P.A. Zeijlmans van Emmichoven

Angle resolved electron spectra from grazing incidence He2+/Cu(110) collisions

Electron spectra from keV collisions of He2+ with a single crystal Cu(110) surface are reported. The spectra are obtained for well defined angles of incidence and detection. From the spectra and from their variation with detection angle the following conclusions are drawn. At vertical velocities of the order of 0.01 atomic units the He2+ neutralizes in two successive steps while it approaches the surface. In the first step He2+ is converted to He+ in the ground state by two competing processes, namely, double capture followed by autoionization or Penning ionization, and direct Auger capture. In the second step He+ is neutralized by Auger capture into ground state He. A model is developed that allows one to calculate electron spectra due to those processes using model functions for distance-dependent transition rates and transition energies. It is shown that the main features of the observed spectra can be well reproduced by the model calculations if it is assumed that the metal does not respond adiabatically to the sudden changes induced upon the emission of fast electrons in the various spontaneous ionization processes.

ON THE NEUTRALIZATION OF SINGLY AND MULTICHARGED PROJECTILES DURING GRAZING INTERACTIONS WITH LIF(100)

Abstract Measurements are reported of scattered neutral fractions for Na, K, Cs and Ne singly and multicharged ions, and of scattered negative ion fractions for incident O, F and B projectiles grazingly incident on LiF(100) as function of projectile velocity. In the case of the Na and Ne incident ions, significant dependence of the scattered neutral fractions on incident charge state is found, which is most pronounced at the lowest investigated velocities. Possible reasons for the observed initial charge state dependence are considered. In addition, results are reported for the target azimuthal dependence of the final neutral fraction observed for grazingly incident 35 keV Cs7+ ions.

Electron emission in slow collisions of protons with a LiF-surface

Abstract Electron spectra have been measured for collisions of protons with surfaces of LiF grown on a tungsten substrate. The measurements were carried out for 5° grazing incidence collisions at energies from 50 eV up to 1 keV. The experimental spectra are compared with theoretical spectra obtained from model calculations. The model is based on the assumption that electron emission is dominated by the mechanism of electron promotion via the 3dσ molecular orbital of the HF − quasi-molecular system. Using theoretically obtained functions for the scattering potential and for the 3dσ orbital energy, and calculating the spectra in semiclassical approximation for a distribution of impact parameters obtained from Monte Carlo simulations for the relevant scattering geometry, good agreement between experimental and theoretical spectra is obtained. This agreement is taken as strong evidence for the assumed ionization mechanism.

Nonlinear screening effects in the interaction of slow multicharged ions with metal surfaces

Abstract Density functional theory is used to treat the problem of screening of highly charged ions in an electron gas when inner shell vacancies are present. Results are presented for N and Ne ions in various electronic configurations. These include information about screening charge densities, induced densities of states in the conduction band, bound energy levels and KLL Auger lines.

Ion-Surface Interactions

The interactions of slow multicharged ions with surfaces is a rapidly developing field, which attracts a lot of attention nowadays. One of the underlying driving forces is the fact that still many ‘unknowns’ exist. Through the input of many groups now working in this field, the understanding of the physics involved increases significantly every year.

Charge transfer and atomic rearrangement in collisions of Ar+(2P12,32) with H2 measured with a new PEPICO apparatus

Abstract A new apparatus designed to perform photoelectron product-ion coincidence (PEPICO) measurements is described. The collision system Ar + ( 2 P 1 2 , 3 2 ) + H 2 is studied as a first test case. Relative cross sections for ArH + and H 2 + formation by the two fine structure states of the ion are measured at three average center-of-mass collision energies, 0.05, 0.075 and 0.125 eV. The results are interpreted in the context of data available in the literature and confirm the importance of a vibronic resonance for the Ar + ( 2 P 1 2 )+ H 2 system at low collision energies.

Sputtering of high-energy metal ions in keV heavy-ion surface collisions

Spectra of high-energy negative and positive metal ions sputtered from Cu(110) and Al(111) by keV noble-gas ions and measured at large recoil angles are presented. We have identified the most important mechanisms that lead to sputtering at these high energies. A comparison with a model based on MARLOWE simulations and on the electron tunneling model shows that for Cu(110) a significant fraction of the high-energy sputtered particles leaves the surface in a positive charge state. At the surface, the fraction of sputtered negative ions is two orders of magnitude smaller. On the outgoing trajectory most of the ions are neutralized by resonance neutralization. The rate for neutralization of the positive ions has been found to be significantly higher than for the negative ions.

Electron emission during collisions of low-energy ions with MgO(1 1 0)

Electron spectra arising in collisions between low-energy He þ and Ne þ ions and bulk MgO(1 1 0) are reported. For both ions the measured spectra are virtually the same and consist of very-low-energy electrons with energies of up to only about 6 eV. Electron emission originating in Auger neutralization of the primary ions close to the MgO surface is considered. The strong Coulomb interaction between primary ions and O 2� cores may significantly reduce the ionization potentials and thereby the final electron energies. Contributions from other neutralization channels can presently not be excluded. 2002 Elsevier Science B.V. All rights reserved.

Instrumentation and experimental procedure for investigation of the electronic and geometrical structure of metal surfaces

An apparatus for studying metal crystal surfaces with ion scattering techniques is described. These techniques are low-energy ion scattering (LEIS) ion-neutralisation spectroscopy (INS) and low-energy recoil scattering (LERS). Angles of incidence and detection as well as the energies of incoming and outgoing particles can be controlled very precisely. Furthermore, coincidence measurements between scattered projectiles and electrons or between scattered projectiles and recoil ions can be performed. Ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED) are available as additional techniques.

Preferential ejection of high-energy sputtered Cu atoms in keV atom - Cu(110) collisions

Abstract Preferential ejection of sputtered target atoms during the bombardment of crystalline solids by ions (atoms) is not restricted to the dominant low-energy part of sputtering. In our MARLOWE simulations for the bombardment of a Cu(110) surface with keV Xe and Kr atoms we have observed focused spots in the angular distribution of the Cu atoms which are sputtered with energies larger than 200 eV. We have found that the sputtered atoms predominantly originate from the first four surface layers. The escaping atoms are blocked and focused by the atoms in the same surface layer or by the atoms nearer to the surface when they travel towards the vacuum. The spot positions roughly coincide with the most open directions in the crystal surface as seen from the atoms of the respective layers. The absolute and relative intensities of the spots depend on the incidence conditions. There is no specific collision sequence associated with the individual high-energy spots.