Hannspeter Winter

Collisions of atoms and ions with surfaces under grazing incidence

Abstract A detailed discussion of the scattering of fast atoms and ions from solid surfaces under a grazing angle of incidence is presented. Theoretical and experimental results are used to demonstrate that collisions employing this scattering geometry provide interesting new phenomena and insights into atom–surface interactions.

Recent developments for plasma edge diagnostics using atomic beams

Abstract Li-atom beams with velocities of 1.5 × 105 cm/s and 1× 106 cm/s have been used to measure ne profiles in the density range 1011–1013 cm−3 from the Li line emission with a spatial resolution of about 1 mm. Injecting in addition a different type of atoms, the ionization rate of which shows a strong dependance on the electron temperature in the interesting range of 5–100 eV (e.g. carbon and helium), also radial Te profiles are obtained. A high intensity 30 keV Li beam is employed to measure radial profiles of impurity ion concentrations (e.g. He, C, 0) by charge exchange recombination spectroscopy. For the different purposes several types of injectors have been developed: thermal beams using ovens, suprathermal beams using laser ablation techniques, and a high energy beam using a plasma ion source in combination with a charge exchange cell. They are located at different poloidal and toroidal positions. The spatial line emission profiles of the beams are recorded either by Si diode array cameras or by a photomultiplier in combination with a scanning mirror so that a spatial resolution better than 2 mm is achieved. The combination of all these systems delivers information about the structure of the TEXTOR boundary layer in poloidal and toroidal direction.

Absolute ionisation cross sections for electrons incident on O+, Ne+, Xe+ and Ari+(i=1,...,5) ions

Absolute ionisation cross sections for electrons incident on O+, Ne+, Xe+ and Ar1+,..., Ar5+ ions have been measured at electron energies Ee between the ionisation threshold Ei and 830 eV. The measurements have been performed with crossed electron and ion beams. The comparison of the data for O+ and Ne+ with results of Aitken and Harrison (see ibid., vol.4, p.1176, 1971) and Dolder et al. (see Rep. Prog. Phys., vol.39, p.693, 1976), respectively, shows agreement within the combined experimental errors. The measured cross sections for ionisation of Ari+ ions can be reproduced within +or-20% of the empirical formula sigma i,i+1=1.4*10-13((ln(Ee/Ei))/(EeEi))(eV)2 cm2 for the charge states i=1 up to i=5.

SPUTTER YIELDS OF INSULATORS BOMBARDED WITH HYPERTHERMAL MULTIPLY CHARGED IONS

The total sputter yield for Au, Si, GaAs, SiO2, MgO, LiF and NaCl bombarded with hyperthermal highly charged Arq+ ions (q = 1-9) has been measured. Only for alkali halides (LiF, NaCl) and to some extent for SiO2 potential sputtering (enhancement of the sputter yield with increasing charge state of the primary ion) has been observed. All other targets showed normal collision induced sputtering. From that result it is obvious that the mechanisms for sputtering can not be explained by the Coulomb explosion model, because in this model insulators like MgO and semiconductors like Si and GaAs should also show charge state dependence of the sputtering yield. Alkali halides and SiO2 are materials which are known for strong electron phonon coupling where electronic excitations in the valence band are localized by formation of self trapped excitons (STE) and/or self trapped holes (STH). During bombardment with highly charged ions the neutralization process in front of, at and below the surface causes the formation of STE and/or STH. Therefore the potential sputtering can be explained as a defect mediated sputtering process which is well known in electron stimulated desorption (ESD) where the decay of STH and/or STE into different colour centers leads at the end to the desorption of neutralized anions. The also created neutral cations are either evaporated (as it is the case for the alkali halides) or have to be removed by momentum transfer by the impinging projectiles. Therefore it is very likely that in the case of SiO2 for very low impact energy mainly only oxygen is enhanced sputtered, the surface is enriched in Si and the potential sputtering effect decreases with increasing ion dose.

