New surface ionization processes

The results of analysis of processes including surface ionization are presented. It is found that these processes do not fit into the classical theory of surface ionization. This concerns the processes with diffusion exchange of particles between the adsorbed layer and the bulk of the emitter, catalytic processes on the emitter surface involving individual centers, as well as processes occurring in the adsorbed layers under illumination, electron bombardment, and in electric fields. We consider the results of analysis of surface ionization of alkali metal atoms and organic molecules on the surface of gold intermetallide (Na _x Au _y ), considerably extending information about this phenomenon.     

Diffusion of particles over strongly anisotropic surface with traps

We investigate surface diffusion in a system of particles adsorbed on a two-dimensional strongly anisotropic lattice. There are two kinds of the lattice sites - ordinary sites and deep traps. Particles adsorbed in the ordinary sites can migrate over the surface, but particles adsorbed in traps are immobile. These particles do not move over the surface and they obstacle also the mobile particles migration (surface defects). Using kinetic Monte Carlo simulations we obtained coverage dependencies of the tracer, jump, and chemical diffusion coefficients. The coefficients are rather sensitive to the defect concentration. Even small admixture of the defects decreases drastically the fast diffusion. The effect is rather specific: strong dependence of the pre-exponential factor on the defect concentration and almost independent activation energy. The defect influence on the slow diffusion is weak. It results in strong decreasing of the surface diffusion anisotropy with the defect concentration. Such unusual behavior of the diffusion coefficients was observed in many experimental investigations of the surface diffusion of lithium, cesium, potassium, and strontium over strongly anisotropic W(1 1 2) and Mo(1 1 2) planes. It was shown that this specific behavior arises exclusively due to the surface anisotropy, and does not depend on the lateral interaction between the particles.     

Photostimulated diffusion of atoms adsorbed on a solid surface

A new light-induced effect—surface photodiffusion—is found in the processes of transfer on a solid surface. The essence of the effect is that the energy of electronic excitation of an adsorbed atom is transformed into that of its motion over the surface. It is shown that, under nonuniform illumination of the surface, the adsorbed atoms are moved away from the illuminated region, with the atomic concentration at the boundary between the illuminated and dark regions exceeding the equilibrium value. The dark and photoinduced diffusion coefficients are measured, and the value of the diffusion barrier height is estimated.     

Diffusion of particles on a heterogeneous surface: A case of adsorption-induced surface reconstruction

The influence of surface reconstruction on diffusion of particles adsorbed on the surface is investigated in the framework of symmetrical four-position model. The analytical expressions for free energy and diffusion coefficients are obtained assuming the lateral interaction between particles is negligibly small.The critical behavior of the system is described by the Ising spin model. The coverage dependencies of the tracer, jump and chemical diffusion coefficients are calculated for some representative temperatures. The dependencies show clearly strong influence of the surface reconstruction on the thermodynamic and kinetic phenomena: diffusion coefficients become anisotropic on the reconstructed surface. To check the analytical results we have used Monte Carlo simulations of the diffusion on this lattice.     

Diffusion on surfaces

A commentary is presented on current progress towards understanding diffusion on surfaces. Recent work by field ion microscopy shows that interactions between diffusing adatoms lead to correlated motions and add unexpected complexity to the elementary diffusion mechanisms. Work on high temperature surface self-diffusion also suggests the importance of interactions between diffusing atoms. Mass transport, catalysed surface diffusion, giant diffusivities, impurity, tracer and adsorbed layer diffusion, diffusion in sintering and other applications, are briefly discussed.     

The study of sputtering effects in oxides and metal— Adsorbed-gas systems using combined analytical techniques

The sputtering of adsorbed O and CO on clean metal surfaces and of solid oxide surface layers has been investigated by various techniques. Aspects of the effect of adsorbed layers on particle excitation and of interatomic binding energy and kinetic energy transfer on the sputtering are studied. The results from various experiments can be explained within collisional models, no thermodynamic models being necessary. The role of recoil implantation is positively demonstrated.     

