Isabelle Derycke

Physisorption in confined geometry

The physisorption of molecules in confined geometry, i.e., in pores of atomic size such as found in zeolites, has been investigated using a simple pairwise‐additive Lennard‐Jones potential and an effective‐medium model. In a spherical geometry, it is found that the equilibrium distance D corresponding to the lowest equilibrium energy is reduced to about 90% of the pair equilibrium distance σe. This originates from the increased dominance of long‐range forces in the condensed state. The enhancement of the physisorption energy due to surface curvature and confinement effects reaches its maximum value of 5.05, relative to the flat surface, when D=0.899σe. This value must be compared to the factor of 8 which was derived previously [D. H. Everett and P. C. Powl, J. Chem. Soc. Faraday Trans. 1 72, 619 (1976); E. G. Derouane, J.‐M. Andre, and A. A. Lucas, Chem. Phys. Lett. 137, 336 (1987)] using a simple van der Waals model neglecting repulsion forces. It is also concluded that molecules can be strongly trapped ...

Phonon surface loss function of ionic-crystal films: A comparison between microscopic and macroscopic approaches

Abstract The surface loss function is an important quantity in the description of reflection electron-energy-loss spectroscopy (EELS) in near-specular geometry. In this paper, lattice-dynamics calculations of the surface energy-loss function associated with optical excitations in ionic-crystal slabs are performed. Test calculations are carried out for both NaF isolated films and NaF slabs onto a semi-infinite substrate. In the latter case, a continuum dielectric theory is used to describe the response of the substrate, emphasis being on long-wavelength optical phonons. We compare the results of the microscopic formulation of the surface loss function of the film with those deduced from the so-called dielectric theory, where a bulk-like dielectric response of the slab is assumed. The legitimacy of the use of a bulk response is a central question in applications of the dielectric theory to a very thin slab. It is shown that the latter provides a valid description of the surface energy-loss function for the wave vectors predominantly probed in EELS when the film thickness exceeds ~ 2 nm (about 10 atomic planes). For very thin, relaxed films (less than 8 atomic layers), the dielectric theory fails in producing the correct frequency positions of the surface energy-loss peaks; by contrast the intensities of the loss peaks are more accurately predicted.

van der Waals interaction at a material wedge

The problem of evaluating the van der Waals attraction energy of an atom or microparticle adsorbed near the edge of a straight material wedge of arbitrary opening angle is considered. In order to disentangle more clearly the effect of the wedge geometry from the material parameters, a simple model is used in which the particle is represented by a harmonic oscillator and the substrate by a perfect conductor. By a straightforward extension of the method of images of classical electrostatics, the dipolar coupling of the particle to the substrate is constructed from which new oscillator frequencies can be determined as a function of its position relative to the edge and of the wedge angle. The van der Waals energy is given by the shift in zero‐point energy of the oscillator coupled to itself via its images. For a physisorbed atom crossing the edge of a wedge material, the detailed shape of the van der Waals energy barrier (respectively, well) offered by a convex (respectively, concave) wedge is displayed and compared with approximate results deduced from a pairwise summation of Lennard‐Jones interactions. The interest of these results for the dynamics and kinetics of sorption by microporous solids is briefly discussed.

Theory of electromagnetic energy transfer in three-dimensional structures

Abstract The practical computation of electromagnetic energy transfer through an inhomogeneous dielectric or conducting film is considered at the level of vector waves multiple scattering theory. Maxwells equations are formulated in the Laue representation and the scattering boundary conditions are implemented using a transfer-matrix technique. This formulation leads to a fast algorithm if all convolution products involved are handled by fast Fourier transform techniques.

Theoretical aspects of scanning tunneling microscopy

Abstract This paper discusses recent theoretical efforts towards developing a strong-coupling theory of tunneling for application to STM, one which is capable of including the perturbation caused by the tip atoms to surface atoms in the image formation at close tip-surface separations. Several examples of tip effects will be shown for illustration, in particular: (1) the asymmetry of the I–V characteristics due to the sharpness of the tip and the 3D nature of the tunneling and (2) the variation of atom visibility on the graphite surface as a function of the tunneling distance.

Three dimensional scattering and scanning tunneling microscope images

Abstract This paper describes the computation of the tunneling current in a scanning tunneling microscope (STM) up to the geometric contact of the tip with atoms in a cluster adsorbed on a flat conducting surface. The calculation accounts for the three-dimensional scattering taking place simultaneously in the top atomic layer of the sample and in the apex of the probing tip. The model is built with the following ingredients: (a) the tip is represented by a cluster of atoms attached to an otherwise planar free-electron metal surface, and (b) the analysed sample is a planar free-electron metal with a local potential corrugation induced by an isolated molecule or adatom. The potential barrier includes the strong bending effect due to the image-charge occuring as the tunneling electron crosses the gap between the tip and the sample. The specific theoretical approach designed to solve this scattering problem exploits the fast Fourier transforms algorithm to construct a transfer-matrix in a mixed real- and momentum-space representation. The theory is used here to simulate the current image of a model-aluminium atom on a free-electron metal substrate using electrons focussed by a single-atom tungsten tip.