Atomic-beam surface scattering

All of these processes have been observed in atomic-beam surface-scattering experiments, during which a beam of atoms of well-defined kinetic energy impinges on a single-crystal surface. The kinetic energy distribution of the back-scattered atoms can be detected by using a correlation chopper velocity selector [18]. 

Atomic and molecular-beam surface-scattering studies reveal efficient energy transfer between the translational, vibrational, and rotational energy modes of the incident molecules and.the surface atoms. 

Two factors determine the intensity of the scattered beam the scattering cross section for the incident ion-target atom combination and the neutralization probability of the ion in its interaction with the solid. It is the latter quantity that makes LEIS surface sensitive 1 keV He ions have a neutralization probability of about 99 % on passing through one layer of substrate atoms. Hence, the majority of ions that reach the detector must have scattered off the outermost layer. At present, there is no simple theory to adequately describe the scattering cross section and the neutralization probability. However, satisfactory calibration procedures by use of reference samples exist. The fact that LEIS provides quantitative information on the... 

Based on the data of hydrogen and oxygen atoms scattering by the surface of ZnO film obtained by the method of atom beams we calculated the degree of ionization of chemosorbed particles on the film using the variation of its conductivity (see Table 3.1) [23]. 

Fig. 4.13. Classical turning point in atomic-beam scattering. When the repulsive potential on a He atom at the sample surface equals the kinetic energy of the He atom, as a classical particle, the He atom is turned back.

The detailed data from He-scattering experiments provide information about the electron density distribution on crystalline solid surfaces. Especially, it provides direct information on the corrugation amplitude of the surface charge density at the classical turning point of the incident He atom, as shown in Fig. 4.13. As a classical particle, an incident He atom can reach a point at the solid surface where its vertical kinetic energy equals the repulsive energy at that point. The corrugation amplitude of the surface electron density on that plane determines the intensity of the diffracted atomic beam. 

The determination of the atomic structure of surfaces is the cornerstone of surface science. Before the invention of STM, various diffraction methods are applied, such as low-energy electron diffraction (LEED) and atom beam scattering see Chapter 4. However, those methods can only provide the Fourier-transformed information of the atomic structure averaged over a relatively large area. Often, after a surface structure is observed by diffraction methods, conflicting models were proposed by different authors. Sometimes, a consensus can be reached. In many cases, controversy remains. Besides, the diffraction method can only provide information about structures of relatively simple and perfectly periodic surfaces. Large and complex structures are out of the reach of diffraction methods. On real surfaces, aperiodic structures such as defects and local variations always exist. Before the invention of the STM, there was no way to determine those aperiodic structures. 

Figure 3 Schematic diagrams of prototypes of gas-surface interactions as can be probed by molecular beams, presented as side views of the surface atoms or cubes. (A) molecular scattering in which parallel momentum is conserved and die surface is represented by hard cubes. (B) molecular scattering from individual surface atoms. (C) molecular scattering in the presence of a strong chemisorption well. (D) molecular scattering for a partially passivated surface, containing specific sites where chemisorption is possible. Note that in this case the interaction is also strongly orientation dependent. From Ref. [1].

Blythe, Grosser, and Bernstein [151 ] have used crossed molecular beams to observe the J = 2 - 0 rotational deexcitation process in D2. A velocity-selected atomic beam of potassium was made to impinge on a modulated Da beam from an effusive (T = I8PK) source. The scattered K atoms were detected by surface ionization on a hot Pt-W ribbon, from which the ions were drawn into an electron multiplier equipped with lock-in amplification. 

A thermal energy atomic beam (20-200 meV) has a wavelength on the order of inter-atomic distances. The atomic beam diffracts from a contour of the surface potential corresponding to the beam energy. This contour is located 3-4 A above the ion cores in the outermost layer of the surface. Atomic beam diffraction patterns are normally interpreted using model surface scattering calculations, where the scattering is described as a Van der Waals interaction. 

Atom or helium diffraction AD Monoenergetic beams of thermal energy neutral atoms are elastically scattered off ordered surfaces and detected as a function of scattering angle. This gives structural information on the outermost layer of the surface. Atom diffraction is extremely sensitive to surface ordering and defects. Atomic structure... 

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