Calculation of structure-sensitive surface peak intensity in mev ion scattering

The surface peak intensity from a silicon crystal has been calculated by the Monte Carlo simulation method assuming a variety of surface structure models and dynamics. Effects of thermal vibration, displacements and relaxation of surface atoms, and also the shadowing effect by adsorbed layers and the effect of multiple scattering of the projectile in penetrating surface layers have been evaluated in the He⁺ ion incident energy range from 0.1 to 2.0 MeV. Since many real surfaces of crystals are covered with adsorbed (deposited) layers or otherwise reconstructed when they are clean, it is of practical usefulness to obtain the calculation results about such surfaces in order to apply the MeV ion scattering technique for surface analyses. The results show the sensitivity and limits of this technique.     

A molecular dynamics study of Lennard-Jones physisorption on W(100)

The physisorption of Xe on W(100) was modeled by Lennard-Jones pair-wise interaction potentials and the dynamics of coverages ranging from one to four adlayers obtained by molecular dynamics simulation. At 115 K, the first two layers were well-ordered and each adsorbed with c(2 × 2) symmetry. Further adsorption produced a surface similar to that of a distorted Xe(100) face. In accord with the work of Broughton and Woodcock [1], the top layers of the three- and four-adlayer coverages were “rough” and had liquid-like diffusion coefficients. The potential energies of all layers other than the first were similar, thus corroborating one of the postulates of BET theory [2]. Generally, the effect of adsorbing a layer was to reduce the entropy of all those beneath.     

Morphological change of D2O layers on Ru(0 0 0 1) probed with He atom scattering

The morphological change of D₂O layers on a Ru(0 0 0 1) surface has been investigated on the basis of He atom scattering. With the increase of D₂O exposure on Ru(0 0 0 1) at 111 K, the intensity of a specularly reflected He beam continuously decreases up to the exposure of 1.0 L (Langmuir). At the D₂O coverage of 1.0 adsorbed layer (∼1.5 L), which is characterized by temperature-programmed desorption measurements, the formation of the (√3 × √3)R30° superstructure as a result of the diffusion of D₂O on the surface was confirmed by He atom diffraction. With the further increase of D₂O exposure, at 2-3 adsorbed layers, the disordered structure was found to be on the surface at 111 K. The morphological change of the disordered layers was observed during annealing, and discussed in detail.     

氧原子在Zr(0001)表面附近的扩散

在密度泛函理论计算的基础上,利用微动弹性带（nudged elastic band）方法研究了氧原子在Zr（0001）表面附近的扩散.首先计算了氧原子从稳定的表面面心立方（SFCC）位置向表面六角密排位置的扩散激活能（0.77 eV）;然后计算了氧原子从稳定的SFCC位置扩散到表面下第1层与第2层之间的八面体间隙位置,再继续向表面下第2层与第3层之间的八面体间隙位置扩散的激活能,在此过程中氧原子需克服两个能垒,其激活能分别为2.14和2.57 eV.结果表明,氧原子在Zr（0001）表面上方的扩散比较容易,而氧原子向Zr（0001）表面下的扩散相对较难. 关键词： Zr（0001）表面 微动弹性带 氧的扩散     

Band structure of Au layers on the Ru(0001) and graphene/Ru(0001) surfaces

The electronic structures of Au monolayers on the Ru(0001) and graphene-coated Ru(0001) surfaces have been calculated by DFT method using the supercell (repeated-slab) approach. The local densities of states (LDOS) and band structures of the monolayer and bilayer Au films adsorbed on the graphene/Ru(0001) and those of free hexagonal Au layers are found to be very similar. This result indicates that the monolayer graphene almost completely screens the Au layers from the Ru(0001) substrate surface, so that electronic properties of Au films adsorbed on graphene are determined predominantly by the electronic structure of the Au adlayers, essentially independent on the electronic structure of the substrate surface.     

Effect of adsorbed Sn on Ge diffusivity on Si(111) surface

The effect of adsorbed Sn as a surfactant on Ge diffusion on a Si(111) surface has been studied by Low Energy Electron Diffraction and Auger Electron Spectroscopy. The experimental dependence of Ge diffusion coefficients on the Si(111) surface versus temperature in the presence of adsorbed Sn atoms has been measured in the range from 300 to 650°C. It has been shown that at a Sn coverage of about 1 monolayer the mobility of Ge atoms increases by several orders of magnitude.      

