J. Suzanne

Transition bidimensionnelle du premier ordre; cas du xénon adsorbé sur la face (0001) du graphite

Submonolayer adsorption isotherms of xenon condensed on the (0001) face of graphite are measured between 85°K and 102°K by Auger electron spectroscopy. A two-dimensional phase change 2D gas ⇄ 2D solid is emphasized. The solid phase is characterized by low energy electron diffraction. It is a two-dimensional crystal in epitaxy on the graphite. The analysis of the adsorption isotherms measured with a sensivity of 1500 of monolayer, i.e. 1010 atoms, allows to determine the integral heat of adsorption at the two-dimensional phase change (5.5 ± 0.1 kcal mole−1). We also deduced from our measurements, the binding energy of an individual atom of xenon on the (0001) face of graphite, the heat and the entropy of fusion of the two-dimensional crystal.

Crystallography and growth modes of thick physisorbed films on graphite

Abstract Electron diffraction of physisorbed layers on graphite has been investigated in the low energy (LEED) and reflection high energy (RHEED) geometries, for several “simple” molecules (Xe, Kr, Ar, Ne, N 2 , O 2 , CF 4 ). For multilayer films, RHEED is shown to be very sensitive to the growth mode, readily distinguishing layer growth (or type I adsorption) from layer plus three-dimensional island growth (or type II adsorption). In the present paper, the RHEED geometry and crystallography of these multilayer films is analysed. We find that Xe, Kr and Ar grow as layers in approximately the √3 × √3 R30° orientation; Ne, however, grows in the layer plus island mode in two domains rotated ±17° (±2°) from this orientation. All the molecular systems investigated grow in the layer plus island mode at suitably low temperatures, with rotated domains. These systems are analysed as far as the data allow. The results are discussed briefly in terms of current theories of adsorption and crystal growth.

A LEED and neutron diffraction study of hexane adsorbed on graphite in the monolayer range: uniaxial commensurate-incommensurate transition

Abstract The structure of a n-hexane molecule (C6H14) adsorbed on graphite has been studied by LEED and neutron diffraction in the range 100 to 177 K. Below 151 K the monolayer has an ordered herringbone structure. The rectangular unit mesh is commensurate 4 × √3 in only one direction. Along the other direction, the lattice constant is 5.3 ± 0.05 A . With increasing coverage, a continuous uniaxial incommensurate to commensurate transition to a 2 × 4√3 phase occurs. At 151 K the ordered structure is replaced by a fluid-like phase, as indicated by a diffuse ring in the LEED pattern. This melting transition appears to be first order. The isosteric heat at monolayer condensation is measured to be 16 kcal/mol via LEED graphite spot intensity isotherms from 160 to 177 K.

CO adsorbed on MgO(100): a high resolution LEED study

The monolayer structure of CO molecules adsorbed on MgO(100) single crystal surfaces cleaved in situ has been analysed by LEED within the 30–56 K temperature range. At T ⩽ 40 K CO forms a 2 × 4 commensurate 2D solid phase. A sharp uniaxial transition occurs above this temperature, along the [10] surface direction which locks the monolayer into a new commensurate 2 × 3 phase stable over a temperature range of 8 K. Above 50 K, this second commensurate phase expands itself uniaxially in a sharp transition toward a solid with disorder increasing with temperature. This succession of transitions is an interesting illustration of the incomplete “devils staircase”.

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

Abstract 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.

Interaction of ammonia molecules with the MgO(100) surface: Application to the measure of the effective ionic surface charge

The interaction of ammonia molecules with the (100) surface of MgO has been investigated on uniform powders and single crystal surfaces using volumetric isotherm and low energy electron diffraction measurements in the temperature ranges of 170–205 and 90–120 K, respectively. The kinetics of condensation and the isosteric heat of adsorption have been measured. The results show that there is a strong repulsive interaction between NH3 molecules within the adsorbed monolayer. No ordered structure has been found by low energy electron diffraction up to monolayer coverage. The comparison of our experimental value of the isosteric heat of adsorption at low coverage and at monolayer completion with the calculated value using semiempirical potentials found in the literature leads to an estimated value of the effective surface ionic charge q/e=1.25.

