Helium atom scattering

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Helium atom scattering ( HAS ) (also helium atom beam scattering ) is a characterization method for surfaces ( surface physics ). It can be used to examine the surface of crystalline substrates in ultra-high vacuum ( UHV ). A helium gas jet is bent at the periodic surface of the single crystals . The quality of the surface as well as the mean terrace width and also the mean terrace height can be determined from the shape of the “diffraction peaks” determined in this way. Furthermore, with the help of HAS, lattice constants , desorption energies and phonons can be investigated.

Apparatus construction

In a helium atom scattering apparatus, high-purity helium (99.9999%) is expanded into a vacuum through a nozzle with a diameter of 10 μm at a high pressure of at least 50 bar. Since the He-He scattering cross-section is large, the expansion leads to a large number of collisions, at the end of which there is a monoenergetic helium beam with an energy width of less than 2%. The energy of this jet can be varied from 20 meV to 60 meV via the temperature of the nozzle  . The helium beam generated in this way is bent on the crystal in the analysis chamber. The diffracted helium beam passes through a path into the detector, where a mass spectrometer ionizes and detects a small part of the helium atoms.

Principle of helium atom scattering

The reflection of the helium atoms from the surface occurs through Pauli repulsion. The helium surface interaction potential can be described with a strongly repulsive and a negligible, far-reaching, attractive part (van der Waals interactions). The resulting total interaction potential has its minimum at a distance of 4 Å (400  pm ) and has typical pot depths of 10 meV. In a first approximation, the potential can thus be viewed in simplified form as a hard wall with periodic corrugation. Helium atoms are scattered on this wall.

A special feature of helium scattering is the large scattering cross-section of the helium atoms with individual adatoms as well as 1D and 2D defects. The scattering cross-sections of the helium atoms go far (about 100 Ų) beyond the extent of the defects. Only places where the surface is free of defects represent a periodic structure for the helium atoms, where they can be bent. Helium atoms that meet imperfections or adatoms are diffusely scattered and do not contribute to the intensity of the diffraction reflections.

Operating modes

A helium jet apparatus is usually used in four operating modes.

  1. Angular distribution: In angular distribution, the crystal is rotated around the polar angle at constant crystal temperature and constant beam energy. Finally, the intensity is plotted as a function of the polar angle, which corresponds to a one-dimensional diffraction image. By rotating the crystal along the azimuth , the other directions of high symmetry of the examined crystal can also be determined.
  2. He-TDS: In helium thermal desorption spectroscopy, the He reflectivity of a surface is examined as a function of its temperature. For this purpose, the temperature of the crystal is increased at a constant angle (usually the zero order diffraction reflex) and constant beam energy. If the surface changes due to desorption of adsorbates, for example, the He reflectivity also changes. Since HAS hides defects, desorption energies of adsorbates that are adsorbed on the perfect parts of the crystal can be determined.
  3. Drift spectra: Here the He reflectivity is determined as a function of the beam energy. Both the crystal temperature and the polar angle are kept constant.
  4. TOF: In the case of time of flight spectra, the jet is pulsed at a constant angle, constant crystal temperature and constant nozzle temperature, so that the time of flight and thus also the kinetic energy of the helium atoms can be determined. A plot of the kinetic energy against the intensity generates a time-of-flight spectrum. Dispersion curves can be determined from a series of time-of-flight spectra that were measured at different beam energies.

Advantages of helium atom scattering

HAS is particularly suitable for examining the surface of crystalline semiconductors and insulators , as a beam of uncharged helium atoms is diffracted, thus avoiding charging effects. Furthermore, a helium beam with a wavelength in the order of magnitude of lattice constants has a very low energy of 20 to 60 meV. This means that the energy compared to the energy of the LEED electrons (70 to 250 eV) is so low that the helium scattering is absolutely non-destructive and thus weakly bound adsorbates can also be examined. Furthermore, HAS is the most surface-sensitive characterization method, as the helium atoms of the beam are reflected before the surface and, as a first approximation, do not experience any electronic excitation.

Disadvantages of helium atom scattering

The disadvantage of helium atom scattering is that an enormous amount of pumping power is required to maintain the UHV. A helium scattering apparatus usually consists of around 20 pump stages.

Finally, the method is limited to the investigation of either small or rigid adsorbates, since helium atoms are slightly inelastically reflected from soft surfaces, according to the Debye-Waller interpretation.

literature

  • Frank Hofmann, J. Peter Toennies: High-Resolution Helium Atom Time-of-Flight Spectroscopy of Low-Frequency Vibrations of Adsorbates . In: Chemical Reviews . tape 96 , no. 4 , 1996, pp. 1307-1326 , doi : 10.1021 / cr9502209 .

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