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.     

Argon (10 keV) scattered from structures,induced by bombarding a Cu(100) surface; ionization and neutralization

The ion fractions, η⁺, of 10 keV argon particles, scattered from a damaged copper surface, are measured with a time of flight spectrometer. The damage was introduced by bombardment with argon ions. The scattering angle was 30°. The results for different angles of incidence, ψ, are reported. For Ψ < 10° the ion fraction is relatively high (～27% for Ψ = 4°) and decreases as Ψ increases. For Ψ = 15° the value of η⁺ is 7%, whereas for 21° < Ψ < 27° the value of η⁺ appears to be constant (～14%). An explanation is given by assuming interatomic ionization as well as neutralization processes along the trajectory of the scattered particles. The number of step-atoms, induced by ion bombardment, is estimated to be about 2 × 10¹⁴/cm².     

Energy and charge distributions of fast hydrogen ions and atoms reflected from Cu in the case of grazing incidence

The energy and charge distributions of protons and hydrogen atoms reflected from the Cu surface in the case of grazing incidence angles are measured at energies of incident particles (H⁺ and H⁰) of 200 and 250 keV. The charged fractions of reflected particles are analyzed. A weak dependence of the neutral fraction of reflected particles on the scattering angle is discovered for incidence angles of 1°–2° and an energy of scattered particles of 60 keV or less. It is shown that the neutral fraction of reflected particles with an energy of 60–80 keV or more is independent of the scattering angle and is determined by the ratio of the cross sections for the electron capture and loss by ions in the material.     

Reconstruction of heterogeneous single-channel neutralization kinetics by the method of fast-ion grazing scattering from metal surfaces

A method is suggested for the unambiguous reconstruction of the heterogeneous slow-ion neutralization kinetics near the surface of a conductor. The method is based on the special features of fast ion grazing scattering with above-thermal energies of translational motion along the normal. It is shown that the angular distributions of fast particles reflected from the surface are related to the slow-ion neutralization rate by a simple algebraic expression. The method allows the reconstruction of the coordinate dependence for the neutralization rate and the interaction potentials. Its possibilities are demonstrated by the example of neutralizing fast He⁺ ions (ion energies E₁≈2 keV and glancing angles θ₀≈0.5°–0.8°) scattered from the A1(111) surface.     

Energy distributions of atomic and molecular ions sputtered by C60 projectiles

In the process of investigating the interaction of fullerene projectiles with adsorbed organic layers, we measured the kinetic energy distributions (KEDs) of fragment and parent ions sputtered from an overlayer of polystyrene (PS) oligomers cast on silver under 15 keV C₆₀⁺ bombardment. These measurements have been conducted using our TRIFT™ spectrometer, recently equipped with the C₆₀⁺ source developed by Ionoptika, Ltd. For atomic ions, the intensity corresponding to the high energy tail decreases in the following order: C⁺(E^−0.4) > H⁺(E^−1.5) > Ag⁺(E^−3.5). In particular, the distribution of Ag⁺ is not broader than those of Ag₂⁺ and Ag₃⁺ clusters, in sharp contrast with 15 keV Ga⁺ bombardment. On the other hand, molecular ions (fragments and parent-like species) exhibit a significantly wider distribution using C₆₀⁺ instead of Ga⁺ as primary ions. For instance, the KED of Ag-cationized PS oligomers resembles that of Ag⁺ and Ag_n⁺ clusters. A specific feature of fullerene projectiles is that they induce the direct desorption of positively charged oligomers, without the need of a cationizing metal atom. The energy spectrum of these PS⁺ ions is significantly narrower then that of Ag-cationized oligomers. For characteristic fragments of PS, such as C₇H₇⁺ and C₁₅H₁₃⁺ and polycyclic fragments, such as C₉H₇⁺ and C₁₄H₁₀⁺, the high energy decay is steep (E⁻⁴ − E⁻⁸). In addition, reorganized ions generally show more pronounced high energy tails than characteristic ions, similar to the case of monoatomic ion bombardment. This observation is consistent with the higher excitation energy needed for their formation. Finally, the fraction of hydrocarbon ions formed in the gas phase via unimolecular dissociation of larger species is slightly larger with gallium than with fullerene projectiles.     

