Scattering of low energy Na+ ions from a clean polycrystalline Ag surface

The energy distributions of low energy (E₀ = 0.4–3.2 keV) Na⁺ ions scattered from a clean polycrystalline Ag surface were measured. The angle between the incident beam and the surface was fixed at ψ = 45° while the scattering angle (θ) ranged from 50 to 130°. The cleanliness of the surface during the measurement was maintained by simultaneous deposition of Ag atoms from an effusion source. The obtained distributions considerably differ from the corresponding distributions of noble ions. Firstly, for all measured values of E₀ and θ, an intensive hump is observed in the high energy part of the distribution. In certain cases this hump is transformed into a peak. Secondly, the low energy part of the distribution is very pronounced, especially for higher values of E₀ and θ.     

The peculiarities of the low-energy ion scattering by polycrystal targets

In the present work, experimental and computer simulation studies of low-energy (E₀ = 80-500 eV) Cs⁺ ions scattering on Ta, W, Re target surfaces and K⁺ ions scattering on Ti, V, Cr target surfaces have been performed for more accurate definition of mechanism of scattering, with a purpose of evaluation of an opportunity of use of slow ions scattering as a tool of surface layers analysis. The choice of the targets was based on the fact that the ratios of atomic masses of target atoms and ions μ = m₂/m₁ were almost the same for all cases considered and greater than 1 (direct mass ratio) however, the difference of binding energies of target atoms in the cases of Cs⁺ and K⁺ scattering was almost twice as much. It has been noticed that the dependencies of the relative energy retained by scattering ions at the maximum of energy distribution versus the initial energy E_m/E₀ (E₀) have a similar shape in all cases. The relative energy retained by scattering ions increases while the initial energy of incidence ions decreases. The curves are placed above each other relative to the binding energies of target atoms, to show what this says about the influence of binding energy on a process of scattering of low-energy ions. The correlation between value of energy change maintained by an ion for different values of E₀ in the case of scattering by targets with different masses of atoms and its binding energies is experimentally established. The contrary behavior of the E_m/E₀ (E₀) dependencies concerning the target atom binding energy quantity E_b for cases with direct (μ > 1) and inverse (μ < 1) mass ratio of colliding particles is established. The comparison of experimental energy distributions with calculated histograms shows that the binary collision approximation cannot elucidate the abnormally great shift in the maxima of relative energy distributions towards greater energy retained by scattering ions.     

Oxygen adsorption on Cu(110): Determination of atom positions with low energy ion scattering

The position of adsorbed oxygen on Cu($\text{1}$10) surfaces was determined with Low Energy Ion Scattering (LEIS). The experiments were performed by bombarding the copper surface at small angles of incidence with low energy Ne⁺ ions (3–5 keV). Measurements of the Ne⁺ ions scattered by adsorbed oxygen showed regular peaks in the azimuthal distribution of the scattered ions due to a shadowing effect. From the symmetry of the azimuthal distributions it follows that the centre of an adsorbed oxygen atom on the Cu(1&#x0304;10) surface lies about 0.6 Å below the midpoint between two neighbouring Cu atoms in a 〈001〉 row. A comparison of the azimuthal distributions of Ne⁺ ions scattered from clean Cu surfaces and oxygen-covered Cu surfaces showed that hardly any surface reconstruction had occurred in the oxygen-covered surfaces. The applied method seems to be an appropriate one for locating adsorbed atoms because it uses only simple qualitative considerations about azimuthal distributions of scattered ions.     

Numerical experiments on low-energy ion scattering at a surface containing adsorbed impurity atoms

The energy distributions of Cs⁺ and W⁺ ions scattered, respectively, at Mo and W single crystals, the surfaces of which contain adsorbed oxygen, nitrogen, and cesium atoms, are calculated via the molecular dynamics method. It is demonstrated that the energy spectra are sensitive to the impurity atoms of a surface. When compared with the energy distributions corresponding to a pure surface, even a partial monolayer of these atoms can modify their shape. A peak in the scattered-ion intensity is commonly shifted to the lowerenergy region if the adsorbate is light and to higher energies if the adsorbate is heavy.     

Ion scattering (Me+ → Me) by monatomic films

Discusses the results of computer simulation of the scattering of metal ions low energy (E ₀ = 2–530 eV) films those metals (Me⁺ → Me) with thickness of few atomic layers on a crystalline substrate, conducted in the framework of the model of multi-particle interactions. The effect of conservation for scattered ions with large values of energy, depending from the film crystal structure, has been realized as in the case of scattering on monocrystalls [1]. It has been determined, that the 3–5 atomic layers coating can save the substrate surface from its interaction with bombarding ions in the energy region E ₀ up to 500 eV.     

