C. Ramseyer

Xe monolayer adsorption on Cu(110): Experiments and interaction calculations

Abstract The isosteric heat of adsorption, the lateral energy, the frequency of the perpendicular monolayer vibrational mode and the sequential occurrence of metastable high order commensurate structures for the Xe monolayer adsorbed on the Cu(110) surface have been determined using helium atom scattering experiments. The same quantities are then calculated on the basis of simple expressions of semi-empirical interaction potentials and the pertinency of such expressions in describing the whole body of experimental data is discussed. It is shown that a fair agreement can be obtained in a first approximation with potentials exhibiting a minimum of three parameters.

Adsorption and structure of N2 on copper(110)

The adsorption and structure of ultrathin physisorbed N2 films on Cu(110) has been investigated in a combined experimental and theoretical study. Using specular helium scattering the heat of adsorption for the unconstrained N2 monolayer was determined to qst = 88 meV per molecule and the 2D heat of condensation to q2D = 5.5 meV. In the coverage range up to one monolayer the nitrogen molecules form a complex (4113) high-order commensurate phase with an oblique, quasi-hexagonal unit cell. Increasing the coverage beyond completion of the unconstrained monolayer, the adlayer first undergoes a phase transition into a weakly compressed phase before the condensation of the bilayer sets in. In the bilayer the N2 molecules are arranged in a slightly distorted hexagonal structure. Using realistic interaction potentials, the structure of the N2 monolayer is investigated by detailed total energy calculations. As a result, the most stable structure is found to be the (4113) phase in agreement with experiment. In addition, the arrangement of the nitrogen molecules has been determined: the unit cell contains seven N2 molecules arranged in a novel type of 7-sublattice pinwheel structure. Molecular dynamics simulations corroborate this structure and further demonstrate its remarkable thermal stability.

Structure and phase transitions of xenon monolayers on Cu(110)

The structure of xenon adsorbed on the Cu(110) surface was determined in a combined experimental and theoretical study. The experimental results were obtained using helium-atom diffraction. In the entire temperature and coverage regime studied (20 K ≤ Ts ≤ 70 K and Θ < 1 monolayer) the xenon adlayer can be described in terms of (n × 2) high-order commensurate (HOC) structures, with n ≥ 7. As a result of the weak commensurability along the [110] direction, a series of uniaxial first-order phase transitions between (n × 2) structures with different n is observed as a function of coverage and annealing temperature. In most cases these transitions are not completely reversible, indicating that the apparent stability of some of the HOC phases might be due to kinetic limitations, i.e. an effective “pinning” of the adlayer by the substrate. Along the highly corrugated [001] direction, the adlayer is in perfect registry with the substrate lattice. Inside the (n × 2) unit cell, the xenon atoms form a quasi-hexagonal array. The experimental data were compared to the minimum free-energy configurations of the xenon adlayer calculated for surface temperatures between 0 and 75 K. These calculations are based on parameterized interaction potentials fitted to the measured thermodynamic properties of xenon on Cu(110). The experimental results, in particular the stability of the various HOC phases and their sequence with temperature, is well reproduced by the calculations assuming a corrugation of the holding potential along the [110] direction of about 4 meV. The energy difference between the most stable HOC structures is found to be quite small, in agreement with the observed “metastability” of the structures. The calculations further reveal that the details of the sequence and the temperature range of stability of the HOC phases strongly depends on the corrugation and the exact lattice misfit.

Image states on a free-electron metal surface covered by an atomically thin insulator layer

Abstract Excited electronic states on a free-electron metal surface are investigated in the case when an atomically thin Ar layer is adsorbed on the surface. It is shown that the dielectric character of the Ar layer allows the existence of image states as well-defined resonances in this system. The image states are confined in vacuum by the Ar layer and decay by electron transmission through the Ar layer. A 3D microscopic description of the Ar layer associated to a wave-packet propagation approach is used to determine the characteristics, energy and lifetime, of the image states in a model Ar/free-electron metal system. The possibility of the observation of these states in a time-resolved experiment is discussed.

Role of water molecules in the KcsA protein channel by molecular dynamics calculations

Molecular dynamics simulations supported by electrostatic calculations have been conducted on the KcsA channel to determine the role of water molecules in the pore. Starting from the X-ray structure of the KcsA channel in its closed state at 2.0 angstroms resolution, the opening of the pore towards a conformation built on the basis of EPR results is studied. We show that water molecules act as a structural element for the K+ ions inside the filter and the hydrophobic cavity of the channel. In the filter, water tends to enhance the depth of the wells occupied by the K+ ions, while in the cavity there is a strong correlation between the water molecules and the cavity ion. As a consequence, the protein remains very stable in the presence of three K+ ions in the selectivity filter and one in the cavity. The analysis of the dynamics of water molecules in the cavity reveals preferred orientations of the dipoles along the pore axis, and a correlated behavior between this dipole orientation and the displacement of the K+ ion during the gating process.

