Th. Fauster

Probing valence states with photoemission and inverse photoemission

Photoemission and inverse photoemission (or bremsstrahlung spectroscopy) are shown to be complementary techniques for probing occupied and unoccupied electronic states, respectively. The momentum of delocalized valence states can be measured as well as their energy if low electron (photon) energies in the 10–40 eV range are used. Thus, energy band dispersions are obtained for bulk, surface, and adsorbate states which cannot be determined by other techniques. A status report of inverse photoemission is given and illustrated by results for bulk states in ferromagnets and silicides, for broken bond states on silicon surfaces, and for unoccupied molecular adsorbate orbitals.

Surface Geometry Determination by Large-Angle Ion Scattering

Abstract The current state of surface structure determination by ion scattering experiments at large scattering angles is reviewed.

Atomically accurate Si grating with 5.73 nm period

A vicinal surface of silicon is found that exhibits an atomically accurate step pattern with a period of 5.73 nm, corresponding to 17 atomic rows per (111) terrace. It can be viewed as reconstructed Si(557) surface, where a triple step is combined with a single Si(111)7×7 unit. The driving forces for establishing regular step patterns are discussed.

Coverage-dependent electronic structure of Na on Cu(111)

Abstract The electronic states of Na on Cu(111) have been studied by high-resolution two-photon photoemission. Image states which are well separated from alkali states are observed at all coverages. An abrupt change of the occupancy of the alkali state is observed at coverages between 0.40 and 0.45 ML. We interpret this as the transition from a more ionic bonding of alkali atoms on the surface to the metallic bond in alkali islands. Other alkali metals on close-packed surfaces show a very similar electronic structure as a function of alkali coverage.

Determination of the Geometry of Sulphur on Nickel Surfaces by Low-Energy Ion Scattering

Abstract In the impact collision mode of low-energy ion scattering with scattering angles close to 180° the surface geometry can be determined by the exploitation of shadowing effects in a simple and direct way. We have studied the scattering of Ne + ions with 5 keV energy off clean and sulphur covered nickel surfaces. For the ordered overlayers Ni(001)-c(2 × 2)S, Ni(110)-c(2 × 2)S and Ni(111)-(2 × 2)S we find — in agreement with other experiments — that the sulphur adsorbs in the hollow site at a distance of 2.2 A to the nearest Ni atom. The advantages of low-energy ion scattering for surface structure determination are the unambiguous identification of the adsorption site, the self-calibration of the technique, the straightforward interpretation of the experimental data and the possibility to study non-ordered structures in real-space.

Surface Structure Determination of the (2x1)0-Cu(110) Reconstruction by Low-Energy Ion Scattering

Using impact collision ion scattering we show, that the reconstruction of the (2 x l)O-Cu(ll0) surface is of the missing-row type where every other [OOl] row of copper atoms is missing. The oxygen atoms are located 0.08 & 0.15 A below the copper atoms of the remaining rows. The second layer copper atoms are shifted laterally 0.12 rf 0.07 A away from the oxygen atoms in a pairing-row type reconstruction. The first layer is relaxed outward over 0.23 + 0.04 A compared to the ideal spacing, whereas the distance between the second and third layer remains unchanged. A critical comparison of our results with the data available in the literature shows that the position of copper and oxygen atoms can be determined with an accuracy of about 0.05 and 0.15 A, respectively.

Growth of ultrathin iron films on Cu( 001): an ion-scattering spectroscopy study

Abstract Low-energy ion scattering has been applied to study the pseudomorphic growth of ultrathin fee iron films on a Cu(001) single-crystal surface. At all coverages — even as low as 0.1 monolayer (ML) — iron is found to about equal amounts in both the surface and the first subsurface layer, since part of the iron atoms are incorporated into the original copper surface. This is particular evident for the 0.1 ML film, which does not exhibit any iron defects, such as steps or adatoms. Instead copper surface defects become visible upon iron deposition. Near 2 ML the substrate is covered for the most part by a relatively smooth bilayer indicating the coalescence of the iron islands. Up to a film thickness of around 6 ML the surface defect density of the iron layers decreases with increasing coverage and raises again at larger coverages. Above ∽ 10 ML the pseudomorphic fee growth breaks down and domains of bcc iron with (110) orientation are formed.

Bilayer growth in a metallic system: Fe on Cu( 100)

Abstract During epitaxial growth of Fe on Cu(100) the normalized Auger signal of Fe was taken as a function of evaporation time. This curve shows evidence for a layer-by-layer growth mode and clearly rules out other possible growth modes. Rutherford backscattering spectrometry was used to determine thicknesses of a one- and two-layer film quantitatively with an accuracy of ± 0.3 ML. These data together with the Auger analysis unveil a bilayer-by-bilayer growth mode for the first two bilayers.

Improved resolution in VUV isochromat spectroscopy

The energy resolution of a VUV isochromat spectrometer employing the traditional energy selective Geiger counter can be significantly improved. The variance of the optical resolution function with the usual CaF2 entrance window is (240 meV)2. With SrF2 we obtain (113 meV)2 at room temperature and (73 meV)2 at 70°C. A direct confirmation of these data, which were derived from a moment analysis of the threshold behaviour of an Au isochromat is provided by a measurement of image potential states at Cu(001).

Spectrometer for momentum‐resolved bremsstrahlung spectroscopy

A spectrometer for momentum‐resolved and polarization‐dependent bremsstrahlung spectroscopy (inverse photoemission) in the vacuum UV range is described and the performance of the apparatus is discussed. A Seya–Namioka monochromator combined with a position‐sensitive channel plate device for parallel detection of a wavelength range of 165 A is used as a photon detector and covers an energy regime from 10 to 40 eV with a resolution of 5 A. Due to the polarization‐dependent reflectivity of the gold‐coated gratings, the monchromator is also inherently an analyzer for the polarization of the detected light. A space‐charge‐limited Pierce‐type electron gun comprising a BaO dispenser cathode is used as an electron source with 0.1‐A−1 momentum resolution. The overall energy resolution of the apparatus is 0.3 eV at 20 eV photon energy. The sensitivity is 1.8×104 counts per Coulomb and eV for the unoccupied s, p bands of a polycrystalline gold sample.