Interaction of low energy (5–10 keV) argon atoms and ions with a Cu(100) surface; Ion fraction,ionization and neutralization

The ion fractions η⁺ of low energy (5–10 keV) argon particles scattered from a Cu(100) surface, are measured with a time of flight spectrometer. Neutral as well as charged projectiles are used. The scattering angle θ is 30°. The results for different angles of incidence ψ and crystal directions are reported. For scattering in the 〈100〉 direction, with a ψ-value of 15° and a primary energy E₀ of 5 and 10 keV, the ion fractions for the quasi single scattering peak, η⁺_QS, are 1.5 and 6.1% respectively. When E₀ is between 5 and 10 keV a reionization process with a constant reionization probability occurs during the violent interaction. This process, but also neutralization along the outgoing trajectory, determines η⁺_QS. With ions as projectiles, an energy difference of about 16 eV is observed between the quasi single scattering peaks in the spectra of all scattered particles and of ions only. The ion fraction for the quasi double scattering peak, η⁺_QD. depends largely upon E₀, indicating that the efficiency of the reionization process increases with E₀. A qualitative discussion of the data is given, using the reionization process and the interatomic neutralization processes along the trajectory of the scattered particles.     

The structure of a stepped copper (410) surface determined by ion scattering spectroscopy

Ion Scattering Spectroscopy applied in the multiple scattering mode is used to determine the structure of a stepped Cu(410) surface. The energy of singly scattered ions is influenced by the presence of neighbour surface atoms. This effect can be used to determine interatomic distances up to about 10Å, as is shown by the results of 8 keV Ar⁺ and 11 keV Ne⁺ scattered through θ = 50°. The edge-edge distance of the stepped copper surface appears to be in accordance with the results of LEED experiments obtained by other investigators. The experiments show a good agreement with the results of the analytical 3-atom model of Poelsema. The energy of the so-called “plateau collision” appears to depend on the effective plateau length l as measured in the plane of incidence. Lengths l between 15 and 60 Å can be determined with an accuracy of 5 Å. Results are shown for 8 and 12 keV Ar⁺, θ = 40° and 60°, and 8 keV Kr⁺ θ = 40°. The experimental dependence of the energy on lis described correctly by a phenomenological model.     

Low energy (E0 < 10 keV) atom and ion scattering of neon from a Cu(100) surface; Ionization and neutralization

The ion fractions η⁺ of low energy (5–10 keV) neon particles scattered from a Cu(100) surface are measured with a time of flight spectrometer. These fractions are obtained for neutral as well as charged projectiles and for different crystal directions. The scattering angle θ was 30°. For a primary energy E₀ of 5 keV neutral projectiles have a value for η⁺ which is 30 times lower than for charged projectiles; these values are 0.15 and 4.5% respectively. For E₀ = 10 keV the values of η⁺ are about the same (~22%). Energy differences up to 22 eV, depending on E₀, are observed between the single scattering peaks in the ion spectra of charged and neutral projectiles but also between the single scattering peak in the spectra of all scattered particles and of ions, with ions as projectiles. A qualitative discussion of these data is given, involving charge transfer processes of noble gas particle and target atom. The data suggest that these neutralization processes can be described more adequately with interatomic neutralization processes along the trajectory than with Auger neutralization by conduction electrons.     

Hyperthermal scattering of Cs from Pt(1 1 1): the effect of image charge on the ion-surface energy transfer

We have studied the energy exchange between hyperthermal (5-100 eV) Cs⁺ projectiles and a Pt(1 1 1) surface by measuring the kinetic energy of the scattered ions. The scattering geometry was chosen to be in-plane with specular scattering angles, and the energy of the scattered ions was analyzed as functions of incidence energy and angle. For low incidence energy (<40 eV), the energy transfer to the Pt surface is substantially enhanced due to the attractive image charge force between Cs⁺ and the surface. The image charge effects are highlighted by the different energy transfer on Pt(1 1 1) and Si(1 1 1) surfaces. Analysis of the experimental results using two- and three-dimensional theoretical models revealed a well depth of 1 eV for the image charge potential. Hyperthermal Cs⁺ ions scatter from Pt(1 1 1) predominantly via double collisions with Pt atoms, though the scattering phenomena are insensitive to the impact site at the surface.     

