Structure of ultrathin oxide layers on metal surfaces from grazing scattering of fast atoms

The structure of ultrathin oxide layers grown on metal substrates is investigated by grazing scattering of fast atoms from the film surface. We present three recent experimental techniques which allow us to study the structure of ordered oxide films on metal substrates in detail. (1) A new variant of a triangulation method with fast atoms based on the detection of emitted electrons, (2) rainbow scattering under axial surface channeling conditions, and (3) fast atom diffraction (FAD) for studies on the structure of oxide films. Our examples demonstrate the attractive features of grazing fast atom scattering as a powerful analytical tool in surface physics.     

Electron scattering by atoms and molecules

Recent advances in the scattering of electrons by free atoms and molecules are considered with an emphasis on the interface between experiment and theory. A survey is made of the theory and methods of measurement and their trend towards the perfect scattering experiment. For atomic targets a detailed coverage is given of the various theoretical models and the results compared with experimental data. For elastic and excitation collisions, the low and high energy regions are reasonably well understood. In the medium energy region, extending from a few times the ionization threshold down to about the ionization threshold there are still difficulties arising from the infinite number of open channels. For ionizing collisions, the three body Coulomb interaction still presents unanswered theoretical problems but has inspired some elegant experimental studies. For molecular targets the additional complexities arising from the nuclear motion, the lack of a centre of symmetry and the evaluation of multicentre integrals are considered. A brief discussion of some current problems and possible future developments is given.     

Anisotropic charge transfer rates in the scattering of oriented atoms or molecules from surfaces

Atoms or molecules scattered from surfaces can undergo charge transfer to or from the surface during the scattering event. It is proposed that when the incident particle is “prepared” in an excited or ionic state which is oriented with respect to the surface, observable anisotropies in charge transfer rates involving the oriented state should occur. The effect is illustrated with two examples.     

Nonequilibrium desorption of atoms and molecules from surfaces

A model, based on concepts from the theory of atom-surface accommodation coefficients, for nonequilibrium desorption of atoms and molecules from surfaces is proposed. We contend that the nonequilibrium effects should be expected to be general properties of not only polyatomic molecule-surface systems, but of all desorbing systems. The model predicts large deviations from the equilibrium dependences, on desorption angle, of (a) the desorbing atomic flux, (b) the average energy of desorbing atoms and (c) the desorbing atomic speed ratio. The deviations are larger when the accommodation coefficient is smaller.     

Interaction kinetics of atoms and molecules on carbon nanotube surfaces

We review recent experimental investigations of the interaction of gases with the surface of single-wall carbon nanotube bundles. We discuss thermal desorption spectra of both non-polar and polar adsorbates for low and high coverage. We show experimental results for diffusion processes of Xe along and within carbon nanotube bucky paper material, which is consistent with a recently proposed coupled desorption diffusion (CDD) model. We further discuss details of the interaction of ammonia with carbon nanotube surfaces, including the experimental investigation of the influence of adsorbed ammonia on the electrical transport properties of carbon nanotubes under ultra-high vacuum conditions.     

Quantum chaotic scattering with CsI molecules

A molecular beam of CsI molecules passed through the electric field created by a pair of charged wires is proposed as a laboratory experiment on quantum chaotic scattering. It is shown that the proposed experiment is realistic.     

Elastic scattering of alkali atoms from iodine atoms and molecules at thermal energies

High-resolution, small-angle measurements of the elastic differential cross sections have been made for the systems K/I and K/I₂. In all cases the structure at small angles is found to arise predominantly from collisions occurring along the adiabatic potential. The off-diagonal matrix element of the electronic hamiltonian between the covalent and ionic states in the K/I system is found to be ～ 6×10^-15 ergs.     

Electron excitation in atoms and molecules by neutron-nucleus scattering

The quantum theory of electron stimulated desorption developed in two preceding papers is presented in a WKB like form and is applied to the desorption of hydrogen and oxygen from tungsten. Retunnelling processes are taken into account and manifest themselves in an imaginary part in the potential of the desorbing particles. This imaginary part together with a non zero width of the wavepacket leads to quantum effects in the motion of the desorbing particles modifying the corresponding classical results of the reduction factor and of the energy conservation. The modifications turn out to be quite large for hydrogen as well as for oxygen increasing the reduction factor by several orders of magnitude. The isotope effect also is changed appreciable. The parameters of the model are determined from corresponding experimental data such as vibrational levels of the adsorbates, the energy distribution of the desorbing particles and the reduction factor.     

Quantum theory of electromagnetic fields interacting with atoms and molecules

This paper reviews the recent achievements in nonrelativistic quantum electrodynamics, especially nonlinear and coherent phenomena. The general properties of coupled radiation and matter are presented within simple models in section 1. The following sections treat in some detail three main aspects of the system and can be read independently of each other. Section 2 discusses some experiments with long-wave-length radiation (r.f.) and atoms. Section 3 presents the quantum theory of a laser and the ensuing photon distributions. Section 4 treats the case of strongly correlated emission of radiation called superradiance. The use of statistical ensembles is briefly discussed in Appendix A, whereas Appendices B, C and D present some technical details of the text.     

