Slow, multiply charged ions incident on solid surfaces interact only with a few atomic sites from the top-most surface layer. A novel technique for probing local spin polarization at surfaces was developed, namely multiple electron capture spectroscopy (MECS). This technique is based on the Auger electron emission from doubly excited He atoms formed in front of ferromagnetic surfaces. The temperature dependence of the spin polarization of Fe(110) was obtained and the results are compared with those from the highly polarized Ni(110) surface.

The relaxation of hollow atoms produced by slow multiply charged ions impinging on surfaces produces characteristic Auger electron spectra. These spectra, which serve as fingerprints of the interaction, can be used to probe local spin ordering at surfaces by relating changes in the intensities of different spin states to local spin polarization at the surface. The area from which the electrons are captured is of the order of a few Angstrom(2), only. The potential of the method is illustrated by He(2+) and N(6+) ions interacting with a ferromagnetic Ni(110) crystal. From the Auger spectra we determine a spin polarization of approximately 90% at room temperature.

The evolution of the vibrational intensity distribution of the singly ionized A (2)Pi state in CO is experimentally examined for photon energy detunings below the adiabatic 0-0 transition of the O 1s -> 2 pi resonance. We have found a strong suppression of the entire vibrational fine structure of this state, leading to its almost complete quenching for certain excitation energies, followed by a partial restoring for larger values of negative photon energy detuning. Our observation, that cannot be rationalized by the known model of a vibrational collapse for energy detuning, may be explained in terms of a Fano interference between the direct and resonant photoionization channels in the presence of strong lifetime vibrational interference.

We report on surface channeling experiments of singly charged ions on single crystal surfaces of Pt(110) and Pd(110). Using a time-of-flight system installed in forward direction we analyze the energy distribution of the scattered projectiles. By variation of the primary energy and the angle of incidence we investigate effects of the perpendicular energy on the channeling features. The perpendicular energy is defined as E⊥=E0sin2ψ with ψ the angle of incidence. In combination with precise azimuthal rotations of the crystal, we are sensitive to axial channeling and obtain information about the limits of axial surface channeling. From a comparison with detailed trajectory calculations we find that axial channeling effects are most pronounced for a perpendicular energy between 5 and 20eV. As a result, we obtain an exemplary channeling map for the interaction of nitrogen ions with the (1×2) reconstructed Pt(110) surface identifying different channeling regimes.

Low-energy ion beams impinging under grazing incidence on surfaces interact inherently with the topmost surface layer(s). The method of electron capture spectroscopy in which the degree of polarization of the light emitted by the neutralized projectiles is analyzed has been used to investigate spin polarization effects at Ni(110) and Fe(110) surfaces. The results hint at strong spin filtering mechanisms. With multiply charged instead of singly charged ion beams one may obtain access to short range spin ordering at the surface. Progress in the development of this new, so-called multiple electron capture spectroscopy technique is discussed and first results for He2+ ions interacting with a Ni(110) surface are presented.

Ion channeling is used to investigate the electronic density corrugation at surfaces by analysing the electronic stopping behaviour of ions scattering grazingly off a clean single crystalline Pt(110)(1 x 2) surface. We use the fact that under these conditions the elastic contribution can be separated from the inelastic energy-loss processes, which are closely related to the electronic density sampled along the projectiles trajectories. Therefore, we have a direct probe of the surface electronic density structure. Here, experiments and their theoretical interpretation for N+ ions at primary ion energies of 1-10 keV are presented. Multi-peak features of the energy spectra, depending on the azimuthal surface orientation, are attributed to different particle trajectories. A detailed analysis of trajectory calculations affords an unambiguous assignment of the observed peak structures to three trajectory classes, each sampling specific energy losses. The comparison of the experimental energy-loss values with results obtained from a surface-adapted energy-loss model for ion scattering that includes the electron density allows us to investigate the electronic surface of the sample. The results are compared with calculated surface electron density contours. Reasonable agreement is obtained, showing that the presented method is suitable to characterize the electronic density structure at crystalline surfaces. Copyright (C) 2005 John Wiley Sons, Ltd.

The magnetism of Ni(110) and Fe(110) surfaces was investigated by electron capture spectroscopy. He+ and He2+ ions impinged on the Fe(110) and Ni(110) surfaces under grazing incidence, and the degree of polarization of the light emitted by the neutralized projectiles was analyzed. Our measurements show that in Ni(110) minority electrons have a higher density of states at the Fermi energy than majority electrons, opposed to the Fe(110) case. From a comparison of our measurements we estimate the ratio between captured minority and majority electrons in Ni(110) to be similar as the ratio between captured majority and minority electrons in Fe(110).

Surface channeling experiments at different energies are used to evaluate inelastic losses. Over a wide range of energies, from 1 keV to 2 MeV, the inelastic losses can be explained consistently with theory based on the density functional ansatz. The data for random surface directions need one parameter, zedge, only. The parameter zedge is an average measure for the decay of the electron density at the surface with respect to the dielectric function. Experiments along the surface channels of the (1 1 0) surface and variation of the azimuthal angle with respect to these directions reveal many new features. The ion energy loss spectra split up into several peaks which can be identified being due to different trajectories. The interpretation of the results based on the density functional theory energy loss data show that the zedge follows laterally the corrugated electronic surface. This corrugation does not coincide with the surface defined by the turning points of the trajectories, i.e. the 'elastic potential surface'.

We report on surface scattering experiments of 75 keV N5+ ions interacting with a single-crystalline Pt(110)(1×2) surface. We use grazing incidence conditions, i.e. the energy perpendicular to the surface is low enough to prevent penetration through the first atomic layers. Incident, azimuthal and scattering angle are varied. We investigate surface step effects by measuring the energy loss and the spatial intensity distribution of the scattered ions. The increase of energy loss with increasing scattering angle is partly attributed to step effects.

