Study of low energy noble gas ion reflection from monocrystalline surfaces; Influence of thermal vibrations of the surface atoms: IV. Possibilities for the estimation of the actual ion-atom interaction potential

The angular and energy distributions of low energetic noble gas ions, reflected over 30° from a copper (100) target, are measured and compared with calculations. In the simulations the chain-model approach was applied. The interaction potential is estimated by fitting the calculated results to the experiments. This procedure is achieved by varying the screening length of the used Thomas—Fermi potential in the approximation of Molière. Information about the actual interaction potential can be obtained in three complementary ways: (1) from the width of the angular distributions of ions, reflected (quasi) singly along a low index direction, (2) from the height ratio of the QD and the QS peak and (3) from the width of the QS peak. The resulting values for the fitted screening lenghts appear to be notably smaller than the theoretical ones. A comparison of the experimental and calculational widths of the distributions as a function of the temperature seems to suggest the presence of correlations between thermal displacements of neighbouring surface atoms.     

Study of low energy noble gas ion reflection from monocrystalline surfaces; Influence of thermal vibrations of the surface atoms: II. Calculational evaluation of the sensitivity for model parameters and possibilities for estimation

The sensitivity of the characteristics of low energy noble gas ion reflection from monocrystalline surfaces for thermal properties of the target atoms has been investigated by computer simulation. In addition the uncertainties in comparing experimental results with calculations, introduced by a not well-known interaction potential, have been examined. The calculations have been carried out for 6 keV Ar⁺ ions reflected from a vibrating Cu〈100〉 chain. To achieve the above presented object we varied the mean square value $\text{u}{}^2$ and the correlation coefficients of the atomic thermal displacements. The used ion-atom interaction potential (a Thomas-Fermi potential in the Molière approximation) has been varied by changing the screening length aF. Under certain conditions the shape of the energy spectra of specularly reflected particles depends pronouncedly on both $\text{u}{}^2$ and aF. The effects are the most pronounced for scattering angles between about 20° and 30°. The angular distribution shows also a distinct and simultaneous sensitivity for the used potential and the target-temperature. A most interesting feature is the occurrence of a QT peak at higher temperatures, resulting from quasi triple collisions from surface “thermal pit” structures. At a given scattering angle the cut-off temperature of this QT peak can be related to the mean square displacements of the involved atoms. This cut-off temperature appears to be (almost) independent from the used potential, allowing an estimation of $\text{u}{}^2$. The intensity of the QS peak and the QD peak depend exclusively on the mean square differences of thermal displacements of neighbouring atoms. Correlated atomic displacements have some influence on the angular distributions and on the QT peak intensity. Possibilities to estimate model quantities are discussed briefly.     

Differential cross section surfaces for low energy scattering of electrons and positrons from rare gas atoms

The differential cross sections for scattering of electrons and positrons from He, Ne, Ar, Kr, and Xe at projectile energies below the inelastic thresholds are calculated using a model potential approach in which the interaction between the projectile and the target atom is partitioned into static, exchange (for electrons), and correlation-polarization parts. Two different forms of the parameter-free correlation-polarization potential are suggested; in both cases the correlation-polarization potential is determined by smoothly matching the asymptotic form of the polarization potential (1/r ⁴) to the correlation potential at the outermost orbital radius of the target atom. The results of angular distributions are presented in the form of contours of constant differential cross sections as well as in the form of differential cross section surfaces in three-dimensional plots. Both of these presentations display the locations of the principal maxima and minima of the differential cross sections as well as the critical points in a very useful manner.     

The influence of correlated atomic vibrations on low energy ion scattering: II. Two-atom,three-atom and “chain” models

The possibilities and limitations of two- and three-atom models for thermal effects in ion scattering are investigated, using a sophisticated “chain” simulation. Qualitative agreement is found. The temperature variation of the QD peak height can be used to determine θ_D. The value for θ_D found from the QT peak height is, under some circummstances, sensitive to the ion-atom potential but can give information on the correlations.     

Desorption of carbonium ions from zeolite surfaces; A comparison of reactive surface ionization with field ionization

In comparison with field ionization on platinum the thermal desorption of surface carbonium ions from zeolite surfaces has been investigated mass spectrometrically. CaY-zeolites — forming carbonium ions — were supported by Pt filaments or Pt emitter tips where hydrocarbons are chemisorbed in a non-ionic form. Thermal ion desorption at very low external fields has been observed with stable carbonium ions of triphenylmethyl compounds Ph₃ C-X. This ion desorption was achieved on CaY zeolite surfaces and occurred in the order X  Br > X  Cl > X  OH; however, no ions could be obtained with X  H and X  COOH up to 825°K. On Pt surfaces high fields were required for desorbing molecular ions.     

A theoretical study of poisoning in heterogeneous catalysis; discussion of the role of electronegativity and a comparison with experimental results of Goodman et al. on CO adsorption and methanation on Ni(100)

The influence of a range of electronegative poisons, (C, S, P, and Cl) on the catalytic activity is considered. Calculations, in which the atoms are described by muffin tin potentials, use a Green function method to study the effects of poisons on the local density of states (LDOS) at the CO adsorption site of a nickel (100) cluster. All poisons studied influence nearest neighbour sites, as indicated by a marked reduction of the LDOS around the Fermi level. S, P, and Cl also affect next-nearest neighbour sites. The effective range of S and Cl, the strongest poisons studied, is shown to be < 5 Å. The importance of electronegativity in determining poisoning strength is shown to be strongly modified by the size of the poison atom, reflected by its vertical distance (d⊥) above the Ni(100) surface. A simple model is developed to compare calculations with experimental studies on nickel by Goodman et al. [Acc. Chem. Res. 17 (1984) 194, Surface Sci. 108 (1981) 64]. The model yields good agreement with experimental results for suppression of CO adsorption activity by carbon and sulphur. The model suggests that the influence of sulphur on methanation should be significant, but less dramatic than observed. Substantial changes to the model fail to generate quantitative agreement with experiment and reasons for the discrepancy are discussed.