ute little to the AES signal. The CO desorption spectra from the annealed layers are very similar to the CO desorptioa spectra from one monolayer of Pt on the Mo(110) surface, suggesting that Pt clustering occurs on top of a single Pt monolayer on Mo(110). The spatial distribution of Pt clusters is estimated from Auger intensity changes following annealing of a 6 monolayer Pt film.