Trajectory and collision related neutralisation in low energy He+ ion scattering

A recent Surface Science Letter by MacDonald and Martin (Surface Sci. 111 (1981) L739) presents new experimental data on the neutralisation of He⁺ ions scattered from a Ni surface, which is interpreted in terms of a large collisional contribution to the neutralisation, not compatible with the usual Auger neutralisation picture. With the aid of new calculations, it is shown here that these data are consistent with an Auger neutralisation model involving local ion-atom charge exchange.     

Correlated electron effects in low energy Sr(+) ion scattering

Resonant charge transfer during low energy ion scattering reveals correlated-electron behavior at high temperature. The valence electron of a singly charged alkaline-earth ion is a magnetic impurity that interacts with the continuum of many-body excitations in the metal, leading to Kondo and mixed valence resonances near the Fermi energy. The occupation of these resonances is acutely sensitive to the surface temperature, which results in a marked temperature dependence of the ion neutralization. We report such a dependence for low energy Sr(+) scattered from polycrystalline gold.     

Theory of ion neutralisation scattering from surfaces

A method for the calculation of the probability of neutralisation of an ion, which is scattered from the surface of a solid is presented. It assumes the ion to move along a classical trajectory and solves for the time evolution operator for the electronic system. For one electron Hamiltonians the solution can be carried out exactly. Results are presented for scattering from a semi-infinite linear chain.     

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.     

Surface analysis with low energy ion scattering

Principles and applications of low energy ion scattering for surface analysis are presented. Basic features are the binary collision concept, the scattering cross-sections and the ion neutralization process. The potential and the limitations of the method are outlined. Some pertinent experimental aspects are considered. In a number of examples the performance of the technique is demonstrated for qualitative and quantitative composition analysis and for studies of surface structures. Finally a few comparisons are made with other techniques, such as AES, LEED, or SIMS.     

Inelastic effects in low energy He+ ion scattering from solid and atomic Pb targets

Inelastic energy losses of low energy He ⁺ ions scattered from a solid Pb surface and gas phase Pb are investigated for forward scattering of less than 12°. The gas phase results give rise to loss peaks characteristic of atom-atom collisions, ionization, and/or excitation of the target Pb atom. In comparison, the surface is found to result in a loss peak at an energy which agrees well with the atomic character of the Pb target, although the loss peak is broad. This feature of the surface is explained to be due to excitation of electrons in the 6s or 6p band. Molecular orbital crossing (MO model) is considered to be a reasonable interpretation of the continuous character of the inelastic energy loss.     

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.     

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.     

On the defect structure due to low energy ion bombardment of graphite

Graphite surfaces cleaved perpendicular to the c axis have been irradiated with low doses of Ar⁺ ions at 50 eV kinetic energy and perpendicular incidence. Scanning tunneling micrographs (STM) of these irradiated surfaces exhibited dome-like features as well as point defects. These dome-like features retain undisturbed graphite periodicity. This finding is attributed to the stopping of ions between the first and second graphite sheets. The possibility of doping semiconductors at extremely shallow depths is raised.     

Surface hydrogen detection by low energy He ion scattering spectroscopy

Hydrogen adsorbed on a Si(100) surface can be detected by angle-resolved LEIS (low-energy ion scattering spectroscopy) which uses a ⁴He⁺ ion beam with incident energies around 1 keV. The scattering peaks from the surface hydrogen are restricted to narrow scattering angles from 0° to 15°, which is in agreement with those expected by the binary elastic collision model.     

Second order density effects in low energy pion scattering

The differences between a full Lorentz-Lorenz potential modification and a ?² approximation are examined for low energy pion-nucleus elastic scattering. It is found that the two methods give comparable results when Coulomb, absorption and kinematic effects are taken into account.     

Comments on Lorentz-Lorenz effects in low energy pion scattering

The differences between a full Lorentz-Lorenz potential modification, and a ?² approximation to this modification are examined for low energy pion-nucleus elastic scattering. The relationship between these approaches and the single-scattering impulse approximation is also investigated.     