Ion survival probabilities for 3 keV Ar+ scattering from La,Yb, and chemisorbed H2, O2, and H2O on La surfaces

TOF spectra of scattered primary and surface recoiled neutrals and ions for 3 keV Ar⁺ bombardment of clean La and Yb and H₂, O₂, and H₂O saturated La surfaces are presented. The spectra are analyzed in terms of single (SS) and multiple (MS) scattering of the primary ions and surface recoiling (SR) of adsorbate atoms. Measurement of spectra of neutrals + ions and neutrals alone allows determination of scattered ion fractions Y. The Y values for the SS event are high for clean La (37%) and lower for adsorbate covered La (32% for H₂, 13% for O₂, and 8% for H₂O); Yb exhibits a similar behavior, i.e. 16% for clean Yb and 5% for O₂ + H₂O covered Yb. Photon emission accompanying the scattering collision has been observed from clean La and Yb and adsorbate covered La. A preferential inelastic energy loss of 15 ± 3 eV for the SS event has been observed for scattered neutrals as opposed to ions for La and H₂ saturated La at 135°. These results are interpreted within the models for Auger and resonant electronic charge exchange transitions during approach or departure of an ion with a surface and the electron promotions occuring during close atomic encounters where the electron shells are interpenetrating.     

Inelastic effect in low energy ion-surface collisions: He+Au,Ag

Energy distributions of He⁺ ions scattered by Au and Ag surfaces are measured by an ISS system with high energy resolution, at a scattering angle of 90° and at incident ion energies ranging from 277 to 977 eV. It is found that the observed peak energies deviate toward the low energy side by several electron-volts with respect to the calculated elastic single collision energies. Both the deviation Q' and the inelastic loss energy Q are plotted as a function of incident ion energy for the Au surface.     

Scattering of Li+ from LEED characterized W(110) and Si (111) surfaces at energies between 2 and 20 eV

Energy and angular distributions of Li⁺ ions scattered from W(110) and Si(111) surfaces have been measured for a wide range of scattering angles and for beam energies between 2 and 20 eV. The collision process can be explained in terms of binary collisions with single surface atoms, if the influence of the attractive part of the interaction potential is taken into account. A square well approximation for the potential makes it possible to predict the form of the energy spectrum as well as the behaviour of the angular distribution of the scattered particles for both systems. The influence of adsorbed atoms and molecules on the scattering behaviour has been investigated. Exposure of the W surface to H₂, O₂ and CO shows that surface coverages exceeding 10% of a monolayer very drastically influence the scattering. The results from the clean surfaces indicate that sufficiently precise measurements of the energy spectra of the scattered particles can yield very detailed information about the form of the interaction potential. The form of the energy spectra also contains information as to the extent of vibrational excitation of the surface atoms.     

The theoretical and experimental study of the ionization processes during the low energy ion sputtering

The process by which atoms are ionized as they are sputtered from a metal surface has been analyzed both theoretically and experimentally. In the theoretical part the expressions for ionization coefficient R⁺ of atoms having the ionization energy much larger than the metal work function have been derived using a molecular orbital method. The effect of the level crossing was estimated in an approximate way. In the experimental part the SIMS experiments on clean Ni and Al surfaces and on Ni surface covered with a submonolayer of adsorbed K, Na and Al are reported. It has been found and it is for the first time reported that the energy distribution of ions sputtered from a submonolayer of adatoms is independent of energy (200–2500 eV) and mass (Ar⁺ Xe⁺ of incident ions and depends only upon the adsorption energy of the adatom. The energy distribution of ions sputtered from bulk samples has been found dependent on the primary ion energy. The measurement of the absolute value of R⁺ has shown that there is a strong correlation between the number of the adatom valence d-electrons and the value of R⁺, the value of R⁺ being smaller for atoms with more d-electrons. These experimental data have been compared with the theoretical expressions and the important role of the mechanism which takes into account the bending of the adatom energy level has been assessed.     

Scattering of plasma-jet ions by a tantalum target

The possibility of recording a chemical reaction occurring on the surface of a metal target in a collisionless plasma jet by analyzing the longitudinal-velocity spectra of jet-plasma ions scattered by the target is shown. The target is a Ta strip with a 3×0.1 mm cross section. The ion energy is 30 eV. It is found that, after rapid heating of the target to 1000 K, the current produced by scattered ions decreases and is then restored in a time of 40 s for nitrogen ions and 60 s for hydrogen ions. The relaxation of the current correlates with the accumulation of an intermediate reagent (probably, TaO₂Na⁺ ions) on the target surface; the reagent participates further in the formation of experimentally observed ions with a mass of 245±1 (probably, TaO₂Na⁺ · H₂O).     

