It provides information about the surface structure and lattice dynamics of a material by measuring the diffracted atoms from a monochromatic helium beam incident on the sample.
The first recorded helium diffraction experiment was completed in 1930 by Immanuel Estermann and Otto Stern[1] on the (100) crystal face of lithium fluoride.
At the time, the major limit to the experimental resolution of this method was due to the large velocity spread of the helium beam.
It wasn't until the development of high pressure nozzle sources capable of producing intense and strongly monochromatic beams in the 1970s that HAS gained popularity for probing surface structure.
Interest in studying the collision of rarefied gases with solid surfaces was helped by a connection with aeronautics and space problems of the time.
Plenty of studies showing the fine structures in the diffraction pattern of materials using helium atom scattering were published in the 1970s.
However, it wasn't until a third generation of nozzle beam sources was developed, around 1980, that studies of surface phonons could be made by helium atom scattering.
These nozzle beam sources were capable of producing helium atom beams with an energy resolution of less than 1meV, making it possible to explicitly resolve the very small energy changes resulting from the inelastic collision of a helium atom with the vibrational modes of a solid surface, so HAS could now be used to probe lattice dynamics.
While there are several techniques that probe only the first few monolayers of a material, such as low-energy electron diffraction (LEED), helium atom scattering is unique in that it does not penetrate the surface of the sample at all!
There are several advantages to using helium atoms as compared with x-rays, neutrons, and electrons to probe a surface and study its structures and phonon dynamics.
As mentioned previously, the lightweight helium atoms at thermal energies do not penetrate into the bulk of the material being studied.
When used at thermal energies, as is the usual scenario, the helium atomic beam is an inert probe (chemically, electrically, magnetically, and mechanically).
Finally, because the thermal helium atom has no rotational and vibrational degrees of freedom and no available electronic transitions, only the translational kinetic energy of the incident and scattered beam need be analyzed in order to extract information about the surface.
Also contained in section B is a chopper system, which is responsible for creating the beam pulses needed to generate the time of flight measurements to be discussed later.
That is, the locations of the diffraction peaks reveal the symmetry of the two-dimensional space group that characterizes the observed surface of the crystal.
Most helium atom scattering studies will scan the detector in a plane defined by the incoming atomic beam direction and the surface normal, reducing the Ewald sphere to a circle of radius R=k0 intersecting only reciprocal lattice rods that lie in the scattering plane as shown here:
Note that the detection of the helium atoms is much less efficient than for electrons, so the scattered intensity can only be determined for one point in k-space at a time.
The intensity of the incoherent elastic peak and its dependence on scattering angle can therefore provide useful information about surface imperfections present on the crystal.