A.W. Kleyn

Negative ion formation and dissociation in scattering of fast O2 and NO from Ag(111) and Pt(111)

Results of scattering of 80–300 eV O2 and NO from Ag(111) and Pt(111) single crystal surfaces are presented. Molecular negative ion formation occurs on both surfaces. For O2 scattering around 80 eV for θi=70° from Ag(111), the O−2 distribution is detected in the specular direction, whereas an off‐specular shift towards the surface is observed for scattering from Pt(111). For the Pt(111) surface, a degree of dissociation is observed that is much larger than for the Ag(111) surface. Both observations are consistent with the low dissociative sticking probability for thermal O2 on Ag(111) reported and the high dissociative sticking probability for thermal O2 on Pt(111). For NO−, the degree of dissociation is approximately equal to the degree of dissociation for O2 at the same Pt(111) surface.

Scaling law for dissociation of fast molecular hydrogen scattered from Ag ( 1 1 1 )

Abstract Experiments on the scattering and ionization of fast (70 eV ⩽ E ⩽ 1 keV) molecular hydrogen from Ag( 1 1 1 ) are reported. In these experiments, ions are, surprisingly enough, rather easily formed. The observed dissociated fraction in the positive-ion signal scales with E θ 2 where θ is the total scattering angle. This scaling indicates the impulsive and atomic nature of the collision between the incoming molecule and the surface atoms. With the help of a simple model where the molecules and the surface atoms are treated as hard spheres, a theoretical explanation is given for the observed scaling.

The adsorption of Ba on Ag(111)

The growth behaviour of barium on silver(111) at room temperature is investigated by AES, XPS, MEIS, TDS, work function measurements and by monitoring the secondary-electron-emission crystal current. It is shown that, for low coverage, initially a 2D monolayer is formed. For higher coverages the overlayer formed resembles a structure formed by Poisson or Stranski-Krastanov growth with small islands. As evaporation continues the islands become very large or the sticking coefficient drops to zero. Annealing the Ag with more than one monolayer of Ba on top shows a large work function decrease of 0.6 eV to an unusually low value of 1.9 eV. The lowering of the work function can partly be explained by the formation of large clusters. Coadsorption experiments with H2 indicate that the H2 sticking coefficient on Ba/Ag(111) is constant and that the H2 adsorbs dissociatively into one binding state. From the experiments an inelastic mean free path of 11+or-1 AA for 351 eV electrons and 17+or-1 AA for 1113 eV and 1119 eV electrons is deduced.

Relativistic effect on the interatomic platinum–oxygen potential and its consequences in high energy O2/Pt(111) scattering

The influence of relativity on the binding in the Pt–O molecule is investigated using density functional calculations and proves to be quite important, not only for the potential well but also for the repulsive wall. Using a Born–Mayer fit to this interatomic gas phase potential, we perform a classical trajectory study on high energy O2/Pt(111) scattering (Ei=80 eV). The Born–Mayer form of the interatomic potential leads to a higher degree of dissociation for O2/Pt(111) than for O2/Ag(111) which is also experimentally found. The role of relativity turns out to be significant. The dissociation mechanism, however, does not change when going from O2/Ag(111) to O2/Pt(111). The molecules were found to first gain primarily rotational energy, which is largely transfered to vibration at the turning point of the second atom, in the case of finally dissociating molecules. Since the calculated dissociation in the case of platinum is still less than found experimentally, we investigate the influence of better fits to...

An ultrahigh vacuum (UHV) apparatus to study the interaction between adsorbates and photons

An UHV apparatus has been designed and constructed for studying the interaction between adsorbates and photons. Since a variety of light sources and experimental geometries are used in these studies, the system was designed to be as flexible as possible. The system is put on a single frame so that it can be transported from one photon source to another. A compact, exchangeable, high-pressure cell is designed to provide easy access for photons to the crystal. To allow the study of small poly-atomics the following combination of detection techniques are installed: thermal desorption spectroscopy (TDS), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and reflection - adsorption infrared spectroscopy (RAIRS). Furthermore, a sample exchange system in combination with liquid nitrogen sample cooling is incorporated. The exchange system allows three samples to be stored in the main chamber. The sample exchange capability does not restrict the cooling capabilities - 85 K within an hour after turning on the liquid nitrogen flow and 85 K within minutes after flashing to 800 K.

