Temperature and concentration effects in electron-stimulated desorption of sodium atoms from sodium layers adsorbed on tungsten coated with a gold film

The yield and energy distributions of sodium atoms upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a gold film are investigated for the first time as functions of the thickness of the gold film, the concentration of deposited sodium, and the surface temperature. It is found that the energy distributions exhibit two peaks, namely, a narrow peak with a maximum at about 0.15 eV, whose intensity continuously increases with increasing temperature, and a broad peak with a maximum at about 0.35 eV, whose intensity either decreases or remains constant with increasing temperature. It is shown that both peaks arise as a result of the same excitation, which gives rise to different channels of electron-stimulated desorption of sodium atoms. Possible mechanisms of electron-stimulated desorption and the kinetics of destruction of the surface coating are discussed.     

Initial ion velocity in electron-stimulated desorption

The effect of electronic-shell repulsion and electron-phonon interaction among atoms of an adsorption complex on the initial ion velocity is considered.     

Energy distributions of ions produced by electron-stimulated desorption

We have measured the initial kinetic energy distributions of ions produced by electron bombardment of various oxides and halides. The instrument used allows ions directly ejected from the sample surface to be distinguished from ions formed by electron impact in the gas phase. Singly and multiply charged positive ions of species present in the matrix as anions and cations were desorbed by high energy ( 11 keV) electron impact. Directly desorbed positive halogen ions show a narrow, low energy peak, consistent with conventional models of electron stimulated desorption (ESD). In addition, some of the cation species exhibited similar narrow energy spectra. Charge states up to +6 were observed for the halides; with the exception of F²⁺ and Cl²⁺, multiple charge states were due to electron impact ionization of desorbed neutrals. Charge states up to +4 were seen for silicon from electron-bombarded SiO₂; energy distributions of Si⁺, Si²⁺ and Si³⁺ showed that these species were desorbed directly from the surface. The energy distributions of O⁺ and O²⁺ ions ejected from SiO₂ are relatively wide, compared to the energy distribution of Si⁺ ions. In contrast, O⁺ ions ejected from TiO₂ have a much narrower energy distribution, like those observed for the halogen ions.     

Modeling of electron-stimulated mobility and disordering of adsorbed particles

The electron-stimulated mobility and the electron-stimulated disordering of adsorbed particles is studied for a two-dimensional lattice gas model on a square lattice using kinetical Monte Carlo simulations. Pairwise nearest-neighbor repulsive interactions are considered which induce c(2 × 2) ordering of the lattice gas at low temperatures around half coverage. Adsorbed particles are allowed to perform thermally activated as well as electron-induced jumps to nearest-neighbor sites. The calculations are performed taking full advantage of the numerical power of a supermassive parallel computer.

It was found that the electron-induced mobility of adatoms causes the complete breakdown of the c(2 × 2) ordering at low temperatures if the fraction of electron-induced jumps exceeds a critical value. The breakdown of the ordering is accompanied by substantial changes of the chemical and tracer surface diffusion coefficients.     

Electron-stimulated desorption of sodium atoms from sodium layers adsorbed on a gold film

The yield and energy distributions of sodium atoms upon their electron-stimulated desorption from sodium layers adsorbed on tungsten coated by a gold film are measured for the first time as functions of the energy of bombarding electrons, the thickness of the gold film, and the amount of adsorbed sodium. The electron-stimulated desorption channel associated with the excitation of core levels of gold is revealed for the first time.     

Characterization of hydrogen species on TiO2 by electron-stimulated desorption

The hydrogen species in TiO₂ have been characterized using the site specificity of Electron-Stimulated Desorption (ESD). Hydrogen is found to be bonded to surface Ti's in hydride-type bonds, to subsurface or bridgebonded O's in a hydroxyl-like bond, or be part of a surface hydroxyl. Hydrogen is found at high levels in the near-surface region as a contaminant and is intimately involved in the reaction of O₂ and H₂O with the defective TiO₂ surface. It is suggested that this phenomenon may occur in a wide range of compounds.     

