Auger neutralization of He ions at an Al(100) surface using isotope effect

He atoms and ions of the isotopes 3He and 4He are scattered with keV energies under a grazing angle of incidence from a flat and clean Al(100) surface. For the two isotopes we investigate Auger neutralization of incident He+ and He2+ ions via fractions of surviving ions. Pronounced effects for the different isotopes are observed which can be attributed to different time scales concerning the neutralization process of He ions in front of a metal surface. From the analysis of the data obtained for singly and doubly charged ions we find evidence that charge fractions for scattering of He+ ions from an Al surface result predominantly from a direct (Auger) electron capture event.     

Surface miller index dependence of Auger neutralization of ions on surfaces

Neutralization of He+ ions in grazing incidence scattering on Ag(111) and Ag(110) surfaces is studied. These measurements reveal the existence of an order of magnitude difference in the probability of ion survival on Ag(110) and Ag(111). The experimental results are discussed in terms of survival from Auger neutralization, whose rates are derived theoretically. Molecular dynamics simulation of scattered ion trajectories is performed and the surviving ion fractions are then calculated using the theoretical Auger neutralization rates, without adjustable parameters. The calculations agree quite well with the experimental data and show that the observed differences in the neutralization probabilities on these surfaces are related to different extensions of the electron density beyond the surface, resulting from different atomic packing.     

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.     

Velocity scaling of ion neutralization in low energy ion scattering

The ion fraction P+ is measured for He+ ions scattered by 129 degrees from a Cu surface. Both the primary energy and the angles of incidence and of exit are varied. From our results we conclude the following: along the incoming and outgoing trajectories, neutralization is due to Auger processes and depends on the normal velocity component v( perpendicular ) only. At higher energies, additional charge exchange is due to collision induced neutralization and reionization, both depending on the total ion energy only. Also in this regime P+ depends on v( perpendicular ), but via a two-valued function of the scattering geometry at fixed energy.     

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.     

New model for ion neutralization at surfaces

We investigate the neutralization of low energy He⁺ ions in close collisions with metal surface atoms. In order to describe the neutralization process as completely as possible, we consider Auger neutralization (AN), resonant neutralization (RN) and resonant ionization (RI). Our calculation agrees well with experimental data and shows that in some metals (like Pd) AN is the dominant process, whereas in others (like Al) RN and RI contribute significantly for energies above the threshold for reionization.     

Energy and charge distribution of helium atoms reflected from a Cu surface under grazing incidence

The energy and charge distributions of helium ions and atoms reflected from a Cu surface at grazing incidence have been measured at the energies 250 and 300 keV. The dependence of the relative number of helium atoms in the scattered beam on the scattering angle at incidence angles less than 2° and scattered particle energies below 150 keV has been discovered. It is shown that, for a theoretical estimation of the charge fractions of particles reflected from the target surface, charge-exchange cross sections used in calculations must be corrected when solids are considered instead of gases.     

Neutralization of low energy helium ions scattered from Au adatoms on Si(111)-√3 × √3-Au

The neutralization of 5 keV He⁺ ions scattered from Au adatoms on the Si(111)-√3 × √3-Au surface was studied by impact-collision ion-scattering spectroscopy (ICISS). The He⁺ ICISS data contained false shadowing features that were actually the result of local neutralization effects. The radially dependent ion-atom neutralization theory of Woodruff, when used in our simulations of the ion scattering results, was reasonably successful in describing the neutralization of the He⁺ ions by the Au atoms. Good agreement for both the [112&#x0304;] and [1&#x0304;10] azimuths was obtained for a neutralization rate R = A exp(− ar), where A and a are 15.5 fs⁻¹ and 1.94 Å⁻¹ , respectively. An Auger neutralization model assuming a planar ion-solid interaction surface was also tested, yielding much poorer agreement.     

Crystal effects in the neutralization of He+ ions in the low energy ion scattering regime

Investigating possible crystal effects in ion scattering from elemental surfaces, measurements of the positive ion fraction P+ are reported for He+ ions scattered from single and polycrystalline Cu surfaces. In the Auger neutralization regime, the ion yield is determined by scattering from the outermost atomic layer. For Cu(110) P+ exceeds that for polycrystalline Cu by up to a factor of 2.5, thus exhibiting a strong crystal effect. It is much less pronounced at higher energies, i.e., in the reionization regime. However, there a completely different angular dependence of the ion yield is observed for poly- and single crystals, due to massive subsurface contributions in nonchanneling directions.     

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.     

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.     

Angular studies of potential electron emission in the interaction of slow ions with Al surfaces

We report energy distributions of electrons emitted from Al surfaces under impact by 1 keV Ar+ and 1-5 keV Ne+ ions. The variation of the energy distributions with the angle of incidence is different for both ions and provides information on the mechanism responsible for electron emission. For Ar+ electron emission results mainly from Auger neutralization, while for Ne+ an important emission mechanism is the decay of plasmon excitations. We find a transition between surface and bulk plasmon excitations as the energy of the ion is increased.     