We report on energy loss measurements and charge state distributions for 60 keV N6+ and 75 keV N5+ ions scattered off a Pt(110)(1x2) single crystal surface. In particular, the influence of surface steps on the energy loss and the outgoing charge states is discussed. The scattering angle and the angle of incidence are varied. We use grazing incidence conditions, i.e., the momentum perpendicular to the surface is low enough to prevent penetration through the first atomic layer. Image charge effects are observed leading to an additional projectile acceleration towards the surface. Outgoing charge states are detected from 1+ up to 3+. Axial channeling conditions are defined by scattering along the azimuthal [001] direction, i.e., the projectiles are guided along the [001]-atomic rows. The energy loss for axial channeling is found to be somewhat larger and the energy distribution to be broadened in comparison with the spectra for planar channeling conditions. The broadening is attributed to the interaction of the projectiles with the side wall potentials of the surface channels. We find a strong increase in the energy loss with increasing scattering angle, which exceeds the calculated contribution of the elastic energy transfer by orders of magnitudes. These increased energy losses are attributed to the interaction of the ions with the surface steps.

The thermal behaviour of the (110) surface of aluminum is investigated by low-energy ion scattering along the 〈11̄0〉 and 〈001〉 azimuthal directions in the temperature range between 300 and 910 K. Surface channeling mode and neutral impact collision ion scattering spectroscopy are used. Surface melting is observed and information on the evolution of the surface structure is gained with the help of simulations performed with the MARLOWE code. Experimental evidence for residual short range order is obtained along both azimuths within the quasi-liquid layer which probably consists of groups of surface atoms in correlated motion.

We present new surface scattering results combining measurements of energy loss and charge state distributions of 0.7–1.4 MeV Nq+ (q=1,2) ions. The energy range is still below the bulk stopping power maximum and charge exchange occurs. The projectiles scatter from a Pt(110)(1×2) single crystal surface under grazing incidence in specular reflection geometry. Comparison of the experimental results with two theoretical models are discussed and the influence of the trajectories on the charge state and the energy loss is investigated.

We have performed surface-channeling experiments with ${\mathrm{He}}^{+}$ ions in the energy range between 0.5 keV and 3.5 keV scattering off a flat Pd(110) surface. The energy-loss spectra are measured by a time-of-flight analysis at grazing incidence for different surface directions. The shape of the energy-loss spectra is found to depend strongly on the azimuthal direction. Along axial channeling directions, the energy spectra are broadened and multipeak structures are found. The analysis of these features allows insight into the inelastic interaction process with the surface electrons, which is found to depend strongly on the projectiles' trajectories. Deviations from a linear dependency of the mean energy loss on the primary energy are observed and partly explained by the introduced surface-channeling model including the analysis of detailed trajectory calculations. The significant broadening of the energy spectra is mainly attributed to trajectory straggling.

We report on surface scattering experiments in the MeV regime. Nq+ (q=1, 2) ions with 0.7–1.4 MeV are scattered off a single-crystalline Pt(110)(1×2) surface under grazing incidence and specular reflection geometry. We investigate the energy loss dependency on the azimuthal angle under variation of the perpendicular energy.

The evolution of thermal disorder on the (110) surface of aluminum is investigated along the <1 (1) over bar0> and <001> azimuthal directions in the temperature range between 300 and 910 K by low-energy ion scattering. Surface blocking and channeling mode and neutral impact collision ion scattering spectroscopy are used. Information is gained about the proliferation of point defects and surface roughness below the surface melting point. Surface premelting is observed using a new technique, i.e., investigating the evolution of the channeling peak with temperature. Experimental evidence for residual short-range order is obtained along both azimuths within the quasiliquid layer which probably consists of groups of surface atoms in correlated motion.

Surface channeling experiments with low energy ions show multi-peak structures. We have performed surface channeling experiments with 1 and 10 keV N+ ions. The energy loss spectra are measured using time-of-flight analysis at grazing incidence for different azimuthal angles. Multi-peak structures in the energy loss spectra strongly depend on the azimuthal direction φ. By comparing with particle trajectory calculations the different peaks are identified as being due to trajectories with different lengths. Hence, the observed peak structures are caused by the surface structure and the total energy loss sampled on different trajectories.

The evolution of thermal disorder on the (110) surface of aluminum is investigated along the 〈11̄0〉 azimuthal direction in the temperature range between 300 and 900 K by low-energy ion scattering. Surface blocking and channeling mode and neutral impact collision ion scattering spectroscopy are used. Information is gained about the proliferation of point defects and surface roughness below the surface melting point. Surface premelting is observed using a new technique, i.e. investigating the evolution of the channeling peak with temperature. Experimental evidence for residual short range order within the quasi-liquid layer is obtained.

We present measured energy-loss spectra of nitrogen ions. which are scattered off a (1 x 2) missing row reconstructed Pt(110) single-crystal surface. The primary energy is varied from below 1 keV up to above 1 MeV. i.e., 0.04v(o)<v<2v(o) with v(o) the Bohr velocity. We use grazing angles low enough in order to have surface channeling at all energies. Experimental results are compared with theoretical energy-loss values obtained from trajectory and stopping power calculations of charged particles scattered under grazing incidence conditions from metallic surfaces. The stopping power is calculated using the scattering theory formalism. Different trajectory classes are found by the calculations and assigned to different contributions in the energy-loss spectra. Regarding the simplicity of the presented model the agreement with the experiment is good.

The energy loss of 21.8 and 1.0 MeV Nq+ ions scattered from a Pt(110) single crystal surface at grazing incidence is measured. Comparison of the experimental data with a modified Bohr–Bethe theory shows good agreement.