Molecular beam surface scattering

Just as in gas phase kinetics, reactive molecular beam-surface scattering is providing important molecular level insight into reaction dynamics. There is no surface reaction for which such studies have proven more illuminating than the carbon monoxide oxidation reaction. For example Len, Wharton and co-workers (23) found that the product CO exits a 700K Pt surface with speeds characteristic of temperatures near 3000K. This indicates that the CO formed by the reactive encounter of adsorbed species is hurled off the surface along a quite repulsive potential. 

MO LCAO methods, 34 136 Molecular-beam surface scattering, 26 26, 27 Molecular Cage, 34 226 Molecular design in cyclodextrin, 32 427 Molecular dynamics diffusion in zeolites, 42 2, 4-6 argon, 42 20... 

Another apparatus that is very useful in studies of the mechanism of catalytic surface reactions is shown in Fig. 17. This is used in a molecular-beam surface scattering experiment (22b) in which a well-collimated beam of the reactant gas or gas mixture is scattered from a crystal surface and the products that are desorbed after a single scattering at a given solid angle... 

Fig. 17. Schematic of the UHV molecular-beam surface scattering apparatus.

A detailed description of molecular-beam surface scattering experiments and the results of these studies are given elsewhere (22b, 23). Here we shall discuss only those studies that are important in verifying the nature of active sites in heterogeneous catalysis. 

Low pressure - High surface temperatures studies (molecular beam - surface scattering)... 

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. 

The supersonic molecular beam technique plays an invaluable role in the development of modem physical chemistry and has contributed greatly to the molecular reaction dynamics [30, 31]. Nowadays, molecular beam technique is indispensable in the fields of photolysis, crossed molecular beam, cluster and molecular beam-surface scattering. 

Yu M L and DeLouise L A 1994 Surface chemistry on semiconductors studied by molecular beam reactive scattering Surf. Sc/. Rep. 19 285-380... 

The present paper seeks to clarify these aspects of the Pd-Br2 system by a modulated molecular beam reactive scattering investigation of the surface reaction kinetics. The object is to identify the kinetic order of all overlayer evaporation pathways and to obtain values for all rate parameters which characterise them. 

Characterization and analysis are performed using the following surface science techniques temperature programmed desorption/reaction (TPD/TPR), pulsed molecular beam reactive scattering (pMBRS) (IRRAS), metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS) and auger electron spectroscopy (AES). First the experimental setup is briefly described, followed by the support preparation and characterization as well procedures utilized in this work. These descriptions include a concise introduction to the underlying physical principles of the applied techniques (including experimental details). 

MSS Molecule surface scattering [159-161] Translational and rotational energy distribution of a scattered molecular beam Quantum mechanics of scattering processes... 

This section discusses how spectroscopy, molecular beam scattering, pressure virial coeflScients, measurements on transport phenomena and even condensed phase data can help detemiine a potential energy surface. 

For example, energy transfer in molecule-surface collisions is best studied in nom-eactive systems, such as the scattering and trapping of rare-gas atoms or simple molecules at metal surfaces. We follow a similar approach below, discussing the dynamics of the different elementary processes separately. The surface must also be simplified compared to technologically relevant systems. To develop a detailed understanding, we must know exactly what the surface looks like and of what it is composed. This requires the use of surface science tools (section B 1.19-26) to prepare very well-characterized, atomically clean and ordered substrates on which reactions can be studied under ultrahigh vacuum conditions. The most accurate and specific experiments also employ molecular beam teclmiques, discussed in section B2.3. 

Lykke K R and Kay B D 1990 State-to-state inelastic and reactive molecular beam scattering from surfaces Laser Photoionization and Desorption Surface Analysis Techniquesvo 1208, ed N S Nogar (Bellingham, WA SPIE) p 1218... 

Engel T and Rieder K H 1982 Structural studies of surfaces with atomic and molecular beam diffraction Structural Studies of Surfaces With Atomic and Molecular Beam Scattering (Springer Tracts in Modern Physics vol 91) (Berlin Springer) pp 55-180... 

It is difficult to observe tliese surface processes directly in CVD and MOCVD apparatus because tliey operate at pressures incompatible witli most teclmiques for surface analysis. Consequently, most fundamental studies have selected one or more of tliese steps for examination by molecular beam scattering, or in simplified model reactors from which samples can be transferred into UHV surface spectrometers witliout air exposure. Reference [4] describes many such studies. Additional tliemes and examples, illustrating botli progress achieved and remaining questions, are presented in section C2.18.4. 

Figure 2. togular distribution of the relative scattered intensity of hydrogm molecules scattered from a) Ni(110) b) M(111). The angle of incidenoe of the molecular beam and ibe azimuthal orlentaticn of the surface are indicated in the figure. 

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