Helium atom scattering
Helium atom scattering ( HAS) (also helium atom scattering ) is a characterization method for surfaces ( surface physics ). It can be used to examine the surface of the crystalline substrate in ultra high vacuum (UHV). Here, a helium gas jet on the periodic surface of the single crystals is bent. From the form of the so- specific " diffraction peaks " can the quality of the surface and the mean terrace width and the average terrace height determined. Can still be examined using HAS lattice constants, desorption and phonons.
Construction of apparatus
In a helium atom scattering apparatus high purity helium ( 99.9999 %) expanded bar in a high pressure of at least 50, through a nozzle of 10 micron diameter in a vacuum. Since the He -He scattering cross section is large, it is used in the expansion to a plurality of joints, at the end of a mono-energetic beam of a helium energy width of less than 2% is. The energy of this beam can be varied over the temperature of the die of 20 meV to 60 meV. Helium beam thus produced is bent in the analysis chamber at the crystal. The diffracted beam helium passes through a running track in the detector, where a mass spectrometer are ionized and detected a small fraction of helium atoms.
Principle of helium atom scattering
For reflection of helium atoms from the surface occurs by Pauli repulsion. The helium - surface interaction potential can be described with a strongly repulsive and a negligible, long-range, attractive content (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. Thus, the potential can be viewed as a first approximation simply as a hard wall with periodic corrugation. On this wall helium atoms are scattered.
A characteristic of the helium scattering is the large scattering cross section of the helium atoms with individual adatoms and 1D and 2D defects. The scattering cross sections of helium atoms go far ( about 100 Å ²) over the extent of the defects out. Only points where the surface is defect-free, provide for the helium atoms is a periodic structure, where they can be bent. Helium atoms that hit vacancies or adatoms are diffusely scattered and do not contribute to the intensity of the diffraction peaks at.
A helium beam apparatus is commonly used in four modes.
Benefits of helium atom scattering
HAS is particularly suitable for the investigation of the surface of crystalline semiconductors and insulators, as a ray of uncharged helium atoms is diffracted and so charging effects are avoided. Further, a helium beam having a wavelength in the range of lattice constant has a very low energy of 20 to 60 MeV. So power is compared to the energy of the LEED electrons ( 70-250 eV) is so low that the helium scattering is completely non-destructive and thus also weakly bound adsorbates can be studied. Furthermore, HAS is the oberflächenempfindlichste characterization method because the helium atoms of the beam will be reflected before the surface and under non- electronic excitation in a first approximation.
Disadvantages of helium atom scattering
A disadvantage of the helium atom scattering, that for maintaining the enormous UHVs pump power is required. Usually a helium scattering apparatus consisted of about 20 pumping stages.
Finally, the method is limited to the study of either small or rigid adsorbates because helium atoms are inelastically reflected light according to the Debye -Waller interpretation of soft surfaces.