Field ion microscope observations of surface rearrangement and oxide formation at clean rhenium surfaces heated in oxygen

The interaction between oxygen and clean, well-ordered rhenium surfaces has been investigated in the field ion microscope over a wide range of temperatures. The surface structure of vacuum annealed specimens was extensively modified by the presence of adsorbed oxygen. At partial monolayer coverages, surface metal atom rearrangement was inhibited, particularly at {112?2}, but at coverages near saturation, facets were produced at {101?0} at a lower temperature than on the clean surface, resulting in the exposure of an increased proportion of closepacked surface structures and suggesting a change in the rearrangement mechanism at high coverages. The production of facets at the major poles was temperature and coverage dependent as a result of the varying influence of adsorbed oxygen on surface free energy. A decrease in oxygen coverage was observed at 1200–1300 K on near-saturated surfaces, which may be associated with the formation of volatile ReO₃ from regions where oxygen concentration exceeds a critical value. There was little evidence of the production of a discrete surface oxide phase formed on specimens heated in the presence of gas phase oxygen, but a reduction in specimen radius as a result of volatile oxide formation was observed at pressures greater than 10^?4 Torr and temperatures above 1400 K. It is concluded that the final structure of rhenium surfaces heated in oxygen is dependent upon the rate of oxide formation, the effective oxygen coverage during oxidation, and the extent of surface rearrangement.     

Metal radii in surface science

The saturation monolayer coverage of an adsorbed metal overlayer on a smooth metal surface is largely determined by the adsorbed metal's radius. Experimental maximum packing densities in two-dimensional metal overlayers are compared with predictions based on several different definitions of metal atom radii: atomic radii, covalent radii, minimum bulk interatomic distance (:2) and the Zachariasen [J. Inorg. Nucl. Chem. 35 (1973) 3487] metal radii. Best agreement is found with the last, which is obtained by assuming that the bulk, pure metal density is obtained from an ideal, hexagonal close-packed structure of spheres of that radius.     

Electron-stimulated desorption of lithium atoms from oxidized molybdenum surface

The yield and energy distributions of lithium atoms upon electron-stimulated desorption from lithium layers adsorbed on the molybdenum surface coated with an oxygen monolayer have been measured as functions of the impact electron energy and lithium coverage. The measurements are performed using the time-of-flight technique and a surface ionization detector. The threshold of the electron-stimulated desorption of lithium atoms is equal to 25 eV, which is close to the ionization energy of the O 2s level. Above a threshold of 25 eV, the yield of lithium atoms linearly increases with an increase in the lithium coverage. In the coverage range from 0 to 0.45, an additional threshold is observed at an energy of 55 eV. This threshold can be associated with the ionization energy of the Li 1s level. At the electron energies above a threshold of 55 eV, as the coverage increases, the yield of lithium atoms passes through a maximum at a coverage of about 0.1. Additional thresholds for the electron-stimulated desorption of the lithium atoms are observed at electron energies of 40 and 70 eV for the coverages larger than 0.6 and 0.75, respectively. These thresholds correlate with the ionization energies of the Mo 4s and Mo 4p levels. Relatively broad peaks in the range of these thresholds indicate the resonance excitation of the bond and can be explained by the excitation of electrons toward the band of free states above the Fermi level. The mean kinetic energy of the lithium atoms is equal to several tenths of an electronvolt. At electron energies less than 55 eV, the energy distributions of lithium atoms involve one peak with a maximum at about 0.18 eV. For the lithium coverages less than 0.45 and electron energies higher than 55 eV, the second peak with a maximum at 0.25 eV appears in the energy distributions of the lithium atoms. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model, in which the adsorbed lithium ions are neutralized after filling holes inside inner shells of the substrate and lithium atoms.     

Ionization of xenon Rydberg atoms at a metal surface

Experiments in which a thermal-energy beam of xenon Rydberg atoms is directed at near grazing incidence onto a flat Au(111) surface are described that provide new insight into charge transfer and electron tunneling during atom/surface interactions. Analysis of the data shows that for the present range of principal quantum number n, 13 < or = n < or = 20, ionization occurs at an atom/surface separation Z(i) = (4.5+/-0.9)n2a0, where n2a0 is the Bohr radius of the atom. This result is in good agreement with the value Z(i) approximately 3.8n2a0 predicted by ab initio hydrogenic theory.     

