Helium Atom Scattering

A short introduction on our work and helium atom scattering is given below starting with the components of a typical helium atom scattering apparatus. Shematic drawing of a helium atom scattering machine: Pure helium gas is used to create an atomic beam in a supersonic expansion whereupon the gas expands from a high pressure region (50 bar) through a small nozzle into a region of vacuum. The expansion gives rise to an adiabatic cooling of the gas whereupon the internal energy of the gas is transformed into translational kinetic energy. About 10 mm from the nozzle the outer part of the expanding gas cloud is separated from the forward moving atoms using a conical skimmer. Further down the beam line a chopper can be located allowing time-of flight experiments by chopping the beam into short pulses. The intense, nearly monoenergetic helium beam is then directed towards a target surface at a particular angle of incidence and the scattered intensity is monitored at a given angle using a mass spectrometer. Therefore the sample is mounted on a manipulator which allows to position and rotate the sample.

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HAS Measurements

1) Elastic Scattering - Surface Structure:

Eleastic measurements i.e. measurements where the helium atoms are scattered elastically at the surface, are used to determine the surface structure.Thus the surface structure is studied by scattering the helium beam at the surface and observing the resulting interference pattern. Interference occurs due to wave-particle duality, which states that the incident helium atoms can be described as waves giving rise to a diffarction pattern just as in the case of X-ray diffraction.

2) Inelastic Scattering - Surface Dynamics:

Inelastic measurements include energy transfer to or from the surface which reveals information about the surface dynamics. Thus the helium atoms are scattered inelastically through excitation or deexcitation of the surface vibrational modes (phonon creation or annihilation).

The inelastic scattering of helium atoms is used to determine surface phonon modes that is collective lattice vibrations at the surface. Thus the helium atom may either loose energy by exciting a phonon or gain energy via energy transfer from a phonon to the helium atom. The scattered helium atom exhibits an energy (velocity) that is lower/higher than the velocity before the interaction. To achieve information about the different velocities in the helium beam a so called time-of-flight measurement is carried out. Therefore it is necessary to modulate the beam by a rotating chopper disk: A slit in the chopper rotates past the beam and chops the beam into narrow bunches of He-atoms. The atoms in the bunch are then propagating to the detector whereby faster atoms tend to arrive earlier at the detector than slower particles.

Why use helium atoms?

Using helium atoms to probe a surface holds several advantages compared with X-rays, neutrons or electrons. Hence the lightweight helium atoms in the energy range of 10-200 meV do not penetrate into the bulk material giving rise to the only surface characterization method that is strictly surface sensitive. This means in addition, that using low energetic helium atoms to probe a surface is truly non-destructive to the sample. Since the helium atoms are neutral they are insensitive to surface charges thus allowing the investigation of insulating materials. Furthermore, as a noble gas, the helium atoms are chemically inert thus offering a completely inert investigation method (chemically, electrically, magnetically and mechenically). Hence a helium beam is capable of studying the surface structure and dynamics of a wide range of materials, including those with reactive or metastable surfaces. The drawing on the right side may give you an impression of the whole apparatus.

Research

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