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Thermal Energy Atom and Molecular Scattering

(Dr. D. Farías, M. Patting, F. John)

photograph of the scattering apparatus

Research Fields

The Helium Scattering Apparatus

Some Results

Research Fields

Diffraction of low energy He- and Ne-beams (20 - 150 meV) is an absolutely nondestructive technique, sensitive to the topmost surface layer only. It is applicable to metals, semiconductors, and insulators and has a unique sensitivity to hydrogen adsorbed on surfaces. Our efforts concern quantitative determinations of the surface corrugation of regular adsorbate systems (especially hydrogen) as well as (adsorbate induced) surface reconstructions. Calculation methods for diffraction intensities with realistic particle-surface interaction potentials are further developed. A recent scientific highlight concerns the experimental proof of anticorrugating effects with He and their absence with Ne. Future research plans include quantitative aspects of rotational excitations and de-excitations upon diffraction of molecular hydrogen and deuterium.

The Helium-Diffraction Apparatus

Beam System

Schematic Drawing of the Apparatus The experiments have been carried out in the UHV-apparatus shown schematically in the figure.
It is equipped with standard LEED and ion gun systems to characterize and clean the surface, in addition to a Kelvin probe for performing work function measurements and a quadrupole mass spectrometer with an axial-beam ion source for recording thermal desorption spectra.
The base pressure in the chamber was typically 3 x 10-11 mbar, reaching 5 x 10-10 mbar with the He beam on.
The crystal is mounted on a standard manipulator, modified to allow azimuthal rotation of the sample as well as heating to 1200 K and cooling to 100 K.

Scattering Geometry

Scattering Geometry The angular distribution of the back-scattered atoms was analyzed with a quadrupole mass spectrometer mounted on a two-axis goniometer. This arrangement allows rotations of 200° in the scattering plane, defined by the beam direction and the normal to the surface, as well as ±15° normal to the scattering plane.
Rotation of the goniometer and data aquisition are performed simultaneously by a computer program; one scan takes about 2 minutes. As it will become apparent, the ability to measure out-of-plane spectra is a valuable asset in interpreting diffraction data. The scattering geometry is schematically represented in the figure on the left.
Angles of incidence and angles of scattered beams are both referenced to the surface normal, whereas the out-of-plane angle is measured from the scattering plane.

Some Results

Anticorrugating Effects

As an experimental example of anticorrugating effects see the figure below including sphere models of Ni(110)c(2x4)H and Rh(110)(1x2)H together with grey scale representations of the corrugation functions derived from He and Ne diffraction.
In the sphere models open large circles denote metal atoms and full small circles H adatoms at the locations determined by LEED. The areas of the corrugations correspond to those of the sphere models. The H atoms have the largest corrugation amplitudes and thus show up as the brightest spots in the grey scale top views. Note that in all He-derived corrugations there occur less bright maxima between the H atoms along [001] (perpendicular to H atom rows) on all H-free metal rows in disagreement with the true atom arrangements shown in the sphere models. In contrast to this, in the Ne-derived corrugations the metal maxima are shifted by a/2 along [1-10] (parallel to H atom row) in agreement with the true surface structures.

For a detailed description about anticorrugating effects see: [K.H. Rieder, G. Parschau, B. Burg: Phys. Rev. Lett. 71, 1059 (1993)] and the references inside.

Open Metal Surfaces: fcc(311)

fcc(311) surface Besides fcc(110,111,100) metal surfaces more open high index fcc(311) surfaces are at the focus of present interest. A special property of the fcc(311) surfaces is the existence of both (111) and (100) microfacets offering a large variety of adsorption sites. The figure on the right shows side (a) and top (b) views of the fcc(311) sphere model. The rectangular and primitive unit cells are indicated. Note the asymmetrical surface structure and the possible adsorption sites in the first and second layer and their coordination (top, brigde, threefold, fourfold).

With He and Ne diffraction we investigated the clean and adsorbate covered surfaces of Ni(311), Rh(311) and Pd(311). We observed the formation of several ordered adsorbate structures with hydrogen and oxygen depending on temperature and coverage, also in combination with the HREELS method. Examples of individual results can be found in the publication list [Ni(311), Rh(311), Pd(311)].

Contact: farias@physik.fu-berlin.de

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