Simultaneous and sequential adsorption of deuterium and nitrous oxide by rhenium

The addition of nitrous oxide to a stream of deuterium passing over a rhenium filament reduced the initial sticking probability of the latter gas from 0.24 to 0.09 when the proportion of N₂O exceeded 40%. For the addition of deuterium to nitrous oxide the equivalent figures were 0.45 and 0.30 when deuterium exceeded 30% of the gas phase. These results are attributed to a competition between the two gases for places in the precursor state on the surface. The replacement of adsorbed deuterium from a saturated layer by the oxygen atom of nitrous oxide proceeded initially with a high probability, 0.27, at room temperature and with each oxygen atom replacing one deuterium atom. However, the reaction was incomplete, about 2 × 10¹⁴ atoms cm^?2 of deuterium remaining on the surface. It is suggested that kinetic rather than thermodynamic factors are responsible for the incomplete reaction, possibly as the result of a high activation energy for the migration of deuterium atoms over an oxygenated rhenium surface.     

Chemisorption of Au on Si（001） surface

The chemisorption of one monolayer of Au atoms on an ideal Si(001) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of the adsorption system of a Au atom on different sites are calculated. It is found that the most stable position is A site (top site) for the adsorbed Au atoms above the Si(001) surface. It is possible for the adsorbed Au atoms to sit below the Si(001) surface at the B_1 site(bridge site), resulting in a Au－Si mixed layer. This is in agreement with the experiment results. The layer projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated.     

Electron-stimulated desorption of europium atoms from an oxidized tungsten surface

The yield of europium atoms in electron-stimulated desorption from Eu layers adsorbed on the surface of oxidized tungsten was studied with a surface-ionization detector as a function of the incident-electron energy, surface coverage by europium, and degree of tungsten oxidation. The yield of Eu atoms measured as a function of electron energy exhibits a distinct resonant character with peaks at electron energies corresponding to europium and tungsten core-level ionization energies. The peaks associated with the europium ionization reach a maximum intensity at europium coverages less than 0.1 and decrease subsequently to zero with increasing coverage, while the peaks due to tungsten ionization pass through the maximum intensity at a monolayer europium coverage. The coverage corresponding to the maximum europium atom yield grows with increasing tungsten oxidation. The results obtained are accounted for by the formation of the europium and tungsten core excitons. In the first case, the particles desorb in the reverse motion toward the surface of the oxidized tungsten; in the second, they desorb as a result of repulsion between the tungsten core exciton and the EuO molecule.     

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.     

Binding of an adatom to a simple metal surface

The density functional formalism of Hohenberg and Kohn is used to investigate the energies, charge densities and forces which hold an adatom on the surface of a simple metal. The valence wavefunction of the adatom is fitted to the Herman-Skillman solutions at large distance and is simplified somewhat in the core region. The field of the ion is represented by the Ashcroft pseudopotential. For the metal the jellium model is used. Detailed calculations are carried out for a sodium adatom on a sodium surface. Simply juxtaposing adatom and surface gives a binding energy of about $\text{1}\text{3}\text{eV}$. This value is approximately twice the surface energy per atom in the close-packed plane. Charge redistributions as determined variationally increase the binding energy by about 10%. The redistribution is primarily a dipole induced on the adatom at close distances, but at somewhat larger distances a prolate quadrupole also appears on the atom. A small amount of charge is also drawn from the metal toward the atom. The equilibrium distance for the adatom turns out to be 1.66 Å from the surface, as compared with 1.52 Å, the observed value for one-half the distance between the close-packed planes. Contour plots of the piling-up of electronic charge between the adatom and the metal are presented.     

