Atom and Molecule Probes

The diffraction of H2 and He beams by the surfaces of alkali halide crystals was observed originally in 1930 and provided direct evidence for the wave-nature of matter. Interest in the technique has revived in recent years and considerable experimental advances have been made, particularly in the production of well-collimated and approximately mono-energetic beams for reviews, including experimental details, see refs. 231, 232. 

A typical model potential used to describe the scattering process 233—235 include an attractive component, such as a square well of depth D and width d and a repulsive component in the form of a stationary sinusoidal hard wall (or infinitely hard corrugated surface ). Then the potential V(r) is given by 

The parameters Cx Cy known variously as diffraction strengths , distortion amplitudes , or corrugation parameters = CxPx) f,ly are the dimensions of the unit mesh, assumed to be rectangular. 

A full theoretical treatment of the scattering problem would involve solution of the Schrodinger equation for the system, with the potential energy expressed in terms of equation (8). This approach, however, is impossible at present since the atom-surface interaction is too strong for treatment to be reliable within the Born or other weak-scattering approximations but considerable progress has been made using both 

If the scattered intensity P is calculated on the assumption of a purely elastic scattering process it will be related to the experimentally observed quantity P T, which contains contributions from inelastic events (phonon excitations), by an expression of the type 

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