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. 

The dynamics of the interactions that lead to polymer degradation in space are often studied with beam-surface scattering techniques in which the dynamical behavior of gas-phase products (both reactive and nonreactive) is examined. Although infrared emission has been used to detect reaction products, the majority of scattering dynamics studies to date have employed mass spectrometric detection.The most detailed dynamical studies on gas-surface interactions relevant to polymer degradation in space have been done in the laboratory of the authors. ° 36-i4i j.g suits from these studies will be summarized in Secs. 3.3 and 3.4, while the experimental methods follow in this section. 

Low pressure - High surface temperatures studies (molecular 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. 

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. 

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