Slow particle-induced electron emission from solid surfaces

Theoretical Concepts and Methods for Electron Emission from Solid Surfaces.- Photon and Electron Induced Electron Emission from Solid Surfaces.- Potential Electron Emission from Metal and Insulator Surfaces.- Kinetic Electron Emission for Grazing Scattering of Atoms and Ions from Surfaces.- Spin Polarization of Electrons Emitted in the Neutralization of He+ Ions in Solids.- Electron Emission from Surfaces Mediated by Ion-Induced Plasmon Excitation.- Slow Ion-Induced Electron Emission from Thin Insulating Films.

Search for projectile charge dependence of kinetic electron emission from clean polycrystalline gold

Abstract Total yields for ion-induced electron emission from atomically clean polycrystalline gold have been measured accurately (total errors ⩽±5%) for normal incidence of C q + ( q ⩽5)-, N q + ( q ⩽6)-, O q + ( q ⩽7)- and Ne q + ( q ⩽9) ions with impact velocities from below 10 5 m/s (exclusive potential emission/PE regime) up to 10 6 m/s (5 keV/amu). Contribution from kinetic emission (KE) to these total yields was studied by taking into account the respective PE yields and precise determination of the KE impact velocity threshold for corresponding singly charged ions. For given ion species Z 1 and impact velocity we have investigated whether KE yields depend on the ion charge state q . From q =1 toward 3 a slight decrease with q has been found, but there are no significant differences in KE yields at higher q values. Results are discussed in view of the q -dependence for relevant KE mechanisms.

Auger Electron Emission from Slow Multicharged Ions Near a Metal Surface

Electron energy spectra from multicharged ion (N5+, N6+, Ar9+) impact on clean tungsten (impact energies ≥ 500 eV) revealed Auger electron emission from the projectile particles. The impact energy dependence of emission yield and kinetic-energy shift for these Auger electrons lead to a fairly detailed explanation of slow multicharged ion neutralization near metal surfaces.

Modelling of fast neutral Li beams for fusion edge plasma diagnostics

We present an experimental and theoretical study on the modelling of fast Li beams which are used for diagnostics of fusion edge plasmas. The atomic collision database utilized for modelling the Li beam attenuation has been revised and extended by detailed investigations of populations resulting from ion impact excitation. We obtain good agreement between measured and calculated Li(nl) populations for an Li beam passing the edge plasma region of the ASDEX Upgrade tokamak and WENDELSTEIN 7 AS stellarator, respectively, at IPP Garching.

Quantitative model for the surface sensitivity in Auger-photoelectron coincidence spectroscopy (APECS)

Abstract The effects of multiple elastic and inelastic scattering on the lineshape in Auger-photoelectron coincidence spectra (APECS) are studied. A simple and at the same time general model is proposed. Model calculations are performed that confirm the well known fact that the surface sensitivity of APECS, when the Auger peak is measured in coincidence with the photoelectron peak, is enhanced as compared to the ordinary (“singles”) spectrum. The results also show how the average depth from which the Auger electrons originate can be selected experimentally by recording them in coincidence with the loss features of the photoelectron spectrum and vice versa. Additional simulations reveal why surface and bulk excitations, that can be clearly distinguished in experimental APECS spectra, are uncorrelated to a high degree. This allows one to consistently eliminate the contribution of multiple bulk and surface scattering from experimental APECS data ultimately providing the true energy distribution at the source.

Surface-induced dissociation of singly and multiply charged fullerene ions

Collisions of singly and multiply charged ions C60z+ (z=1,2,3,4,5) with a hydrocarbon-covered stainless steel surface have been investigated; product ions of fragmentation and pickup reactions were determined as a function of the collision energy (100–500 eV) and the projectile charge z. All ions scattered off the surface are singly charged. The extent of fragmentation increases with the collision energy and the projectile charge. However, the increase of fragmentation with the charge of the projectile is less pronounced than expected from a full conversion of electronic energy, gained in the neutralization process, into internal energy of the ion.