Adsorption of SO2 on Li atoms deposited on MgO (1 0 0) surface: DFT calculations

The adsorption of sulfur dioxide molecule (SO₂) on Li atom deposited on the surfaces of metal oxide MgO (1 0 0) on both anionic and defect (F_s-center) sites located on various geometrical defects (terrace, edge and corner) has been studied using density functional theory (DFT) in combination with embedded cluster model. The adsorption energy (E_ads) of SO₂ molecule (S-atom down as well as O-atom down) in different positions on both of O⁻² and F_s sites is considered. The spin density (SD) distribution due to the presence of Li atom is discussed. The geometrical optimizations have been done for the additive materials and MgO substrate surfaces (terrace, edge and corner). The oxygen vacancy formation energies have been evaluated for MgO substrate surfaces. The ionization potential (IP) for defect free and defect containing of the MgO surfaces has been calculated. The adsorption properties of SO₂ are analyzed in terms of the E_ads, the electron donation (basicity), the elongation of S-O bond length and the atomic charges on adsorbed materials. The presence of the Li atom increases the catalytic effect of the anionic O⁻² site of MgO substrate surfaces (converted from physisorption to chemisorption). On the other hand, the presence of the Li atom decreases the catalytic effect of the F_s-site of MgO substrate surfaces. Generally, the SO₂ molecule is strongly adsorbed (chemisorption) on the MgO substrate surfaces containing F_s-center.     

Interactions of charged dust particles in clouds of charges

Two charged dust particles inside a cloud of charges are considered as Debye atoms forming a Debye molecule. Cassini coordinates are used for the numerical solution of the Poisson-Boltzmann equation for the charged cloud. The electric force acting on a dust particle by the other dust particle was determined by integrating the electrostatic pressure on the surface of the dust particle. It is shown that attractive forces appear when the following two conditions are satisfied. First, the average distance between dust particles should be approximately equal to two Debye radii. Second, attraction takes place when similar charges are concentrated predominantly on the dust particles. If the particles carry a small fraction of total charge of the same polarity, repulsion between the particles takes place at all distances. We apply our results to the experiments with thermoemission plasma and to the experiments with nuclear-pumped plasma.     

Au钝化Si(100)表面的电子特性

用TB-LMTO方法研究单层的Au原子在理想的Si(100)表面的化学吸附.计算了Au原子在不同位置的吸附能,吸附体系与清洁Si(100)表面的层投影态密度, 以及电子转移情况.结果表明, Au原子在吸附面上方的A位(顶位)吸附最稳定, Au钝化Si(100)表面可以取得明显的钝化效果, 这一结论与实验事实相符合.     

Electron-stimulated desorption of sodium atoms from germanium-coated tungsten

The yield of sodium atoms and energy distribution upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a germanium thin film are measured under variations in the electron energy, the sodium coverage, and the surface temperature by the time-of-flight method with the use of a surface ionization detector. It is revealed that the electron-stimulated desorption of sodium atoms occurs via three channels, namely, a channel involving ionization of adsorbed sodium; the most efficient channel, which is produced by the germanium ionization; and a channel associated with the formation of tungsten excitons, which brings about desorption of NaGe molecules.     

Thresholds and hysteresis of surface ionization under nonuniform conditions

The influence of nonuniformity of the atom arrival rate, of the emitter temperature and of the adsorption properties of its surface on characteristics of thresholds and hysteresis of the surface ionization is analyzed. It is shown that, unlike the uniform conditions, in the threshold zone the ion current changes with the emitter temperature monotonically and not in a form of a jump ; the width of hysteresis is substantially diminished ; the slope of the first threshold curve is not obligatory equal to the heat of ion adsorption; for a polycrystalline surface the critical temperature for hysteresis to vanish is related to the minimum work function and not to the effective work function.     

Adsorption de l'éthyléne et de l'acétylène sur des films de rhénium evaporés sous ultra-vide; Hydrogénation de l'éthylène: II. Discussion des résultats expérimentaux et interprétation

A qualitative model is proposed in order to explain our experimental results on ethylene chemisorption on evaporated rhenium films and hydrogenation of ethylene (part I). The surface must present at least two kinds of surface sites (A and B). The second type (B), either preexists on the surface, or is induced by the adsorption phenomenon itself. On the most energetic ones (A), dissociation of ethylene and hydrogen is complete. Adsorption of ethylene is characterized by a sticking coefficient value of 0.1 if they are free and 1 if they are hydrogen covered. On sites B, ethylene is adsorbed without full dissociation (sticking coefficients equal to 0.015). independent on adsorption temperature. Hydrogen desorption is due to full dissociation of ethylene on the surface and a displacement reaction while ethane is produced by reaction between non-dissociated adsorbed ethylene and hydrogen in the gas phase. The same Rideal-Eley mechanism applies for hydrogenation of ethylene in quasi-stationary conditions, along with a self-poisoning mechanism involving dehydrogenation leading to C₂H₂ non-hydrogenable adsorbed species.     