Structure of N2 adlayers on the highly corrugated Cu(110)–(2×1)O surface

Abstract The adsorption and structure of molecular nitrogen on the reconstructed Cu(110)–(2×1)O surface has been studied by He-diffraction and temperature programmed desorption. Owing to the “added row” reconstruction, the Cu(110)–(2×1)0 surface exhibits a large corrugation along the [1 1 0] direction, i.e. perpendicular to the Cu–O rows. This has a marked influence on the adsorption properties and structure compared with the bare Cu(110) surface. The N 2 molecules initially adsorb in a lattice gas phase which is stable up to rather high density. In this phase the desorption proceeds via first-order desorption kinetics. With increasing coverage the lattice gas eventually condenses into a (4×3) commensurate phase where the molecules are much more weakly bound. Potential calculations corroborate the existence of a low density phase in which the N 2 molecules adsorb along the troughs between Cu–O added rows with only negligible lateral interactions. The (4×3) phase is found to contain up to eight N 2 molecules per unit cell, half of which occupy sites on top of the Cu–O added rows. A possible novel desorption channel involving a metastable bi-molecular precursor is proposed that could provide a consistent explanation of the N 2 desorption data.

THEORETICAL STUDY OF THE SUM FREQUENCY GENERATION IN MOLECULAR ADSORBATE :NH3/MGO(100)

We present a theoretical study of the sum frequency generation (SFG) for the ammonia molecule adsorbed on the MgO(100) surface. This particular system could be a model for experimentalists in order to test this nonlinear technique, mainly for two reasons. First, the ammonia molecules can be considered in a first approximation as isolated on the surface due to repulsive lateral interactions. Second, the symmetry of ammonia and its particular equilibrium configuration on this surface allow us to separate the SFG response in a part which depends only on the non linear susceptibility of the molecules and an other part that characterizes the experimental setup. As a consequence, the bandshifts and widths of the SFG and infrared (IR) signals can be directly compared. Within the accuracy of the vibrational dependence of the molecular parameters, our calculations predict that the SFG signals connected to the symmetric vibrational modes should be more intense than the antisymmetric ones, in consistency with the IR...

Dynamics of commensurate (N×2;N=3,4) and incommensurate phases of an argon monolayer adsorbed on MgO(100)

Abstract Calculations of the dynamics of three different geometries of an Ar monolayer adsorbed on the MgO(100) substrate are developed within the harmonic approximation in order to determine the dispersion relations, the vibrational spectrum and the inelastic neutron scattering cross section. Several energy loss peaks are obtained for the (3×2), (4×2) commensurate and hexagonal incommensurate geometries. Their shapes and energies strongly depend on the geometry of the adsorbed phase and they can be interpreted as the result of unequivalent adsorption sites for the monolayer atoms. The comparison with the first available inelastic scattering spectra for the (3×2) structure shows that it is possible, at high resolution, to exhibit the various peaks and to interpret them with the present calculations.

Modelling the Cu mono-atomic wire formation on Pt vicinal surfaces using kinetic Monte Carlo simulations

Heteroepitaxial growth of the Cu?Pt system is investigated using kinetic Monte Carlo simulations based on a semi-empirical description of metal?metal interactions to interpret recent experiments devoted to the formation of mono-atomic copper wires on the steps of the vicinal Pt surface. We show that step decoration occurs for the narrow temperature range [250, 300]?K in agreement with growth experiments. An exchange mechanism leading to interlayer diffusion at step edges which could strongly influence the temperature range for which the perfect Cu wires are observed is also introduced in our model. In addition, we find that an activation barrier higher than 0.6?eV for this exchange process is necessary to reproduce the experimental features observed by Gambardella et al. We also show that this process, when active, is responsible for a modification of the scaling law governing the island density evolution on the step.

Confined phonons in finite-size admonolayers

The dynamics of a confined discrete monolayer adsorbed on a corrugated substrate with steps is analyzed through a minimum set of parameters required to characterize the confinement with respect to the infinite system. Using typical values for the force constants which link the layer sides to the substrate, we give analytical conditions for the occurrence of specific modes having a strong amplitude close to the sides of the system. We thus distinguish side modes which have a quasi-acoustical behavior and obey the same features as Rayleigh waves for surfaces from localized modes with a quasi-optical behavior which lie above the bulk bands of the layer. It is concluded that these localized modes should be observed in inelastic helium scattering experiments.