Photon emission from rare gas ion bombardment of metal surfaces

Metal surfaces (Mg, Cu, Zr, Mo) are bombarded with He⁺, Ne⁺ and Ar⁺ in the energy range of 400 eV to 8 keV. Radiation from scattered projectiles and sputtered target particles is observed between 200 and 700 nm. It is shown that most of the radiating particles originate from surface collisions. Auger neutralization, resonance tunneling and direct electron transitions are the important electronic processes involved.     

The measurement of energy spectra of neutral particles in low energy ion scattering

An apparatus is described for low energy (0.1–10 keV) ion scattering (LEIS) experiments. A time of flight (TOF) spectrometer is incorporated in the system to be able to measure the energy of particles in the neutral state after scattering. The energy resolution ΔE/E of the TOF spectrometer is discussed and found to be 0.5% (FWHM). This is sufficient for our scattering experiments. An electrostatic analyzer (ESA) is used to measure the energy of scattered ions [ΔE/E=0.5% (FWHM)]. Experiments show that in general the ion dose needed to obtain a TOF spectrum (2×10¹⁰ ions/cm²) is much smaller than the dose needed for an ESA-spectrum (6×10¹³ ions/cm²). The ion spectra measured with the TOF spectrometer, by subtracting the neutral yield from the total yield, as well as with the ESA are found to agree quite well. This provides a way to calibrate the TOF spectrometer. The determination of the ion fraction of scattered particles is discussed [10 keV⁴⁰Ar⁺ on Cu(100), scattering angle 30°]. It is shown that the TOF spectrometer is able to measure light recoil particles (e.g. hydrogen) from a heavy substrate. In the analysing system is, in addition to the TOF spectrometer, also incorporated a stripping cell to measure the energy of neutral scattered particles. An energy spectrum of neutral scattered particles measured with both methods is shown.     

Inelastic energy losses in single and double collisions of keV-energy Ar<Superscript>+</Superscript> ions with Be,C, Al,Si, Ge,and in targets

Inelastic energy losses Q in scattering of Ar⁺ ions with an initial energy of E ₀ = 5 keV from Be, C, Al, Si, Ge, and In targets are determined using low-energy ion scattering spectroscopy. In spite of the fact that Ar⁺ ion beams are used in the overwhelming majority of applied studies devoted to analysis of the elemental composition and structure of materials for modern electronics, information on inelastic losses for these bombarding particles is scarce. It is shown that the knowledge of the value of Q makes it possible to correctly interpret the energy spectra of particles emitted during ion bombardment of the surface.     

Ion survival probabilities for 3 keV He+, Ne+, and Ar+ scattering from La and adsorbate covered La surfaces

TOF spectra of scattered neutrals and ions for 3 keV He⁺, Ne⁺, and Ar⁺ bombardment of La and adsorbate covered La surfaces show that the scattered ion fractions are 21.1% and 10.7% for Ar⁺ on clean and adsorbate covered La, respectively, and < 1% for all of the other systems. These results are consistent with a model in which Auger and resonant neutralization (AN and RN) transitions govern the ion survival probability.     

Reionization contributions in low-energy scattering of He,Ne and Ar ions from Sn,Au, and BaTiO3

The proportion of reionized neutrals to the total scattering yield of rare-gas ions is reported for primary energies E₀ from 0.1 to 2.0 keV. The measurements were made for several ion-target combinations by comparing the ion yield obtained from incidence of ions with that obtained from incidence of neutrals. The ration of the reionized neutrals to the total ion yield is strongly dependent on the combination of projectile ion and target atom. In some combinations, e.g. Ne on Sn for E₀ > 1 keV and Ar on Ba for E₀ > 0.5 keV, the reionized neutrals constitute the main part of the total ion yield, while the proportion of reionized Ne⁰ scattered from Au is very small and increases gradually with increasing E₀.     