Dynamics of ice layers deposited on MgO(001): Quasielastic neutron scattering experiments and molecular dynamics simulations

The dynamical behavior of a thin film of ice Ih deposited on MgO(001) surface has been investigated both experimentally and theoretically. Incoherent neutron quasielastic scattering experiments, using uniform MgO powders, show that a quasiliquid water layer of monolayer thickness exists at T=265 K. The translational mobility of this layer, with a diffusion coefficient Dt=1.5×10−5 cm2 s−1, is close to that of liquid water. At T=270 K, the thickness of the quasiliquid layer increases to about two layers, showing no appreciable change in the Dt value but an increase of the rotational mobility from 6×109 s−1 to 1.2×1010 s−1. Classical molecular dynamics simulations are performed to determine the translational and orientational order parameters and diffusion coefficients of the supported ice film as a function of temperature within 190 and 270 K, and to compare the results with those obtained for bulk ice. It is shown that the whole supported ice film is much more disordered than bulk ice, with melting tempera...

Structure and dynamics of ice Ih films upon HCl adsorption between 190 and 270 K. I. Neutron diffraction and quasielastic neutron scattering experiments

Neutron diffraction and quasielastic neutron experiments are performed to investigate the effect of HCl adsorption on the structure and dynamics of an ultra-thin ice Ih film (5 H2O bilayers thick) deposited on a crystalline MgO(001) substrate. Three HCl coverages have been studied 0.3, 0.6, and 1 monolayer (ML) in the temperature range 190–270 K. At 0.3 and 0.6 HCl monolayer, no mobility is measured at T⩽220 K. A translational mobility, which is a signature of a liquid phase, is observed at T=250 K. This phase occurs 15 K below the surface melting temperature of the bare ice film. The fraction of mobile molecules represents 30% (0.3 ML HCl) and 45% (0.6 ML HCl) of the film. At 1 HCl monolayer and T=220 K, HCl–dihydrate coexists with ice Ih, whereas at T=250 K the ice film becomes amorphous and only 9% of the film is mobile. The results are discussed within the context of atmospheric chemistry.

Structure and thermodynamics of acetylene C2H2 on MgO(100)

Abstract The structure and thermodynamics of C 2 H 2 monolayers adsorbed onto MgO(100) single crystal surfaces have been studied by LEED experiments between 40 and 94 K. The surface was prepared by cleaving a crystal in situ under ultrahigh vacuum conditions. The monolayer presents an ordered commensurate (2 × 2) structure. We propose a unit cell containing two molecules and having two glide planes. Furthermore, adsorption isotherms and adsorption isobars were performed in order to establish a phase diagram and to determine the isosteric heat of condensation at half monolayer coverage.

Structure and dynamics of ice Ih films upon HCl adsorption between 190 and 270 K. II. Molecular dynamics simulations

Classical molecular dynamics simulations are carried out between 190 and 250 K on an ultrathin ice film doped by HCl deposition with a coverage varying from 0.3 to 1.0 monolayer. These conditions are similar to those defined in the experiments described in the companion paper. Within the assumption that the hydracid molecule remains in its molecular form, the order parameters and the diffusion coefficients for the H2O molecules are determined in the HCl doped ice film, and compared to the experimental data. The residence times of HCl at the ice surface are also calculated. Below 200 K, the HCl molecules are found to remain localized at the ice surface, while above 200 K, the HCl diffusion inside the film is easy and leads to a strong disorder of the ice structure. Although the formation of hydrates cannot be interpreted by the present calculations, the lowering of the ice melting temperature by 15 K measured in neutron experiments for an HCl doped ice film is qualitatively explained by simulation results.