Cutoffs and shadows in classical scattering of atoms from surfaces

A commonly used model for the scattering of atoms from crystalline surfaces is that of a classical hard sphere interacting with a hard, corrugated surface. This model is simple enough to permit the study of the effect on the scattering of variation of parameters of the model. We examine here how changes in the amplitude and symmetry of the corrugations, the size of the sphere, and the depth of an attractive well near the surface modify the scattering. We show, in particular, how edge effects due to shadowing of parts of the surface and second hits by the sphere distort the classical rainbow structure. We show the results as topological and intensity plots for special cases. The edge effects give new information on the structure of the surface. For suitable real systems, experimental measurement of the angular and energy dependence of the fraction of the scattering that is elastic may show some of the features described.     

Quasi-elastic lineshapes for atoms and simple molecules undergoing jump rotation on surfaces

We apply an analytic model for discrete jump rotation of a species moving around a circle, to quasi-elastic scattering from single crystal surfaces. Results for several molecule-like geometries are presented and the subsequent form of the intermediate scattering function is identified. These functions give clear signatures, enabling different forms of jump rotation to be distinguished. A simple data reduction is discussed and we show how the true ISF will appear within this simplification.     

Competition between electron and phonon excitations in the scattering of nitrogen atoms and molecules off tungsten and silver metal surfaces

We investigate the role played by electron-hole pair and phonon excitations in the interaction of reactive gas molecules and atoms with metal surfaces. We present a theoretical framework that allows us to evaluate within a full-dimensional dynamics the combined contribution of both excitation mechanisms while the gas particle-surface interaction is described by an ab initio potential energy surface. The model is applied to study energy dissipation in the scattering of N(2) on W(110) and N on Ag(111). Our results show that phonon excitation is the dominant energy loss channel, whereas electron-hole pair excitations represent a minor contribution. We substantiate that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.     

Quantum tunneling for separation of atoms and molecules from a mixture

We suggest a technique to separate specific types of isotopes, atoms, and molecules from a mixture. The mechanism is based on the tunneling phenomenon of slowly moving molecules released from a magneto-optical trap. We argue that the scheme suggested can be realized in the laboratory.     

Threshold resonances in the diffraction of atoms and molecules by surfaces

We examine the problem of threshold resonances in the diffraction of atomic and molecular beams by solid surfaces. We show that the divergence in the slope of the intensity as a function of incident polar angle appears generally in all diffraction peaks. However, for interactions with realistically soft repulsive parts we find that the resonances are weakened to the point where they will be difficult to observe experimentally.     

The scattering of thermal He atoms at ordered and disordered surfaces

The use of thermal energy atom scattering (TEAS) as a surface investigation tool is reviewed. Specific properties of thermal He scattering, which are particularly advantageous in surface analysis, are discussed; the main causes which hampered so far the broad application of this technique to the investigation of ordered surfaces are analysed. The recent development of TEAS based on the exploitation of the large cross section for He diffuse scattering of adsorbates and vacancies is reviewed. This development extends the capabilities of TEAS to a straightforward investigation of various aspects of disordered surfaces so far hardly accessible to other methods. Examples concerning the lateral distribution of adsorbates and defects, the mutual attraction and repulsion, 2D-phase transitions, mobility and surface diffusion are given.     

Atomic band structure effects on resonant scattering of atoms from solid surfaces

Bound state resonances related to the band structure of adsorbed atoms and their usefulness for determining the periodic components of atom-solid interaction potential are theoretically investigated. A variety of specular intensity patterns associated with bound state resonances near the Brillouin zone boundaries are exhibited. The (10) and (11̄) bound state resonances give rise to two split specular minima with the splitting depending essentially on v₁₀ for a fixed beam energy; however, the detailed features are dependent on other periodic components. For incidence along a crystal symmetry direction, symmetrization of basis states not only makes numerical computation very efficient, but also implies that there is only one specular minimum for a pair of bound states which are equivalent by symmetry. The (01) and (10) resonances along and near the x = y direction are presented to illustrate the symmetrization principle. The depth of one of the specular minima decreases and finally vanishes as the symmetry direction is approached. The single specular minimum corresponds to a resonance with the bound state which is a symmetric linear combination of (01) and (10) states in a potential well of v₀ + V₁₁. As expected, the shift in positions of specular minima caused by the periodic surface potential increases with decreasing beam energy.     

A puzzle in the inelastic scattering of atoms from surfaces

The insensitivity of helium atoms to possible inelastic scattering involving bulk phonons that terminate at the surface is explained in terms of the experimental parameters that need to be incorporated into the scattering theory.     

Equations for scattering of fast electrons by polyatomic molecules

The correct mathematical procedure is described for obtaining the Fourier representation of the equations for scattering of fast electrons by vibrational subsystems of polyatomic molecules with vibrational anharmonism taken into account.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 93–96, January, 1996.