Low energy alkali ion scattering investigation of Au nanoclusters grown on silicon oxide surfaces

The atomic and electronic structures of Au nanostructures grown by deposition onto various silicon oxide surfaces were probed with low energy alkali ion scattering. Charge state-resolved time-of-flight spectra of scattered 2 keV ³⁹K⁺ ions were collected from Au deposited onto an untreated Si wafer with a native oxide, a thermally grown oxide surface, and atomically-clean Si(111). It is shown that nanoclusters form on both oxides, but not on the clean Si. A quantitative analysis of the ion scattering spectra indicates that the nanoclusters are initially flat, two-dimensional structures that start to develop a second layer at about 0.5 Å of deposited Au and then form three-dimensional islands. The neutral fraction of scattered 2 keV ³⁹K⁺ ions decreases with deposition indicating changes in the quantum state occupancy with cluster size. The shapes of the clusters differ on the native and thermal oxides, leading to shape-dependent neutralization.     

Energy spectra of 6–32 keV neutral and ionized Ar and He scattered from Au targets; ionized fractions as functions of energy

The neutralization of ions is an important aspect of low energy ion scattering for surface analysis. Electrostatic energy analyzers (ESA) have been used almost exclusively in such work, and information on charge neutralization efficiencies is needed for quantitative interpretation of ESA data. In the past, the occurrence in low energy ion spectra of surface peaks and low backgrounds due to scattering from inside the solid has been attributed to preferential neutralization of ions which penetrate beyond the surface. In the work to be described, a time-of-flight technique was used to measure energy spectra of both neutral and ionized Ar and He scattered at 90° from a polycrystalline gold target. Incident energies of 6–32 keV were used. The energy spectra of neutral Ar scattered from polycrystalline gold exhibit sharp surface peaks, and double scattering shoulders, over this entire energy range. For He there is a gradual downward slope toward lower energy rather than a sharp surface peak. The behavior in both cases is attributed to large scattering cross-sections which cause a loss of beam particles during penetration. A calculation using a $\text{1}\text{r}{}^2$ potential illustrates this effect as a function of energy for helium. In the present experiments we find that the ion fraction of scattered argon does indeed depend on depth of penetration. This is in contrast to the behavior of He and H at higher energies, e.g. 100 keV, in which cases the charge state depends on emergent velocity but not on depth of penetration. The characteristic shapes of ion scattering spectra in this energy range appear to result from both neutralization and beam attenuation inside the target.     

Scattering of F atoms and anions on a TiO2(1 1 0) surface

Results of a study of energy losses and electron transfer processes for grazing scattering of fluorine atoms and anions scattering along different azimuthal orientations of the TiO₂ crystal are presented. We observe strong variations in the overall intensity of scattered particles which are due to channelling effects. The energy losses do not show strong variations as a function of crystal azimuth except for the case of scattering along the (0 0 1) direction between the bridging oxygen atom rows, where we also observe differences in the energy losses of scattered ions and neutrals. We attribute this to the fact that larger F⁻ survival occurs for trajectories staying farther from the surface, when also the energy losses remain small. The overall characteristics of energy losses are attributed mainly to trajectory effects due to scattering in regions of different electron density. Measurements of the ratio of scattered ions to the total scattered flux, i.e. the ion fractions which reflect electron capture and loss processes, show that these are not the same for incident anions and atoms. A strong difference for scattering along the (0 0 1) direction is observed, where at low incident energies a strong survival of incident ions occurs. These results are tentatively discussed in terms of non resonant electron capture at lattice O⁻ sites and electron loss into the conduction band or by collisional detachment with bridging O atoms.     

