Aj Algra

Low energy (E0 < 10 keV) atom and ion scattering of neon from a Cu(100) surface; Ionization and neutralization

Abstract 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.

THE ION FRACTIONS OF 2-10 KEV LITHIUM, SODIUM AND POTASSIUM SCATTERED FROM A COPPER (100) SURFACE

Abstract The ion fractions of lithium, sodium and potassium scattered from a copper (100) surface have been measured as a function of several experimental parameters. The ion fraction appears to be almost independent of the final energy, i.e. independent of the type of scattering trajectory at or below the surface. This means that the alkali ions can be used very well in several cases when experimental data are to be compared with theoretical scattering models. Typical values of the ion fraction are: 63% for lithium, 76% for sodium and 99% for potassium (values are given for a primary energy of 5 keV). A comparison of the results has been made with a theory based on ideas developed for the adsorption and scattering of very low energy alkali particles. A qualitative agreement has been found for the dependence of the ion fraction on the ionization energy of the alkali atoms and the (slight) variation with the primary energy.

MEASUREMENT OF ENERGY-SPECTRA OF NEUTRAL PARTICLES IN LOW-ENERGY ION-SCATTERING - APPARATUS AND EXPERIMENTS

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.

Interaction of low energy (5–10 keV) argon atoms and ions with a Cu(100) surface; Ion fraction, ionization and neutralization

Abstract The ion fractions η + of low energy (5–10 keV) argon particles scattered from a Cu(100) surface, are measured with a time of flight spectrometer. Neutral as well as charged projectiles are used. The scattering angle θ is 30°. The results for different angles of incidence ψ and crystal directions are reported. For scattering in the 〈100〉 direction, with a ψ-value of 15° and a primary energy E 0 of 5 and 10 keV, the ion fractions for the quasi single scattering peak, η + QS , are 1.5 and 6.1% respectively. When E 0 is between 5 and 10 keV a reionization process with a constant reionization probability occurs during the violent interaction. This process, but also neutralization along the outgoing trajectory, determines η + QS . With ions as projectiles, an energy difference of about 16 eV is observed between the quasi single scattering peaks in the spectra of all scattered particles and of ions only. The ion fraction for the quasi double scattering peak, η + QD . depends largely upon E 0 , indicating that the efficiency of the reionization process increases with E 0 . A qualitative discussion of the data is given, using the reionization process and the interatomic neutralization processes along the trajectory of the scattered particles.

THE STRUCTURE OF A STEPPED COPPER (410) SURFACE DETERMINED BY ION-SCATTERING SPECTROSCOPY

Abstract Ion Scattering Spectroscopy applied in the multiple scattering mode is used to determine the structure of a stepped Cu(410) surface. The energy of singly scattered ions is influenced by the presence of neighbour surface atoms. This effect can be used to determine interatomic distances up to about 10A, as is shown by the results of 8 keV Ar + and 11 keV Ne + scattered through θ = 50°. The edge-edge distance of the stepped copper surface appears to be in accordance with the results of LEED experiments obtained by other investigators. The experiments show a good agreement with the results of the analytical 3-atom model of Poelsema. The energy of the so-called “plateau collision” appears to depend on the effective plateau length l as measured in the plane of incidence. Lengths l between 15 and 60 A can be determined with an accuracy of 5 A. Results are shown for 8 and 12 keV Ar + , θ = 40° and 60°, and 8 keV Kr + θ = 40°. The experimental dependence of the energy on l is described correctly by a phenomenological model.

ARGON (10KEV) SCATTERED FROM STRUCTURES, INDUCED BY BOMBARDING A CU(100) SURFACE - IONIZATION AND NEUTRALIZATION

The ion fractions, η+, of 10 keV argon particles, scattered from a damaged copper surface, are measured with a time of flight spectrometer. The damage was introduced by bombardment with argon ions. The scattering angle was 30°. The results for different angles of incidence, ψ, are reported. For Ψ < 10° the ion fraction is relatively high (∼27% for Ψ = 4°) and decreases as Ψ increases. For Ψ = 15° the value of η+ is 7%, whereas for 21° < Ψ < 27° the value of η+ appears to be constant (∼14%). An explanation is given by assuming interatomic ionization as well as neutralization processes along the trajectory of the scattered particles. The number of step-atoms, induced by ion bombardment, is estimated to be about 2 × 1014/cm2.

Ion fractions of low energy Ne (E0 < 10keV) scattered from a copper single crystal

Abstract Measurements of the ion fraction using the time of flight (TOF) technique, of low energy Ne (5–10 keV) scattered from a Cu (100) face are reported. The scattering angle θ is 30°. The scattering plane is in the 〈100〉, 〈110〉, 〈210〉 and 〈310〉 directions respectively. The ion fraction as a function of the final energy of the scattered particles has about the same shape as the ion spectrum: single and double collisions can be distinguished. Also there is a small difference between the energies of the scattered ions and neutrals in the single collision peak. The ion fractions indicate an increasing re-ionization during the scattering process as function of E 0 .

THE POSITION OF OXYGEN ADSORBED AT THE STEPS OF A COPPER (410) SURFACE STUDIED WITH LOW-ENERGY ION-SCATTERING

The adsorption of oxygen on the steps of a copper (410) surface has been studied with Low Energy Ion Scattering. The adsorption kinetics suggest that oxygen adsorbs dissociatively. The linear decrease of the logarithm of the scattering signal from oxygen with the bombarding dose indicates that there is only one type of adsorption site. The cross section for desorption with 5 keV Ne + is 4.5 × 10 −15 cm 2 . From the angular distributions of the projectiles scattered from the copper atoms and the adsorbed oxygen atoms, it is found that the oxygen atoms are adsorbed at the hollow sites of the steps between 0.2 A below and 0.7 A above the terrace plane and protruding 0.2–0.7 A from the edges. A computer simulation study gives qualitative support for these findings, but fails in giving a more precise oxygen position because of a lack of knowledge of the vibrational motion and the interaction potential of the atoms involved. At the saturation level of adsorption the coverage is 0.25 of a monolayer, corresponding to an occupation of all available step sites.

Edge atom depression on stepped Cu(410)

Abstract The position of the edge atoms of a stepped Cu(410) surface has been measured by Ion Scattering Spectroscopy using 21 keV H + . The edge atoms are depressed 5.0±1.5% of the copper lattice spacing, corresponding to 0.18±0.05 A.

Structure analysis of solid surfaces by multiple scattering of low energy ions

Abstract Multiple scattering effects in low energy ion scattering (LEIS) can be related to the structure of the bombarded solid surface. Some examples of the use of multiple scattering phenomena for structure analysis of copper (100) and (410) surfaces are discussed.