E.P.Th.M. Suurmeijer

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.

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.

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.

INSTRUMENTATION FOR THE STUDY OF ION REFLECTION AND SECONDARY ION EMISSION UPON ION BOMBARDMENT OF ATOMICALLY CLEAN AND SMOOTH MONOCRYSTALLINE METAL SURFACES.

An apparatus has been built for the study of energy distributions of reflected and secondary ions emitted from smooth and atomically clean monocrystalline metal surfaces upon bombardment with very low intensity noble gas ion beams (<10 nA cm-2) in the energy range 0.5-10 keV. This paper is related to the description of the apparatus construction. Basic design features and operation principles are discussed. Experimental procedures are also described, among which those having to do with the generation and maintenance of ideal target conditions are considered to be especially important. The system performance is discussed on the basis of some preliminary experimental results.

Ultrahigh vacuum target manipulator for use in ion channelling and reflection experiments

A versatile specimen manipulator for use in UHV is described, based on the principle of a ball joint. It allows rotation of the target around three mutually independent perpendicular axes with high accuracy. Two rotations are computer-controlled; the third one is operated manually. The manipulator as a whole can be shifted horizontally and rotated with respect to its base flange. The target can be heated up to 500 degrees C. Cooling facilities can be provided for.

A COMPARISON BETWEEN TIME-OF-FLIGHT AND STRIPPING CELL METHODS USED IN LOW-ENERGY ION-SCATTERING

In low-energy noble gas ion scattering (LEIS) neutralisation plays an important role. Comparison of energy spectra of neutral scattered particles with ion spectra, and ion fractions derived from these data, provides information on neutralisation. To measure the energy of neutrals in the energy range (2-10 keV) a time-of-flight (TOF) spectrometer was used as well as a stripping cell in front of the electrostatic analyser. The results of both methods were compared for argon and neon neutrals of 5-10 keV with helium, nitrogen and argon as stripping gas. To investigate the properties of the cell neutral particles were used, obtained by scattering from a Cu(100) surface.

On the Scattering of Low Energy H+ and He+ Ions from a (001) Copper Surface

Energy spectra and angle dependent yield distributions of 3–9 keV H+ and He+ ions scattered from a (001) face of a copper single crystal were measured. Spectra recorded for scattering planes making small angles with a direction on the surface are influenced by the focusing action of the surface semi-channels in this direction. Computer simulations were carried out to understand the main regularities of this focusing effect.

Gas pressure dependence of prebreakdown conduction in high vacuum

Abstract Secondary-electron pulse-height distributions induced by 29 keV positive ions impinging on a stainless steel converter of a Daly detector have been investigated. Some factors are discussed which influence the shape of this distribution and may sometimes determine the background current which remains after proper discrimination. In our case this background current is about 10−19 A for a detector vacuum of about 2 × 10−8 torr. The following factors are considered: (1) low-energy radiative background pulses generated by ion bombardment of the converter, and (2) ejection of electron clusters consisting of 1, 2, 3, etc. electrons from the converter when relatively high gas pressures (≿ 10−6 torr) prevail in the conversion chamber. It is suggested that the former effect is due to radiative de-excitation of excited particles emitted from the converter. The latter effect appears as a set of more or less pronounced peaks superimposed on the pulse-height distribution measured at lower pressures. The intensity of these peaks strongly depends on the gas pressure in the conversion chamber (∝ p2.7). A simple model is suggested which qualitatively accounts for this so-called “gas effect” and a new method is described for calibrating the system energy scale by means of it. The measured pulse-height distributions are compared with a Poisson distribution corrected for instrumental broadening. The good agreement enables us to obtain fairly precise values for the secondary-electron emission coefficient γ ¯ . For 29 keV argon ions incident on 304 stainless steel under an angle of 52° with the surface normal γ ¯ appears to be about 7.8.