Cross sections for electron-stimulated desorption of alkali-metal atoms from adlayers on oxidized tungsten

Earlier cross-section measurements of electron-stimulated desorption (ESD) of Li, Na, K, and Cs atoms from adlayers on oxidized tungsten are analyzed with respect to substrate temperature and degree of oxidation. It is conjectured that the ESD cross sections are determined by the ratio of the rates of neutral alkali-metal re-ionization on the surface and of relaxation of the O⁺ charge in the substrate. A comparison with experiment revealed the dependence of the re-ionization rate of an alkali-metal adatom on its size and mass, as well as the dependence of the O⁺ charge relaxation rate on substrate temperature and degree of oxidation. The relation of the charge relaxation time to the substrate band structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1491–1497 (August 1997)     

Kinetics of desorption from adsorbed layers formed by two-dimensional islands and by a two-dimensional gas

The kinetics of desorption from the adsorbed layers formed by two-dimensional (2D) islands and by a 2D gas of single atoms is considered for the case when the flux of adsorbate atoms from the gaseous phase reaches the surface. The effect of this flux on the desorption kinetics is analyzed. It is shown that this effect can be disregarded when the islands are in equilibrium with the surrounding 2D gas. The possibilities of determining the energy required for the removal of atoms from 2D islands to a vacuum are considered for the case of the absence of equilibrium in adsorbed layers. The conditions under which such a determination is possible are formulated, and the corresponding recommendations for experimental studies are worked out.     

Samarium atom yield under electron-stimulated desorption from oxidized tungsten surface

The yield of samarium (Sm) atoms under electron stimulated desorption from Sm layers adsorbed on the surface of oxidized tungsten was studied as a function of incident electron energy, surface coverage by samarium, degree of tungsten oxidation, and substrate temperature. The measurements were conducted by the time-of-flight technique with a surface ionization detector in the substrate temperature interval from 140 to 600 K. The yield vs. incident electron energy function has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, which may be related to excitation of the Sm 5p and 5s levels. The Sm yield is a complex function of samarium coverage and substrate temperature. Sm atom peaks occur only in the Sm coverage range from 0 to 0.2 monolayers (ML), in which the yield passes through a maximum. The shape of the yield temperature dependence is a function of Sm coverage. For low Sm cover-ages (<0.07 ML), the yield decreases slowly with the temperature increasing to 270 K, after which it drops to zero at temperatures above 360 K. At higher coverages, the Sm yield passes through a maximum with increasing temperature and additional peaks appear at electron energies of 42, 54, and 84 eV, which can be assigned to excitation of the tungsten 5p and 5s levels. These peaks are most likely associated with desorption of SmO molecules, whose yield reaches a maximum at an Sm coverage of about 1 ML.     

On the isotope effect in the relaxation model of electron-stimulated desorption

The dependence of the ESD yield of ions on their mass has been studied. The influence of short-range electronic-shell repulsion on the isotope effect is discussed. The isotope effect is shown to be depressed with increasing non-Coulombic initial velocity. Fiz. Tverd. Tela (St. Petersburg) 40, 764–767 (April 1998)     

Electron-stimulated desorption of cesium atoms from cesium layers adsorbed on tungsten coated by a gold film

Physics of the Solid State - The yield and energy distributions of cesium atoms escaping in electron-stimulated desorption (ESD) from cesium layers adsorbed on tungsten coated by a gold film have...     

Study of lithium adsorption on the TiO2 surface by electron-stimulated desorption

This paper reports on a study of electron-stimulated desorption (ESD) of O⁺ and Li⁺ ions from titanium dioxide as a function of the preheating temperature T and of the concentration of lithium adsorbed at 300 K, which was carried out with a static magnetic mass spectrometer combined with a retarding-field energy analyzer. For T>1500 K, the TiO₂ surface undergoes irreversible rearrangement. At temperatures from 300 to 900 K and at lithium coverages Θ<1, the ESD cross sections of the O⁺ and Li⁺ ions vary in a reversible manner with temperature, while for lithium coverages Θ>1, the changes in the Li⁺ and O⁺ ESD cross sections become irreversible. For θ<1, the appearance threshold of the Li⁺ and O⁺ ions is 25 eV, whereas for θ>1, the ESD threshold of Li⁺ ions shifts to 37 eV.     