Total internal reflection ellipsometry: ultrahigh sensitivity for protein adsorption on metal surfaces

Total internal reflection ellipsometry (TIRE) is used to study adsorption of human serum albumin and fibrinogen on thin gold films. TIRE shows very high sensitivity for protein monolayers adsorbed on metal surfaces when surface plasmon resonance effects are utilized. The measured data, expressed in ellipsometric angles psi and D are of several orders of magnitude larger in comparison with those from similar experiments performed with traditional ellipsometry. TIRE in spectral mode opens a new path for precise studies of organic layers adsorbed on metal surfaces, with a potential for resolving the adsorbed layer microstructure.     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

THE APPLICATION OF HIGH RESOLUTION SOLID STATE NMR IN ADSORPTION SYSTEMS

High resolution solid state NMR techniques(such as MAS,CP/MAS andCRAMPS)were employed to study the nature of organic molecules adsorbed onporous solids. A magic angle spinning system was achieved for sealed samples with a spinning speed from 2KHz to 4.2KHz.Using this technique,high resolution ~1HMAS spectra of organic molecules and H_2S adsorbed on charcoal were obtained.EXperimental results suggest that for high coverage of adsorbed organic molecules,the spectral lines were resolved very well.But for low coverage,the spectrallines could not be separated completely.As the organic molecules condensed in     

Model and theory for the determination of diffusion coefficients by Auger electron spectroscopy measurements and an application to oxygen diffusion along the [0001] and [101¯0] axes in single crystal zirconium

A simple hopping model of the diffusion of adsorbed species from a surface into the bulk of a material has been formulated and solved mathematically. The difference in the energy barriers for an atom moving between the atomic layers at the surface and in the bulk are explicitly considered. This model is also capable of describing the initial stages of diffusion, something that conventional solutions of the continuum diffusion equation cannot handle. Auger electron spectroscopy has been used to measure the dissolution rate of oxygen from Zr(0001) and Zr(101¯0) surface into the bulk. Satisfactory results were obtained by applying our model to the diffusion data for these two zirconium surfaces for two different heating schedules: (i) rapid temperature ramp-and-hold and (ii) continuous linear heating with respect to time. The resulting Arrhenius expressions for diffusion are: D = (0.115 ± 0.031)exp[(−44.45 ± 4.82)kcal/RT]cm²/s along Zr[0001] and D = (1.07 ± 0.26)exp[(−46.18 ± 4.22)kcal/RT]cm²/s along Zr[101¯0].     

X-ray rocking curves on inhomogeneous surface layers on Si single crystals

Two cases of inhomogeneous surface layers are considered — diffusion layers as well as implanted layers. The parameters of the layers are investigated by means of an X-ray rocking curve analysis. In this paper the concentration profile of a diffusion layer in Si is determined from X-ray rocking curves, the rocking curves on implanted layers will be discussed in the next paper. A graph is constructed for determining approximate values of surface concentrationC ₀ and diffusion lengthL from subsidiary maxima on the X-ray rocking curve. The dependence of the shape of the rocking curves on the type of the concentration profile and on the values ofC ₀ andL was shown. The influence of crystal thickness and curvature is studied theoretically and experimentally. As an example of using this method a rocking curve of a crystal with a boron diffusion layer is measured and the parameters of the concentration profile are determined. The parameters found are proved by multiplied measurement of rocking curves after anodic oxidation and by comparing these rocking curves with theoretical ones.     

Surface diffusion in an adsorbed system with competing order parameters

A spin-1 lattice gas model with stochastic dynamics is developed and applied to the surface diffusion in an adsorbed system possessing an internal degree of freedom. Both the chemical diffusion coefficient (D) and the relaxation rate (T) associated with the internal degree of freedom are examined. The coverage dependence of these quantities is studied within the conventional approximation, with the various static averages estimated by a Monte Carlo method. A mean field theory treatment is also given. Particular emphasis is given to the effects that the various phase transitions of the system have upon the behavior of D and T. The ordering of the internal degree of freedom is found to have an important influence on diffusion, leading to a highly reduced diffusion coefficient in the ordered regions. The connection to surface diffusion in adsorbed systems with solid orderings is pointed out. The importance of thermodynamic properties in determining the behavior of the diffusion coefficient in such systems is indicated.