Accurate He-Ag(110) interaction potential determination by selective adsorption

The first accurate energy level measurements of a He-metal interaction potential are reported for the Ag(110) surface as obtained from selective adsorption data. The fitting of the experimental data with a suitable phenomenological potential leads to a well depth of 6.0 ± 0.1 meV. The potential curve is considerably softer than those obtained for He interaction with either insulators or semiconductors and is in good agreement with theoretical predictions.     

Study of the mechanism of the negative secondary ionic emission by selective superficial oxidation and adsorption

With the aid of a speed analyser mass spectrograph able to function in an ultra vacuum, it was shown in previous work by conducting a study of the speed of ejection of secondary ions O^? emitted under the impact of ions K⁺ by copper target, that under certain experimental conditions at least two different origins of this emission could be demonstrated. The hypothesis that we thus adopted was that the O^? ions having the greatest ejection speed could originate from the layer adsorbed on the surface, whereas the O^? ions having the weakest ejection speed could originate from the oxide. In order to confirm this hypothesis, we conducted the following experiments.We oxidized a copper target with a natural isotope ¹⁸₈O of oxygen. The spectrogram of the negative secondary emission involved then, in addition to the ions of the adsorbed layers (H^?, C^?, CH^?, ¹⁶₈O^?, ¹⁶₈O^?,...) the ion ¹⁶₈OH^?, which could only originate from a chemical compound formed at the time of the reaction of the oxidation.In a second series of experiments, we introduced molecular oxygen ¹⁸₈O₂ into the system, beginning with an initial pressure of 10^?8 torr up to a pressure of 10^?5 torr. The mass spectrogram again contained, over and above the aforementioned ions, the ion ¹⁸₈O^?. Here this ion could only originate from the adsorbed layers, since the experimental conditions favored the chemical and physical adsorptions.In addition, the energy analysis of the ions ¹⁸₈O^? obtained in the two instances allows us to say that the ion O^? originating from the rupture of the chemical junction Cu₂O has a weaker speed of ejection than the ion O^? originating from the adsorbed layer of gas. In conclusion, the experiment shows that the energy transfer from the incident particle to the ejected particle, by means of the atoms in the target, leads to a speed of ejection which is discrete but weak, in the case of the particle originating from the rupture of a chemical junction; whereas in the case of particles originating from the adsorbed layers of gas, the speeds of ejection are greater and have a widespread distribution.     

A quantitative approach to ionic adsorption

Binding properties of multi-atomic systems with ionic bonds are calculated by using a recently developed method based on the density functional formalism. The charge density is obtained from a superposition of the respective atomic densities where the charge transfer between these atoms is chosen such that the total energy attains a minimum. The kinetic energy of the electrons can to a very good approximation be calculated by means of a modified Thomas-Fermi-v. Weizsäcker expression. The molecules NaCl and NaW and a NaW₄ cluster have been treated as model systems for ionic interaction. Moreover, we have computed the binding properties of a Na atom adsorbed on a W(100) surface. This particular problem is the primary subject of the present study. The calculations yield binding energies, binding distances, vibrational frequencies, and induced dipole moments.     

Electrode kinetic phenomena of solid state ionic devices

With the development of functional materials with high ionic and combined ionicelectronic conductivity, the engineering of interfaces became prerequisite. Technological applications demand combinations of materials with different electrical properties to form junctions. The occurring kinetic electrode phenomena play a significant role to the overall performance and may be rate determining. We show here the significance of the equilibration between platinum and the solid electrolyte with respect to the response time in potentiometric oxygen sensors. Fundamental aspects of voltage generation are discussed as well, and the response time is correlated to the equilibration along the contact zone between platinum and the electrolyte. A common problem of potentiometric devices for gas sensing applications is the cross sensitivity to species other than those under detection. The proposed kinetic method based on the theta concept is investigated for selective detection in the presence of a multiple number of complex gases by employing a single electrochemical cell. Information from current-voltage plots is converted to generate complex plane plot for the Fourier coefficients. The various polarizations under the applied electric perturbation are modelled and compared to experimental data for the determination of the rate determining processes.     