Concentration fluctuations in adsorbed layers

The temperature and coverage dependence of the mean square concentration fluctuations in a small open domain of an adsorbed layer is discussed for various situations. It is shown that fluctuations decrease with increasing temperature and reach a limiting value when attractive interactions predominate, but increase and reach a limiting value when repulsive interactions predominate, if a single non-ideal two -dimensional phase exists. Deviations from ideal gas behavior are strongest at half coverage. At very low coverage (low particle concentration) and very high coverage (low hole concentration) ideal behavior is approached. If the layer consists of a two phase system, for instance a two-dimensional liquid or solid in equilibrium with a two-dimensional gas, fluctuations far below the critical temperature are dominated by fluctuations in the partition between phases. As the critical temperature is approached fluctuations first decrease because the mean concentrations in the two phases approach each other, and then increase very sharply near T_c. Detailed calculations for the single phase situation are given for several approximations: a dilute gas; a mean field approximation; a lattice gas in both the Bragg-Williams and the BethePeierls-Weiss approximations. The latter which takes some account of correlations between adsorbate particle positions seems to explain reasonably the presently available experimental observations on chemisorbed layers.     

Modeling migration of adsorbed atoms in dense multicomponent adsorbed layers

Within the framework of a simple (i.e., ignoring lateral interactions) lattice model, surface migration of adsorbed atoms of a crystalline substance A is considered for the case in which the surface is densely covered by the adsorbed atoms of a noncrystallized substance B. Simulation of the migration, performed by means of the Monte-Carlo method, demonstrates a sharp decrease in the root-mean-square displacement of the adsorbed atoms of A with an increase in the degree of covering by adsorbed atoms of B in the case where these adsorbed atoms exhibit weak migration mobility. This effect is associated with strong correlations in the directions of individual jumps of an adsorbed atom of A. The results of the simulation are applied to estimate the influence of the degree of covering of a surface with an impurity on the temperature at which a transition is accomplished from an nascent to a stepwise-layered mechanism of growth in silicon in molecular-beam epitaxy. V. D. Kuznetsov Siberian Institute for Technical Physics. Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 89–94, June, 1998.     

A simple model for ordering in adsorbed layers

Order-disorder phase transitions in adsorbed and confined fluids with directional interactions are studied using lattice density functional theory. A new model is developed that is capable of predicting both order-disorder and condensation phase transitions. For systems with weak interactions, the results of this model are compared with both lattice Monte Carlo simulation data and simple isotherm theories that are commonly used to fit experimental data. The results show that these simple isotherms are incapable of duplicating complex behaviour exhibited by anisotropic molecules. When sufficiently strong interactions are present, confined fluids with directional interactions may spontaneously form ordered structures. It is shown that the ordering predicted by this model can result in the assembly of long chains. Such ordering has been observed experimentally in magnetorheological fluids in a magnetic field. It is shown that the orientation of the chains predicted by this model can be controlled by adjusting the molecule-surface interaction. It may be possible to create nanoscale devices that exploit this type of molecular switching.     

Acoustic study of adsorbed liquid layers

Interference measurements of small variations in the velocity and attenuation of surface acoustic waves (SAWs) are used to investigate water layers up to 15 nm thick adsorbed on the surface of a lithium niobate crystal. The frequency dependence of the relative variation of the SAW velocity with the adsorption of water vapor is measured in the range from 40 to 400 MHz. Acoustic techniques are used to experimentally estimate the frequency dependence of the dielectric constant of adsorbed water and its dipole relaxation frequency along with the dependence of the adsorption layer thickness on the water vapor pressure in the surrounding medium. A simple expression is proposed for calculating the dispersion of the SAW velocity in a solid loaded with a thin liquid layer.