Dissociative scattering of CF+3 from a barium-covered Ag(111) surface

Abstract The dissociative scattering of CF + 3 ions from a barium-covered Ag(111) surface has been studied for incident energies between 100 and 500 eV. The negatively charged scattering products only showed F − and the positives showed CF + and C + fragments. No negatively charged molecular ions are observed. The results are compared with those on scattering from a clean Ag(111) surface. The lowering of the work function facilitates resonant neutralization into the excited 3s Rydberg state of the neutral molecule. Consequently, the spectral features associated with Auger neutralization are not observed.

Classical trajectory study of fast H2 scattering from Ag(111)

Abstract Classical trajectory calculations of H2 scattering at energies of 70–200 eV from Ag(111) have been performed using a HFS pair-potential. The results show specular scattering for both H2 and H atoms, formed in the collision. The angular width of the atoms is much wider. The dissociation observed has an impulsive origin; in the collision with the surface energy is transfered predominantly to the rotational degree of freedom giving rise to a scaling with Eθ2, where θ is the total scattering angle. For most incoming angles the collision is governed by the interaction between the incident molecule and a single Ag atom. The experimentally obtained spectra for product ions in neutral H2 scattering are reproduced quite well, although the calculated angular and energy broadening are smaller. The ionization step and dissociation step are decoupled. It is confirmed that in experiments using incident H2+ dissociation is mainly caused by an electronic process.

A comparative study of Ar/Ag(111) potentials

The potential energy surface for the Ar/Ag(111) system is studied within the local density approximation. Results for N atom clusters, where N=4, 10, and 19, are compared with the potentials obtained with slabs with summation of pair potentials. This last method predicts the threefold hollow site as equilibrium site. However, the larger clusters find the top site as adsorption site. On the top site, the adsorption height and energy are calculated as 3.2 A and 0.10 eV, respectively. The physisorption energy is in reasonable agreement with experimental data, in spite of the LDA level of theory used. Thus the Ag(111)–Ar system is a new example where LDA appears to describe a van der Waals system with reasonable accuracy. A correction to the sum of pair potentials is given that effectively reproduces the calculated potentials at threefold hollow site and top site.

A three-axis goniometer in an UHV molecular beam experiment

A sample manipulator with six degrees of freedom, which is part of a molecular beam scattering apparatus used to study gas - surface interactions, is described. Incidence parameters such as the angle of incidence and the azimuthal angle can be set and particles leaving the surface both in and out the plane of incidence can be measured (to within accuracy). The latter is achieved, under computer control, by a combination of the movement of the manipulator and the movement of the detector. Cooling of the sample to liquid temperatures and heating to at least 1600 K is possible. A mathematical relation among the positions of the manipulator and of the detector, the required incidence angle of the particles and the final angle of the scattered particles to be measured is derived.

Divertor conditions relevant for fusion reactors achieved with linear plasma generator

Intense magnetized hydrogen and deuterium plasmas have been produced with electron densities up to 3.6u2009×u20091020u2009m−3 and electron temperatures up to 3.7u2009eV with a linear plasma generator. Exposure of a W target has led to average heat and particle flux densities well in excess of 4u2009MW m−2 and 1024u2009m−2 s−1, respectively. We have shown that the plasma surface interactions are dominated by the incoming ions. The achieved conditions correspond very well to the projected conditions at the divertor strike zones of fusion reactors such as ITER. In addition, the machine has an unprecedented high gas efficiency.