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Helium Atom Scattering - Time of Flight Spectroscopy

(Dr. W. Theis, H. Tröger)


Research Fields

The Helium Scattering Apparatus

Some Results

Research Fields

Helium Atom Scattering is a versatile tool in investigating structural and dynamical properties of surfaces. If one deals with insulators, materials sensitive to irradiation of light or certain particle beams (as LEED, e.g.), or with adsorbate systems in the regime of physisorption, HAS possibly becomes one of the most powerful methods. The low energies of the thermal He atoms well below 100 meV make HAS absolutely surface sensitive (only the first layer is visible) and completely non-destructive. Information about the surface structure and the corrugations of well ordered crystalline species or systems with defects are obtainable as well as structural properties of adsorbates even in the submonolayer-range. Time of Flight Spectroscopy allows one to determine low energetic surface vibrations as, e.g., the Rayleigh phonon branch or low-lying optical modes over the entire Brillouin zone and vibrational properties of adsorbate systems.

Employing HAS we made investigations of the (111) surfaces of two insulating materials of the Fluoride structure: CaF2 and BaF2. In the case of CaF2 we were able to show that irradation with electron beams causes point-like defects. Another focus was the adsorption of water: A surprising p(4x4)-structure of nanoclusters on the perfect surface at low temperatures was observed. Defect-induced adsorption on an electron-irradiated surface was found to occur even at room temperature. Concerning BaF2 two optical vibrational branches were observed and could be explained with a simple force-constant model.

The current topics of our work are the growth of Fe on MgO(100) and the magnetic properties of thin Fe films.

The Helium Scattering Apparatus

HAS-TOF-Apparatus HAS-TOF-Apparatus

All experiments were carried out in the vacuum system shown in the above figures. More detailed ones are available by clicking on them. The major components of the apparatus are the high vacuum beam generation part (I-III, the differential pumpinge stage III is not visible in the photography), the UHV sample preparation and analysis chamber (IV) and the three stage flight path and detector system (V-VII).

The Beam Generation Part

The creation of a highly expanded, nearly monoenergetic beam of He atoms (less than 1 meV energy width) takes place mainly in the stages I and II. A self-constructed He reservoir and nozzle system provides He gas with stagnation pressures up to 100 bar. The temperature of the system can be varied via a 2-stage coldhead between 30 K and 300 K, which corresponds to beam energies of 5 to 60 meV. An electron microscopy aperture (5 mu diameter) is used as nozzle, through which the He expansion into the high vacuum of stage I is realised. A conical shaped aperture between stages I and II (Skimmer) extracts the centre part of the He beam. After this extraction the expansion is nearly finished and the He atoms, now moving with the same velocity, do not interfere with each other any more. Stage II also contains a multi-slit chopper system, which creates beam pulses necessary for the TOF measurements, and an aperture system, which defines the beam diveregence. Stage III is a differential pumping stage with a gate valve.

The Sample Analysis Chamber

In all experiments the scattering angle (angle between incoming beam and detector arm) remains fixed. So for changing the momentum transfer the sample has to be rotated polar, which changes the angle between surface normal and both incoming and outgoing beam. Furthermore, the sample has to be carefully placed and adjusted in the scattering centre. This is now reached by a 6-axis manipulator (X,Y,Z and polar, azimuthal and flip-rotation) with double-differentially pumped rotation flanges. The precision of the polar rotation is better than 0.01 deg. The sample temperature can be varied between 150 K and 1000 K. The sample analysis chamber is additionally equipped with LEED/AES, sputter-facilities, a metal evaporator and a crystal cleaver.

Flight Path and Detector Part

The flight path is connected via a flexible hose to the sample recipient. Four different ports (a-d) can be used, so that the scattering angle can be varied from 56 deg to 180 deg. Even out-of-plane movement of the hole detector arm is possible. All flight path stages are separately pumped by turbo molecular, diffusion and rotation pumps. The detector consists of a Balzers QMG 420 mass spectrometer, which is mounted in the last stage (VII). It can be fine adjusted.

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This page is made by Dr. G. König. Contact: wolfgang.theis@physik.fu-berlin.de
(Last modified: 12.03.1999)