Interpretation of selective adsorption in atom-surface scattering

The elastic theory for resonant atom-surface scattering is shown to explain simply the occurrence of both minima and maxima at resonance and the strength and shape of the absorption lines, on the assumption that the repulsive part of the potential is of the form V(z?ζ), where ζ is the surface corrugation and V can be regarded as a hard wall at the locus of turning points. Explicit formulae are obtained in the semiclassical limit. For a simple sinusoidal corrugatrion with rectangular symmetry, the signature of an isolated resonance is determined unambiguously by the parity of ¦m¦+¦m' ? m¦?¦m'¦+¦n¦+¦n' ?n¦$\text{}\text{?}$s|n'¦, where (m,n) is the resonant vector and (m',n') the scattering vector, in reciprocal lattice units. The predicted signatures of isolated resonances agree with experiments and more elaborate calculations for the systems He/LiF and He/Graphite. Calculated splittings in the surface band structure also show reasonably good agreement. An alternative formulation of resonant scattering, as suggested by Wolfe and Weare, can be based in an appropriate sorting of successive orders of distorted wave perturbation theory. For small corrugations there is good correspondence between the two approaches.     

Computational investigation of rules governing selective adsorption intensities

Rules previously derived by Wolfe and Weare governing selective adsorption scattering intensities are investigated computationally via the attractive corrugated wall (ACW) model of Harvie and Weare. These rules, which correlate features of the scattering intensity with the form of the atom surface potential, correctly predict intensity features seen in the ACW calculation.     

Evaluation of the adsorption potential distribution function from observed adsorption isotherms

A new method for the evaluation of the adsorption potential distribution function ?(U) from observed adsorption isotherms is proposed. The method is based on the idea that ?(U) may be represented by an exponential higher degree polynomial, thus allowing for skew distributions and distributions with more extremes. The various parameters of the ?(U) proposed are to be adjusted with the method of least squares. The method is applied to adsorption isotherms at 77 °K of nitrogen and krypton on non-porous chromia, aeroal and aerosil. The distribution functions of nitrogen on these hydrated surfaces show two extremes, whereas the corresponding distributions for krypton have only one maximum. The second maximum in ?(U) in the case of nitrogen is attributed to the strong interaction of the nitrogen quadrupole with the dipolar field of the surface hydroxyl groups.     

Diffraction and selective adsorption in the corrugated hard wall model

Numerical calculations are carried out for the elastic scattering of thermal energy atoms from a perfect crystalline surface with a square unit cell. The surface is treated as an infinitely-repulsive corrugated hard wall with an attractive square well in front of the hard wall. Closed, as well as open and bound, channels are included in the calculation, and multiple scattering effects are treated explicitly. It is concluded from these numerical studies that the single scattering approximation is inadequate, that closed channels must be included in any numerical calculations, and that selective adsorption can readily be understood within the framework of this simple model. Moreover, selective adsorption minima as well as maxima are found in this calculation without the necessity of invoking inelastic processes. The general quantitative agreement of the calculated results with the HeLiF diffraction experiments, the relative ease of handling more complicated corrugated surfaces, and the small amount of computer time required for these calculations suggest that the hard wall model is ideally suited in a parameterization scheme.     

Polymers near metal surfaces: selective adsorption and global conformations

We study the properties of a polycarbonate melt near a nickel surface as a model system for the interaction of polymers with metal surfaces by employing a multiscale modeling approach. For bulk properties, a suitably coarse-grained bead-spring model is simulated by molecular dynamics methods with model parameters directly derived from quantum chemical calculations. The surface interactions are parametrized and incorporated by extensive quantum mechanical density functional calculations using the Car-Parrinello method. We find strong chemisorption of chain ends, resulting in significant modifications of the melt composition when compared to an inert wall.     