Hydrogen isotope replacement reactions on rhenium

The existence of a mobile equilibrium at room temperature between part of the hydrogen adsorbed on rhenium and gaseous hydrogen is demonstrated by the easy exchange of isotopes between the adsorbed layer and the gas phase. The adsorbed gas is desorbed as a mixture of homonuclear molecules (of H₂ or D₂)and of the isotopically mixed species (HD). However, the replacement reactions are not symmetrical; there is a greater proportion of HD in the desorbed gas when deuterium is replaced by hydrogen than in the converse reaction. This kinetic isotope effect is attributed to differences between the zero-point energies of the various hydrogen containing species.Quantatitive agreement between the shapes of the experimentally observed desorption curves and calculated curves is obtained if the zero-point energy of the bond between a surface rhenium atom and deuterium is assigned the value 2.6 kcal mole^?1.     

A density functional study of atomic hydrogen and oxygen chemisorption on the relaxed (0001) surface of double hexagonal close packed americium

Ab initio total energy calculations within the framework of density functional theory have been performed for atomic hydrogen and oxygen chemisorption on the (0001) surface of double hexagonal packed americium using a full-potential all-electron linearized augmented plane wave plus local orbitals method. Chemisorption energies were optimized with respect to the distance of the adatom from the relaxed surface for three adsorption sites, namely top, bridge, and hollow hcp sites, the adlayer structure corresponding to coverage of a 0.25 monolayer in all cases. Chemisorption energies were computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at the fully relativistic level (with spin-orbit coupling SOC). The two-fold bridge adsorption site was found to be the most stable site for O at both the NSOC and SOC theoretical levels with chemisorption energies of 8.204 eV and 8.368 eV respectively, while the three-fold hollow hcp adsorption site was found to be the most stable site for H with chemisorption energies of 3.136 eV at the NSOC level and 3.217 eV at the SOC level. The respective distances of the H and O adatoms from the surface were found to be 1.196 ?and 1.164 ?. Overall our calculations indicate that chemisorption energies in cases with SOC are slightly more stable than the cases with NSOC in the 0.049–0.238 eV range. The work functions and net magnetic moments respectively increased and decreased in all cases compared with the corresponding quantities of bare dhcp Am (0001) surface. The partial charges inside the muffin-tins, difference charge density distributions, and the local density of states have been used to analyze the Am-adatom bond interactions in detail. The implications of chemisorption on Am 5f electron localization-delocalization are also discussed.     

Photoionization microscopy of hydrogen atom near a metal surface

We have studied the ionization of Rydberg hydrogen atom near a metal surface with a semiclassical analysis of photoionization microscopy. Interference patterns of the electron radial distribution are calculated at different scaled energies above the classical saddle point and at various atom-surface distances. We find that different types of trajectories contribute predominantly to different manifolds in a certain interference pattern. As the scaled energy increases, the structure of the interference pattern evolves smoothly and more types of trajectories emerge. As the atom approaches the metal surface closer, there are more types of trajectories contributing to the interference pattern as well. When the Rydberg atom comes very close to the metal surface or the scaled energy approaches the zero field ionization energy, the potential induced by the metal surface will make atomic system chaotic. The results also show that atoms near a metal surface exhibit similar properties like the atoms in the parallel electric and magnetic fields.     

Electron state density and electron diffusion coefficient in energy space in nonideal nonequilibrium plasmas

We suggest a model for a hydrogenic low-temperature nonequilibrium nonideal plasma that allows the kinetic parameters of the plasma to be calculated by the method of molecular dynamics by taking into account the interparticle interaction. The charges interact according to Coulomb’s law; for unlike charges, the interaction is assumed to be equal to a constant at a distance smaller than several Bohr radii. For a system of particles, we solve the classical equations of motion under periodic boundary conditions. The initial conditions are specified in such a way that the electrons have a positive total energy. We consider the temperatures 1-50 K and densities n = 10⁹?10¹⁰ cm^?3 produced in an experiment through laser cooling and resonant excitation. We calculate the electron state density as a function of the plasma coupling parameter and the electron diffusion coefficient in energy space for highly excited (Rydberg) electron states close to the boundary of the discrete and continuum spectra.