Multiple scattering and neutralization aspects of low energy noble gas ions incident on a Cu (110) crystal

Experiments have been done with 1 keV Ne⁺ ions bombarding a Cu (110) single crystal in a (111) plane. From the measured energy spectra of the scattered ions the minimum and maximum scattering angles for multiple scattering on the surface are determined. These minimum and maximum scattering angles were also calculated using a computer model. The parameters for an interatomic potential function between ion and metal atom are determined by comparison of the experimental and calculated scattering angles. A neutralization model for an energetic ion at an atomic surface is given. The reliability of this model is tested by comparison of the results of computer calculations and the measured peak intensity distributions as a function of the scattering angle.     

Dependence of scattered ion yield on the incident energy: Ne on pure gallium and indium

An experimental study of the ion yield dependence on the incident energy (0.4-2.2 keV) for Ne⁺ isotopes scattered at 120° from pure gallium and indium targets has been carried out by mass-resolved ion-scattering spectrometry. For both two targets, the ion yield curves exhibited a broad maximum below 0.8-1 keV (with a lower position for Ne⁺ on In) followed by a monotonous decrease yield without any oscillatory features. The energy dependence of ion-survival probability was explained as a complex interplay of the Auger neutralization with the characteristic velocity v_c=(0.9±0.1)×10⁷ cm/s for Ne⁺ on Ga, and the collision-induced neutralization and reionization. The later ones were significant processes at the energies larger than 0.8-1.0 keV or, in terms of the distance of closest approach, d?0.5-0.55 Å; the collision-induced neutralization was more effective than the inverse process. No visible influence of isotope effect on charge exchange was found. The ion-survival probability versus the inverse ion velocity displayed an independence on the mass of Ne⁺ projectiles.     

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.     

Luminescence in collisions of low-energy ions with graphite and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Excitation of H⁺, H₂ ⁺, H₃ ⁺, He⁺, and Ar⁺ ions by impact on graphite and Al₂O₃ was investigated by means of emission spectroscopy in the 50–1000 eV energy range of the projectiles. Emission of Balmer series from excited neutral hydrogen is observed for both targets. In addition, for the Al₂O₃ target a continuum emission is observed. The continuum probably originates from excited M_nO_m molecules produced in the collision cascade, when surface atoms bound by ionic bonds are released after the bond breaking caused by neutralization. The spectra obtained under Ar⁺ -bombardment show Ar II lines emitted by backscattered ions.     

Survival of He+ ions during grazing scattering from a Ag(111) surface

He+ ions as well as neutral He atoms with keV energies are scattered under a grazing angle of incidence from a clean and atomically flat Ag(111) surface. From a comparison of ion fractions observed after scattering of He+ ions and He atoms we find for energies below some keV small but defined fractions of ions that have survived the complete scattering event with the surface. This feature allows us to clear up the microscopic interaction scenario for Auger neutralization of He+ ions at a Ag(111) surface. The Auger neutralization rates are 2 to 3 orders of magnitude smaller than conventional rates derived from experiments for He+-metal systems and agree with recent calculations.     

Secondary ion emission from a GaAs single crystal upon bombardment with Bi<Stack> <Subscript> <Emphasis Type="Italic">m</Emphasis> </Subscript> <Superscript>+</Superscript> </Stack> cluster ions

Secondary-ion mass spectra and energy distributions upon bombarding a gallium arsenide single crystal using Bi_m⁺(m = 1–5) cluster ions with energies of 2–12 keV are investigated. The gallium cluster ion yield grew nonadditively with the number of atoms in the cluster projectiles. A quasi-thermal component found in the energy spectra of secondary Ga⁺ and Ga₂⁺ ions is indicative of the occurrence of the thermal spike mode upon cluster ion bombardment. The quasi-thermal component in the yield of atomic Ga⁺ ions upon bombardment with Bi₂⁺–Bi₅⁺–ions is 35–75%.     

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.