Planar channeling potential in heavy ionic crystals

The planar channeling potential in finite-size heavy KCl-like crystals is studied.     

An optical potential for rotationally mediated selective adsorption (Feshbach) resonances in scattering of molecules from smooth crystal surfaces

An expression is derived for the strength of the optical potential for describing the attenuation of HD molecules, undergoing rotational Feshbach resonances on smooth metal surfaces. The optical potential is assumed proportional to the repulsive component of the molecule-surface potential and its strength is determined by equating the optical potential to the Debye-Waller factor with no adjustable parameters. The good fit of the recently published experimental results of Yu et al. (1982) for HD-Ag(111) indicates that the attenuation can te attributed to phonon interaction.     

On the electronic potential and ionic relaxation at metal surfaces

Some results, obtained by a simple method which gives the main effect of the ions on the surface potential of metals, are advanced. A general expression is given for the change in the surface dipole barrier. The ionic relaxation of (110) and (111) faces of Al are obtained self-consistently, tending to support one of the two current interpretations of LEED diagrams.     

Evidence of ionic adsorption on C12E6 non-ionic micellar solutions

Summary In this paper we present conductivity measurements on different concentrations of C₁₂E₆ non-ionic surfactant solutions as a function of temperature and buffer ionic strength. The experimental data were analysed by means of Looyenga equation. The isothermal slopes of the linearized Looyenga equation depend on the ionic strength, showing saturation effect. Our results indicate that one of the basic hypotheses of the mixture equations is not fulfilled. Adsorption of light ions at micellar interface is necessary to justify the experimental data. Then for non-ionic surfactant solutions the ion adsorption prevents any explicit computation of the micellar hydrated volume. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

The role of ionic quadrupolar deformation in atom-surface potential

We show that the quadrupolar deformation of surface ions gives appreciable contribution to the potential energy between the atom and an ionic surface. Calculations are presented for a number of alkali halides. The validity of Fumi and Tosi crystal radii in the surface region is discussed in connection with the depth and the corrugation parameter.     

Rotationally mediated selective adsorption in rigid rotor/rigid surface scattering

We present a detailed investigation of a new effect in molecule/surface scattering, rotationally mediated selective adsorption, which has been first observed by Cowin et al. (J. Chem. Phys. 75 (1981) 1033) in rotationally inelastic HD/Pt (111) scattering. It is due to resonance between an asymptotically closed rotational channel and a vibrational bound state of the molecule/surface system. Exact close-coupling and diffractionally sudden calculations are performed for HD scattered from a rigid, flat and weakly corrugated surface. Both variation of the collision energy and variation of the incident angle are considered. In the latter case we include the averaging over the collision energy of the initial beam and compare qualitatively our results with the experiment. Taking into account the resonances below they j = 0 → 1 threshold we are able to explain all of the experimentally observed resonances and to assign them with the appropriate rotational-vibrational quantum numbers. The largest deviation between experimental and theoretical resonance angles is ~ 1.6° at a collision energy of E = 109 meV. An interesting interference effect is found in the case of weak surface corrugation. Although the diffractionally sudden approximation generally gives only poor results, it also reproduces this effect and provides a simple explanation of it in terms of the Breit-Wigner representation of the S-matrix.     

Successive ionic layer adsorption and reaction of ZnSe shells for ZnO nanowire-based dye-sensitized solar cells

Thin ZnSe layers were deposited on ZnO nanowires by a novel successive ionic layer adsorption and reaction technique in order to solve recombination problems in ZnO nanowire-based dye-sensitized solar cells (DSSCs). Cell efficiency increased from 0.1 to 1.3–1.4% with the deposition of a 9- to13-nm-thick ZnSe shell on ZnO nanowires due to a large increase in J_SC. The dramatic increase in J_SC and cell efficiency is due to the facilitation of electron transfer related to ambipolar diffusion by the formation of a type II band alignment and the suppression of recombination in the presence of the ZnSe shell.