Coherence during scattering of fast H atoms from a LiF(001) surface

The coherence for diffraction effects during grazing scattering of fast hydrogen and helium atoms from a LiF(001) surface with energies up to some keV is investigated via the coincident detection of two-dimensional angular distributions for scattered projectiles with their energy loss. For keV H atoms, we identify electronic excitations of the target surface as the dominant mechanism for decoherence, whereas for He atoms this contribution is small. The suppression of electronic excitations owing to the band gap of insulators plays an essential role for preserving quantum coherence and thus for the application of fast atom diffraction as a surface analytical tool.     

Diffraction of fast atomic projectiles during grazing scattering from a LiF(001) surface

Light atoms and molecules with energies from 300 eV to 25 keV are scattered under a grazing angle of incidence from a LiF(001) surface. For impact of neutral projectiles along low index directions for strings of atoms in the surface plane we observe a defined pattern of intensity spots in the angular distribution of reflected particles which is consistently described using concepts of diffraction theory and specific features of grazing scattering of atoms from insulator surfaces. Experimental results for scattering of H, D, 3He, and 4He atoms as well as H2 and D2 molecules can be unequivocally referred to atom diffraction with de Broglie wavelengths as low as about 0.001 Angstroms.     

Surface structure and electron density dependence of scattered Ne ion fractions from the Si(1 0 0)-(2×1) surface

The magnitudes and azimuthal anisotropies of 4 keV Ne⁺ scattered ion fractions from the Si(1 0 0)-(2×1) two-domain surface have been measured by means of time-of-flight scattering and recoiling spectrometry. The absolute values of these ion fractions as well as their dependence on surface structure and electron density have been determined. By investigating the trajectories of the scattered Ne⁺, a clear correlation is demonstrated between these experimentally observed surviving ion fractions of Ne⁺ and the fraction of ions that scatters from the topmost layer of the surface. This is interpreted in terms of a model in which the neutralization probability of Ne⁺ is proportional to the local substrate electronic charge density.     

Spin-polarized electrons in collisions of multicharged nitrogen ions with a magnetized Fe(001) surface

We report on the first spin-resolved energy spectra for the emission of electrons during grazing scattering of 150 keV multicharged nitrogen ions from a magnetized Fe(001) surface. A substantial spin polarization for KLL Auger electrons emitted in the final stage of the neutralization sequence during the interaction of multicharged ions with a metal surface is observed. We conclude from our data that the projectile L shell is dominantly populated by electrons from the conduction band of the target. For low energy electrons we find an increase of their spin polarization with an increase of the projectile charge.     

Low-energy ion-induced electron emission from a MgO(100) thin film: the role of the MgO-substrate interface

We present a detailed study of the electron emission from a thin MgO(100) film on a Mo substrate, bombarded with slow He+, Ne+, and Ar+ ions. Neither the high absolute number of emitted electrons per incoming ion nor the electron spectra can be due to Auger neutralization of the incoming ions at the MgO surface alone. Therefore, an additional mechanism is proposed: holes created in the MgO film are transported to the MgO-substrate interface where they give rise to an Auger neutralization process involving two electrons from the metal substrate conduction band.     

An investigation of the Si(111)−(√3 × √3)R30°−Ag surface by Li impact collision ion scattering spectroscopy

The Si(111)−(√3 × √3)R30°−Ag surface has been investigated using the technique of Li⁺ impact collision ion scattering spectroscopy. Typical LEED √3 domain sizes were estimated to be on the order of 150 Å for a 1 ML coverage of Ag, with the √3 structure persisting for coverages of Ag up to 35 ML. Silver islanding was found to influence the appearance of the 5 keV Li⁺ ICISS angular scans even for 1 ML coverages of Ag deposited at 480°C. A detailed structural analysis of the Si(111)−√3−Ag surface (0.25 ML deposition) involved the comparison of 5 keV Li⁺ ICISS experimental data along the [11 ], [ 10] and [2 1] azimuths with computer simulations of the scattered ion intensities based on previously proposed models for the √3 surface. Nine structurally different models were tested, and only the missing-top-layer (MTL) and the honeycomb-chained-trimer (HCT) models were found to be consistent with all the experimental results. An estimate of 0.4 Å for the maximum downward vertical displacement of the Ag atoms with resect to the surface Si atoms in the MTL model is made. The effects of increased thermal vibrational amplitude in the simulation of Si---Ag shadowing effects is also discussed. The interpretations of previous noble gas ICISS results are shown to be inconsistent with the present alkali metal ICISS study of the √3 surface.     

Interaction of silver atoms with iridium and with a two-dimensional graphite film on iridium: Adsorption,desorption, and dissolution

The initial stages in the interaction of silver with the (111)Ir surface and with a two-dimensional graphite film (2D GF) on (111)Ir were studied by high-resolution electron Auger spectroscopy in ultrahigh vacuum. The growth mechanisms of silver films and the desorption fluxes of Ag atoms were determined, and their desorption energies estimated. It was found that the Ag desorption fluxes from a 2D GF on Ir and from a thick silver film on the pure metal are similar and considerably (an order of magnitude) smaller than the sublimation fluxes from bulk silver at the same temperatures. The activation energy for desorption from a submonolayer film varies from 3.2 eV for coverage θ=1 to 3.7 eV at θ ～ 0. It was shown that silver atoms do not penetrate into the substrate bulk throughout the temperature range covered (300–1800 K).