Helium atom scattering , metallic

The ionic insulators discussed in some detail in the previous section have closed shell electronic configurations similar to the noble gases and electronic distributions which are localized around the electronic core. The principal interactions are Coulombic, although their polarizabilities appear to influence greatly the response of the electronic distribution to surface lattice vibrations. For other materials, particularly metals and some layered compounds, the conduction and valence electrons are best thought of as somewhat delocalized if not entirely free. These electrons are what the helium atoms scatter from, and their states of motion are significantly modulated by the vibrations of the atomic cores. Thus, for these materials HAS is very... 

The close-packed (111) surfaces of the noble metals were among the first surfaces studied by HAS [97], Diffractive helium atom scattering shows that these surfaces have hexagonal symmetry and are rather flat with very small corrugation in the electronic density sampled by the helium atoms. They are also found to be relaxed by only a few percent and unreconstmeted with the exception of Au(lll), which exhibits a (23 x 1) structure [98]. 

As in the case of the noble metals above, it is the sensitivity of the helium atom scattering to the distribution of electrons in the conduction energy states which has led to the detailed explanation of the surface properties of these metals. Intriguingly, for the W( 110) and Mo(llO) surfaces, it is still uncertain why saturation with hydrogen leads to just the opposite consequences, namely giant phonon anomalies, whereas the clean surfaces have normal dispersion curves [113]. 

Graham, A.P. The low energy dynamics of adsorbates on metal surfaces investigated with helium atom scattering. Surf. Sci. Rep. 2003, 49,115. 

These predictions have been verified by experiments on low-energy helium atom scattering by metal surfaces [112]. The helium atom repels the Bloch electrons, because their wave function must become distorted to preserve orthogonality to the wave functions of the closed shells of helium. The repulsion potential appears in practise to be proportional to the local free electron density. Thus a helium atom scatters like a ping-pong ball from the electron cloud of the metal, and this allows to probe the distribution of electrons in the cloud, that is, the profile of the electronic tail and its lateral corrugation that usually follows the periodicity of the surface crystal plane [112]. 

Based on the first-principles study of helium adsorption on metals (Zaremba and Kohn, 1977), Esbjerg and Nprskov (1980) made an important observation. Because the He atom is very tight (with a radius about 1 A), the surface electron density of the sample does not vary much within the volume of the He atom. Therefore, the interaction energy should be determined by the electron density of the sample at the location of the He nucleus. A calculation of the interaction of a He atom with a homogeneous electron distribution results in an explicit relation between the He scattering potential V r) and the local electron density p(r). For He atoms with kinetic energy smaller than 0.1 eV, Esbjerg and Nprskov (1980) obtained... 

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,... 

Fig. 6.9. Illustrating the principle of the alpha-particle deflection (scattering) experiments. Positively charged helium atoms are directed at a metal film, and its eWecf on the particles observed with detecting devices placed at various angles. The angle () is a measure of the deflection.

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). 

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