Study of the angular and energy distributions of Na+ and K+ ions which have passed through thin copper films

The angular and energy distributions of alkaline Na⁺ and K⁺ ions which have passed through thin Cu films in different crystal states are studied. The ion energy E₀ is varied from 10 to 40 keV, and the incidence angle. ranges from 0° to 60°. The angular aperture of the detector is ～0.5°, which allows the form of the angular distribution of ions which have passed through the solid thin films as a function of the energy, the angle of primary-ion beam incidence, and the layer thickness to be studied in detail. It is shown that, in the range E₀ = 10.40 keV, the energy loss ΔE of those ions that have passed increases linearly as the energy of incident ions increases. The energy loss increases with increasing ion mass in the case of singly charged ions. The surface amorphization of single- and polycrystalline films leads to an increase (by 150–200 eV) in the energy loss caused by the diffuse propagation of ions and to loss-peak broadening. It is probable that surface amorphization is accompanied by an increase in the number of atoms experiencing multiple collisions with atoms of the film, which leads to an increase in the average energy loss by ions that have passed through films.     

Surface analysis by ion-electron spectroscopy; Silicon target

The energy spectra of secondary elections emitted from a Si(111) surface due to bombardment by 6 keV He⁺ and O⁺₂ ions have been examined. The fine structure in the spectra is explained on the basis of a novel mechanism of creation of Auger electrons at the surface. There are two stages of interaction between incoming ions and the substrate via adsorbed atoms. In the first stage, due to a level promotion mechanism, vacancies in the adsorbed atoms are created. In the second stage, Auger neutralization processes accompanied by the emission of electrons from a solid with characteristic energies take place. These electrons provide a good indication of the degree of coverage of the silicon surface with contaminant atoms. The energy losses of escaping electrons are also discussed.     

Features of the energy distribution of secondary particles upon the ion bombardment of graphite

The secondary emission of carbon atoms from the (0001) plane of graphite nanocrystallites bombarded with argon ions with energies of 1 and 10 keV and the incidence angle α = 45° is investigated. The unusual oscillating energy distributions of secondary C⁺ ions with main maxima E _max in the range of 40–60 eV and peaks corresponding to the energies E ₁ ≈ 20, 30, 70, 80, and 100 eV have been revealed. The C⁺ ion yield decreases, the energy spectrum increases, and the maximum E _max shifts to larger energies E ₁ with increasing emission angle (with respect to the normal to the surface). The secondary-ion emission from the (0001) face of graphite is numerically simulated with allowance for the charge exchange of secondary ions to obtain a qualitative explanation of the observed results.     

Sputtering of metals at ion-electron irradiation

It has been found that, in contrast to the commonly accepted opinion, simultaneous irradiation by 15-keV Ar⁺ ions and 2.5-keV electrons at temperatures above 0.5T _m (T _m is the melting temperature) induces much larger sputtering of metallic copper, nickel, and steel than irradiation only by Ar⁺ ions. The effect increases with the temperature. At T = 0.7T _m, the sputtering coefficients in the case of ion-electron irradiation are more than twice as large as the sputtering coefficients in the case of irradiation by Ar⁺ ions. The experiments on the sublimation of copper show that the sublimation rate in the case of the heating of a sample by an electron beam is higher than that in the case of heating in an electric vacuum oven. The revealed effects are explained by the electron-induced excitation of adatoms (atoms stuck over the surface, which appear owing to ion bombardment). Excited adatoms have a smaller binding energy with the surface and are sputtered more easily.     

Two-dimensional observation of N2+ and N+ scattering patterns from Cu(001)

Low-energy (<1 keV) N₂⁺ and N⁺ ion scattering from the Cu(001) surface was directly observed by using a two-dimensional detection system. The two-dimensional pattern of the scattered ions changed when the primary ion energy was increased from 200 to 600 eV. The scattering peak shifted toward the [110] azimuth, to which the incident plane was not parallel, above 600 eV. The difference of ion-survival probabilities between N₂⁺ and N⁺ was discussed from the full width at half maximum of the angular distribution of scattered ions.     

Resonance fluctuation-electromagnetic energy losses during the glancing reflection of a neutral atomic beam from the smooth amorphous surface of a solid

The energy losses of a Cs⁺ion beam are theoretically studied during its glancing reflection from a smooth amorphous surface of a dielectric or a semiconductor and films made of these materials on a metallic substrate. The conditions of resonance fluctuation-electromagnetic interaction between neutralized Cs atoms and surface polaritons are considered for surfaces where the effects of interest seem to be the most significant. Calculations indicate that, at the optimized initial glancing angle and the Cs⁺ ion beam energy (ψ_in = 0.1–1.0 mrad, E ₀ ～ 50–100 keV), the fluctuation-electromagnetic forces substantially contribute to the total energy losses and this contribution has characteristic dependences on the temperature, particle velocity, and material parameters.