Relativistic effects in low energy electron scattering from atoms

Recent relativistic calculations on low energy elastic scattering of electrons from atoms are reviewed. Two aspects are emphasized : mathematical transformations and numerical methods used in the solution of the Dirac equation and the summation of the partial wave series ; and the results of relativistic calculations, in particular the behaviour of the cross sections and spin polarizations for different atoms, different energies of the incident electron, and in different approximations. The effects of the screening of the nucleus by the atomic electrons, electron exchange (including the use of approximate exchange potentials) and the distortion of the atom by the incident electron are discussed. Detailed results for cross sections and spin polarizations are given for mercury and the inert gases. The differences arising from use of the Dirac equation instead of the Schrödinger equation are considered.     

Quasi-triple collisions in low energy ion scattering; A possibility to determine surface debye temperatures

Calculations and measurements of energy spectra of 10 keV Kr⁺ ions, scattered from Cu(100) face at different temperatures, are reported. One of the observed phenomena is the existence of a new peak. From the temperature behaviour of this peak we obtain the surface Debye temperature.     

The neutralisation of low energy He+ scattered from Ni

The neutralisation of low energy He⁺ scattered off polycrystalline Ni is investigated using three-dimensional angular resolving ion scattering spectroscopy. It is found that upto 98% of the neutralisation occurs during the collision event.     

On low energy pion-pion scattering

Pion-pions andp-wave amplitudes are investigated by means of dispersion relations for the inverse partial wave amplitudes. Possible low energy zeros of thes-waves are taken into account and their positions as well as other parameters are determined by the imposition of the Balachandran-Nuyts crossing constraints and Martin's conditions and by postulating the existence of the ? meson. Very narrow bands of solutions are obtained, where theI=J=0 phase shifts reach 90° and thes-wave scattering lengthsa ₀ vary between 0.13 and 0.23m _π ^-1 and the ratiosa ₀/a ₂ between —1.8 and —3.5. It is shown that inelasticity or the passage through 180° of δ ₀ ⁰ near 1 GeV do not change much the predictions in the low energy region.     

The structure of oxygen on W(001) by low energy ion scattering

The adsorption of oxygen on the W(001) surface has been studied by LEED, AES, and low energy alkali ion scattering in order to obtain a more detailed understanding of the short-range surface structure. Exposure at room temperature results in oxygen adsorption near bridge sites for all coverages below 0.6 ML as determined by the ion scattering incidence angle dependence. Above 0.6 ML, the oxygen adsorbs in four-fold hollow sites. When the oxygen-covered surface is annealed to 950 K, the surface reconstructs, leaving vacancies in the W lattice. This is confirmed by the observation of ions scattered from second layer W, through the vacancies, at relatively low angles of incidence. The oxygen atoms do not adsorb in the position vacated by the missing W atoms but instead adsorb atop the second W layer.     

Local neutralisation effects in esdiad

A simple model of local neutralisation effects, previously applied with some success to interpretation of 1 keV He⁺ ion scattering data, is used to investigate the possible role of such effects in defining the shape of ESDIAD emission lobes. Qualitatively, normal emission lobes and azimuthally dependent patterns from high symmetry sites and off-normal emission lobes from low symmetry terrace sites and step sites are found in agreement with experiment. Quantitatively these features are broad for low index site emission indicating that the process contributes little to the observed angular dependences. For step sites, however, sharp emission lobes are predicted indicating that local neutralisation processes may complicate significantly the interpretation of data from vicinal surfaces.     

Thermally enhanced neutralization in hyperthermal energy ion scattering

Neutralization probabilities are presented for hyperthermal energy Na+ ions scattered from a Cu(001) crystal as a function of surface temperature and scattered velocity. A large enhancement in neutralization is observed as the temperature is increased. Velocity-dependent charge transfer regimes are probed by varying the incident energy, with the most prominent surface temperature effects occurring at the lowest energies. The data agree well with results obtained from a model based on the Newns-Anderson Hamiltonian, where the effects of both temperature and velocity are incorporated.