Melting of the first monolayer of Ar adsorbed on the basal face of graphite

We have measured adsorption isotherms of Ar on graphite from 67 to 77 K in the range of coverages at which the monolayer freezes. The transition appears continuous and shows some similarity with a λ transition. The large increase of the isothermal compressibility at freezing seems to indicate that it cannot be accounted for by the Kosterlitz-Thouless-Halperin-Nelson-Young theory.     

On the orientational alignment of neon adsorbed on basal plane graphite

The Novaco-McTague theory of orientational alignment is discussed for neon adsorbed on basal plane graphite. A degenerate case of the theory which occurs for a √7 superlattice is evaluated and is shown to lead to a novel registry energy.     

Density profiles of a chemically reacting fluid adsorbed on the graphite basal plane

Density profiles for chemically reacting hard spheres near the crystalline surface of the graphite basal plane are investigated by using the integral equation approach. The dependence of the profiles on the density of the bulk fluid, the degree of dimerization and the fluid-solid interaction potential is studied. It is shown that the density profiles of this chemically reacting fluid near the crystalline surface exhibit new features, compared with the case of a hard sphere fluid near a structureless wall.The authors wish to express gratitude to the CONACYT of México (Grant No. 4186-E9405 and el Fondo para Cátedras Patrimoniales de Excelencia) for financial support of the project. S.S. thanks KBN for partial financial support (Grant No. 2P30307705). We would like to express our thanks to A. Malijevsky for valuable discussions and comments. O.P. is grateful to C. Lozada Garcia for helpful discussions.     

Heat capacity study of the first monolayer of adsorbed methane on grafoil

The heat capacity of CH₄ adsorbed on Grafoil below monolayer completion has been determined experimentally for the first time. The specific heat at constant density shows two characteristic features, very broad, weak anomalies at 47.6 K (around one third of a monolayer) and (for occupation numbers below two thirds of a monolayer) very narrow, strong peaks at 56.35 K. The anomalies at 47.6 K are interpreted as being due to a registry — out-of-registry transition of the two dimensional (2D) solid CH₄ adsorbate into an incommensurate expanded structure. The anomalies at 56.35 K define the 2D triple point of adsorbed CH₄. For higher occupation numbers the liquid-solid phase boundary shifts to higher temperatures. The present data make partial re-definition of the 2D-CH₄ phase diagram necessary.     

Fluid-registered solid transition in a monolayer of carbon monoxide adsorbed on the basal face of graphite

Adsorption isotherms of CO on the basal face of graphite have been measured, showing a first order transition within the dense monolayer. There is high a priori probability that this transition is the analogue of the fluid √3 × √3 R 30° transition of the isoelectronic molecule N₂ adsorbed on the same substrate. The comparison between the thermodynamic properties of both transitions, using in particular a corresponding states law scaling, very strongly supports this conjecture.     

Orientational epitaxy of an incommensurate neon monolayer adsorbed on graphite

The structure of a monlayer of neon adsorbed on the (0001) surface of a graphite single crystal has been studied at T? 10 K by using high resolution Low Energy Electron Diffraction. The incommensurate monlayer is rotated by ?17° with respect to the (√3 × √3) 30° commensurate structure for a mean overlayer interatomic spacing d? 3.24 Å. Thus result agrees with the prediction of Novaco—McTague theory.     

Equilibrium configuration of an N2 monolayer adsorbed on graphite

A nitrogen monolayer adsorbed on graphite is found to be unstable with respect to a registered one sublattice structure. A four sublattice structure, which stabilizes the monolayer, is postulated. Phonon dispersion curves for the four sublattice structure are presented.     

Localised to non-localised transition in a krypton monolayer adsorbed on graphite

It is shown theoretically that a solid krypton monolayer can be constrained to lie in registry with a graphite adsorbent. This provides an explanation for a step in the experimental isotherm of krypton on graphite at 90.1 K.     

