B. Poelsema

CHARGE-EXCHANGE OF LOW-ENERGY HE IONS AND ATOMS SCATTERED FROM A COPPER SINGLE-CRYSTAL

Abstract Ion yield measurements are presented for 3–10 keV He ions and atoms bombarding a (100) surface of a copper target. The scattering angle is 30°. The difference between the ion and atom scattering results is explained by charge exchange processes occurring during the particle-surface interaction. An accurate description of the experimental results can be given by a collision model which takes into account Auger neutralization as well as ionization and neutralization resulting from a violent collision between an incoming particle and a surface atom. From the atom beam experiments we are able to measure the characteristic velocity for Auger neutralization V c and the ionization probability P i . We measured the energy dependence of V c and P i in the energy range of 3–10 keV. The observed values of V c and P i agree with estimations deduced from ion beam experiments.

STUDY OF LOW-ENERGY NOBLE-GAS ION REFLECTION FROM MONOCRYSTALLINE SURFACES - INFLUENCE OF THERMAL VIBRATIONS OF SURFACE ATOMS .1. COMPUTER-SIMULATION OF ENERGY AND SPATIAL DISTRIBUTIONS

Abstract The influence of target temperature on the energy and spatial distributions of low energy noble gas ions, multiply reflected from a monocrystalline surface, has been studied by computer simulation. The results are obtained by using the chain model; this simplification is a good approximation when the surface is bombarded along low index directions. The simulation model is outlined briefly. The consequences of different target temperatures on the shape of energy spectra of specularly reflected particles and on the shape of spatial distributions if the total scattering angle, resp. angle of incidence is fixed, are discussed qualitatively. Calculations of 6 keV Ar + ions reflected from a vibrating Cu 〈100〉 chain are discussed using these qualitative considerations. The shape.of both the energy and the spatial distributions appears to be distinctly temperature dependent under certain conditions.

STUDY OF LOW-ENERGY NOBLE-GAS ION REFLECTION FROM MONOCRYSTALLINE SURFACES - INFLUENCE OF THERMAL VIBRATIONS OF SURFACE ATOMS .4. POSSIBILITIES FOR ESTIMATION OF ACTUAL ION-ATOM INTERACTION POTENTIAL

Abstract 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 Moliere. 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 SURFACE ATOMS .3. QUALITATIVE COMPARISON OF EXPERIMENTAL AND CALCULATIONAL RESULTS

Abstract The validity of explaining the thermal effects using the chain model approach is investigated. For this purpose a copper (100) surface was bombarded along a 〈100〉 and a 〈110〉 direction, with low energetic noble gas ions. The comparison is troubled by charge exchange effects, which affect strongly the shape of the experimental energy distributions. At highej target temperatures the calculated temperature dependences agree with the experimental observations. At target temperatures below about 400 K discrepancies arise, which can be ascribed satisfactorily to the appearance of surface defects. The angular distributions depend strongly on the occurrence of charge exchange processes. They can be corrected for these effects using a model proposed earlier by the authors.

STUDY OF LOW-ENERGY NOBLE-GAS ION REFLECTION FROM MONOCRYSTALLINE SURFACES - INFLUENCE OF THERMAL VIBRATIONS OF SURFACE ATOMS .2. CALCULATIONAL EVALUATION OF 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 u2 and the correlation coefficients of the atomic thermal displacements. The used ion-atom interaction potential (a Thomas-Fermi potential in the Moliere 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 u2 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 u2. 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.

ADSORPTION STUDY OF HYDROGEN ON A STEPPED PT (997) SURFACE USING LOW-ENERGY RECOIL SCATTERING

Abstract Information on the binding energy of hydrogen adsorbed on a stepped Pt(S)−[9(111)×(111)] surface can be obtained by bombarding this surface with low energy noble gas ions and by measuring the yield of recoil hydrogen. The angle between the incident ions and the detected recoil hydrogen particles is fixed at 30°. The applied technique appears to be so sensitive that the dose of primary ions required for measuring an energy spectrum amounts to only 2×10 −4 of a monolayer. By a suitable choice of the scattering geometry one may distinguish between the adsorption properties of step edges and terraces. Step edge properties are probed preferentially by directing the ion beam step-upwards at glancing incidence, whereas terrace sites are probed mainly under specular conditions (e.g. parallel to the steps). The yield of recoil hydrogen is measured at constant equilibrium hydrogen pressure as a function of the sample temperature for both geometries. The shapes of the isobars are distinctly different. For step edge sites a coverage independent binding energy of 93 kJ/mol is measured and for terrace sites we deduce an initial value of 75 kJ/mol gradually decreasing with increasing coverage to 58 kJ/mol at θ ≅ 0.45.

