Beam scattering

The nature of reaction products and also the orientation of adsorbed species can be studied by atomic beam methods such as electron-stimulated desorption (ESD) [49,30], photon-stimulated desoiption (PDS) [51], and ESD ion angular distribution ESDIAD [51-54]. (Note Fig. VIII-13). There are molecular beam scattering experiments such... 

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. 

In a third step the S-matrix is related to state-selected reaction cross sections a., in principle observable in beam scattering experiments [28, 29, 30, 31, 32, 33, 34 and 35], by the fiindamental equation of scattering theory... 

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... 

Fluendy MAD and Lawley K P 1973 Applications of Molecular Beam Scattering (London Chapman and Hall)... 

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... 

Figure B2.3.1. Schematic diagram of an idealized molecular beam scattering experiment.

A molecular beam scattering experiment usually involves the detection of low signal levels. Thus, one of the most important considerations is whether a sufficient flux of product molecules can be generated to allow a precise measurement of the angular and velocity distributions. The rate of fonnation of product molecules, dAVdt, can be expressed as... 

With spectroscopic detection of the products, the angular distribution of the products is usually not measured. In principle, spectroscopic detection of the products can be incorporated into a crossed-beam scattering experiment of the type described in section B2.3.2. There have been relatively few examples of such studies because of the great demands on detection sensitivity. The recent work of Keil and co-workers (Dhannasena et al [16]) on the F + H2 reaction, mentioned in section B2.3.3, is an excellent example of the implementation... 

Siska P E 1973 Iterative unfolding of intensity data, with application to molecular beam scattering J. Chem. Rhys. 59 6052-60... 

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. 

It would be of great interest to experimentally verify these new results of phonon modes, MSB s and relaxations by suitable methods, such as electron-energy-loss-spectroscopy or thermal helium beam scattering. 

Figure 2. X-ray and neutron beam scattering intensities with MCY, with addition of three- and four-body corrections.

Copper clusters, as reported by the Rice group(lc), do not react with hydrogen. Hydrogen chemisorption on copper surfaces is also an activated process. Surface beam scattering experiments place this barrier between 4-7 kcal/mole(33). This large value is consistent with the activated nature oT hydrogen chemisorption on metal clusters, and the trend toward bulk behavior for relatively small clusters (>25 atoms in size). 

The kinetics of NO on Pt surfaces have been explored by many authors both in TPD and beam scattering types of experiments. In a series of sophisticated modulated beam scattering experiments, Serri efo/. (referred to subsequently in the text as STC) attempted to reconcile the apparently disparate results reported previously"- " . In their experiments they noted that for... 

More informative are the stochastic trajectory simulations run by Muhl-hausen et al. (M WT), on empirical interaction potential surfaces for scattering and desorption Although the major thrust was to understand the direct beam scattering results of NO/Ag(l 11), extension of these calculations allows for comparison to the desorption of NO from Pt(lll) Important insights derived from the NO/Ag(lll) calculations were ... 

It is impractical to attempt to explore conventional TPD dynamics for NO/Pt(l 11) at values of Tg > 400 K since this would require extreme heating rates. Two techniques have been utilized to extend state-resolved dynamics to higher Tg. These include trapping/desorbing beam scattering and surface reactions (i.e. as observed for the NO-NHj coadsorption system). 

Fig. 5. Rotational temperatures ofNO desorbing from Pt(l 11). The data are representative of data published for (x) neat thermal desorption , ( +) thermal desorption in the presence of coadsorbed C0 ° (solid squares) and (solid triangles) trapping/desorption in molecular beam scattering, (open triangle) reaction limited desorption from NO-NHj complexes, (open circle) and (open square) NHj oxidation reactions. The solid line is for full accommodation. The dashed curve represents results for translational energy measurements in direct inelastic scattering ...

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