Helium scattering

HELIUM-SCATTERING STUDIES OF THE DYNAMICS AND PHASE TRANSITIONS OF... 

The surface charge density of Al(lll) has been well characterized by first-principles calculations as well as helium scattering experiments. The asymptote of the corrugation amplitude Az of equal-LDOS surface contours follows an exponential law, as obtained from a first-principles calculation of the electronic structure of the Al(l 11) surface (Mednick and Kleinman, 1980) ... 

The corrugation of the charge density on metal surfaces can be obtained from first-principles calculations or helium scattering experiments. The theory and the experiments match very well. A helium atom can reach to about 2.5-3 A from the top-layer nuclei. At that distance, the repulsive force between the helium atom and the surface is already strong. The corrugation at that distance is about 0.03 A, from both theory and experiments. For STM,... 

On the domain boundary between the fee stacking and the hep stacking, the Au atoms are squeezed out from the original position to make two ridges per unit cell. The consequences of this model are in good agreement with their helium scattering data. In 1985, STM was already 3 years old. It was believed at that time the ultimate resolution of STM on metals was 6 A, which would be insufficient to resolve the atomic structure on Au(l 11). 

The reason for the large difference between the values of A for positrons and electrons at an energy of 2 eV is that for positrons the s-wave phase shift passes through zero at the Ramsauer minimum and the dominant contribution to the cross section therefore comes from the p-wave, which is quite strongly peaked in the forward and backward directions. In contrast, there is no Ramsauer minimum in electron-helium scattering, and the isotropic s-wave contribution to aT is dominant at this energy. 

The earliest attempts to partition aT for positron-helium scattering were those of Griffith et al. (1979b), before the advent of any partial cross section measurements, and of Coleman et al. (1982), when the first inelastic scattering data became available. The most detailed study is... 

Until quite recently, helium was the simplest atomic target used in experimental studies of positron collisions. It is also the simplest atom for which the wave function is not known exactly. Accordingly, positron-helium scattering has attracted considerable theoretical attention, and detailed comparisons have been made between the experimental measurements of the scattering parameters and the corresponding theoretical results obtained using a wide variety of approximation methods. 

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