CHARGE-EXCHANGE OF LOW-ENERGY HE+ IONS (LESS-THAN 10 KEV) SCATTERED FROM A (100) FACE OF A COPPER SINGLE-CRYSTAL

Abstract Yield measurements are presented for 2–10 keV He ions reflected over 30° or 60° from a (100) surface of a copper target. This yield is measured for different values of the angle of incidence and of the primary energy. The experimental results are explained by charge exchange processes occurring during the ion-surface interaction. From symmetry arguments we conclude that two types of charge exchange processes may be distinguished: (a) The ion interacts with the free electrons of the metal. This may result in an Auger-type neutralization, (b) The ion interacts with a single surface atom. This may result in ionization or neutralization, similar to that which occurs in ion-free atom collisions. A simple model, which takes into account Auger-neutralization and the ionization which results from a violent collision with a surface atom, can partly explain the experimental results. It appears that for a more complete description, the neutralization resulting from the ion-surface atom interaction must also...

SCATTERING OF LOW-ENERGY HELIUM IONS AND ATOMS FROM A COPPER SINGLE-CRYSTAL - ELASTIC AND INELASTIC EFFECTS

Abstract The scattering of 4–10 keV Helium ions from a copper surface can be described reasonably well with elastic, single collisions, even for fairly small scattering angles. However, in a more refined analysis of the experimental results, deviations from this behaviour are observed. Measurements of the energy and the width (FWHM) of the surface peak have been performed. The general behaviour can be explained by differences in inelastic energy losses for scattering from an ideal surface and from surface structures (surface damage). By comparing measurements in which the target is bombarded along different directions, it is shown that multiple scattering effects have only a minor influence. Additional information about the inelastic processes is obtained by performing scattering experiments with a primary atom beam. It appears that, for large angles of incidence, the energy of the reflected ions is reduced about 20 eV if the primary beam consists of atoms instead of ions. This difference is almost indepe...

NEUTRALIZATION AND IONIZATION OF LOW-ENERGY HELIUM IONS SCATTERING FROM A COPPER SURFACE

Abstract Ion yield measurements are presented |of 2–10 keV helium ions scattered from a copper (100) face. The scattering angle is 30°. The results are explained using a charge transfer model originally proposed by the authors. The model takes into account Auger neutralization as well as ionization and neutralization resulting from a violent collision. The present results are compared with previous] experiments in which a primary atom beam was used. Within the experimental errors the results |of both experiments can be explained using the same charge exchange parameters. A second result of the present investigation is that more convincing evidence is given for the occurrence of neutralization during the violent collision by a non-Auger process. It is probable that this process, as well as the ionization process, results from an interaction between the helium particle and the copper L shell electrons.

DETERMINATION OF SURFACE DEBYE TEMPERATURE USING QUASI-TRIPLE COLLISIONS IN LOW-ENERGY ION SCATTERING

Abstract Energy spectra of heavy low energy noble gas ions, specularly reflected from a monocrystalline surface into small scattering angles, show the existence of a quasi-triple collision peak. The occurrence of this QT peak results from scattering from “three atom pit” structures, present at the surface due to thermal vibrations of the target atoms. The temperature behaviour of this peak provides the possibility to estimate the magnitude of the atomic thermal displacements. This possibility is, to a great extent, independent of uncertainties introduced by a lack of knowledge about charge exchange effects and the actual interaction potential. In this investigation the surface Debye temperature of a copper (100) surface is obtained by comparing experimental and calculational results of 10 keV Kr+ ions scattered along different low index directions at the surface. The temperature dependences of the QT and the QD peak in experimental energy distributions of ions, reflected along these azimuthal target directions, agree remarkably with the calculations, using the just obtained Debye temperature. The calculated energy distributions exhibit qualitatively well the main characteristics of the experimental ones.