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

The submonolayer structural properties of methane on graphite have been studied by means of low energy electron diffraction in the range 30–67 K, allowing the determination of the orientational epitaxy of the CH₄ monolayer adsorbed on graphite single crystals. Thermodynamic adsorption properties have also been measured, and are compared with earlier volumetric measurements recorded in the temperature range 77–84 K.     

Atom-solid scattering: He + graphite basal plane

Theoretical studies of scattering of atoms by solid surfaces and calculations of atomic band structure of adsorbed atoms are presented. Inelastic effects on the intensities of diffracted beams are considered within the framework of the optical model potential. The atom-solid potential used for carrying out the numerical calculation is the sum of Lennard-Jones 12-6 pair potentials for the He-graphite system. Resonances with bound states of adsorbed atoms are shown to enhance the diffraction into some of the open channels and inelastic scattering. The degree of enhancement for a certain process depends on the coupling between that process and the resonant bound state and the coupling between the bound state and the incident beam. For first order bound state resonances, minima in the specular intensity may result from a large increase in some of the diffracted intensities or enhancement of inelastic scattering.     

Adsorption of krypton on the basal plane of graphite: LEED and Auger measurements

The adsorption of krypton on (0001) graphite has been studied by LEED and Auger. Stepwise isotherms are observed and thermodynamic quantities such as the latent heat of two dimensional adsorption and the binding energy of a krypton atom are determined. The mean free path of electrons in krypton is measured. The LEED pattern of the krypton layer shows a √3 × √3 superstructure. Some vibrational properties are examined by LEED and possible implications of the experimental findings are discussed. The potentials and limitations of the techniques used in the present work are critically examined with respect to other techniques.     

Effect of structural misalignments on the temperature variation of basal plane electrical conductivity of graphite

The amount and extent of structural misaligments in natural graphite crystals have been determined, and the temperature variation of the basal plane electrical conductivity (σ_⊥) of naturally occurring graphite has also been studied from 300 to 90 K. The conductivity (σ_⊥) has been found to obey a law σ_⊥α($\text{1}\text{T}$) down to a certain temperature θ (θ varying from sample to sample), below which the variation deviates from linearity towards lower values of σ_⊥. This behaviour, which was earlier thought to be a characteristic of graphite and whose origin could not be traced, has been shown to be an effect of the structural misalignments usually present in natural samples of graphite.     

Selective adsorption of 3He and 4He on the basal plane surface of graphite

Atomic beam scattering was used to measure the binding energies of selectively adsorbed states of two isotopes of helium on the basal plane of graphite. The results are used to determine semi-empirical potential parameters, and the ground state energies are compared with values derived from adsorption studies for both isotopes.     

NMR study of n-dodecane adsorbed on graphite

In this brief contribution we demonstrate that 1H and 2H NMR spectroscopy can be an effective method of investigating adsorption from liquids at the solid-liquid interface. The method is illustrated here with the adsorption of a simple alkane adsorbed on graphite, in particular the system n-dodecane and graphite at coverages of 1 and 5 monolayers. Static single-pulse proton nuclear magnetic resonance and static quadrupolar echo deuterium nuclear magnetic resonance spectra were recorded for both coverages. The experimental NMR results presented here show features clearly consistent with earlier calorimetric and neutron scattering work and demonstrate the formation of solid adsorbed layers that coexist with the bulk adsorbate with both isotopes. This ability to probe both deuterated and protonated materials simultaneously illustrates that this experimental approach can be readily extended to investigate the adsorption behaviour of multicomponent mixtures.     

X-ray diffraction on chlorine adsorbed on graphite

Cl₂ layers adsorbed on Grafoil have been investigated by X-ray diffraction between 78 and 285 K. Mono- and submonolayers crystallize in a parallelogram lauttce, incommensurate with the (001) graphite surfaces, the molecules oriented almost perpendicular to the surface. The structure is closely related to that of the (001) planes of three-dimensional solid chlorine. For coverages above the monolayer completion bulk crystalization sets in.