Molecule-surface collisions

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. 

Kasemo B 1996 Charge transfer, electronic quantum processes, and dissociation dynamics in molecule-surface collisions Surf. Sci. 363 22... 

The number of surface collisions at p=l bar and T = 300 K is thus rcoii-surf = 1-08 X 10 m s for hydrogen and 2.88 x 10 m s for nitrogen. Since there are typically 1.5 x 10 surface atoms per m, a surface atom will on average be hit a billion times per second under ambient conditions. This, however, does not necessarily mean that the gas molecule reacts, particularly if the reaction is an activated process. 

Table 4.2 gives decomposition lifetimes extracted from the flow tube data. Keep in mind that these kinetic data are for conditions in which a combination molecule-buffer gas, molecule-molecule, and molecule-wall collisions occur. The wall surface in the flow tube was quartz, and the slight discoloration observed indicates that there was initially some decomposition on the walls. Note, however, that there was no build-up of material on the walls beyond the initial transparently thin carbonaceous coating, and no products (e.g., polymer fragments) were observed that might be expected from reaction on the walls, or from bimolecular reactions. It appears that the decomposition is dominated by true unimolecu-lar reactions however, collisions with the walls are undoubtedly important in energizing the molecules for dissociation. 

Transport of the gas to the surface and the initial interaction. The first step in heterogeneous reactions involving the uptake and reaction of gases into the liquid phase is diffusion of the gas to the interface. At the interface, the gas molecule either bounces off or is taken up at the surface. These steps involve, then, gaseous diffusion, which is determined by the gas-phase diffusion coefficient (Dg) and the gas-surface collision frequency given by kinetic molecular theory. 

In Eq. (PP), N is the gas concentration (molecules cm 3), um is the average molecular speed in the gas phase, R is the gas constant (J K 1 mol ), T the temperature (K), and M is the molecular weight (kg) of the gas. The normalized rates, i.e., divided by the rate of gas-surface collisions in Eq. (PP), will be referred to as conductances, T, for reasons that will become apparent shortly. However, the reader should keep in mind that these conductances just reflect the speeds of the individual processes. 

Evaporation of liquid water forms water vapor which is a gas in the closed container. After a while, water vapor molecules start collisions with each other and with the water s surface then they turn into water. Therefore, evaporation and condensation are a reversible process in a closed container. Reversible processes are represented by Irriversible processes are represented by... 

We have already mentioned (expressions 30—33) the widely used LEPS surface for atom-diatom reactions. This may be regarded as purely empirical or semi-empirical in any modification in which some integrals are evaluated. Another system for which fairly elaborate potential functions have been used is for non-reactive atom-diatom scattering. The experiment for which the potential is designed is the change of rotational or vibrational state of a diatomic molecule by collision with a third atom, and also the quasi bound states, which may be observed spectroscopically, of van der Waals molecules such as Ar—H2 (133). 

A collision between a gas molecule and a surface sometimes leads to a heterogeneous reaction. We will obtain an expression for the rate of molecule-wall collisions. 

Incidentally we note that resonances do exist, however, in gas-surface collisions in which, as a consequence of the infinite mass of the solid, J is always zero resonances are indeed one major source of information on the gas-surface interaction (Hoinkes 1980 Barker and Auerbach 1984). Likewise, resonances are prominent features in electron-atom or electron-molecule collisions (Schulz 1973 Domcke 1991) the extremely light mass of the electron implies that only partial waves with very low angular momentum quantum numbers contribute to the cross section. 

Diffusion in macropores occurs mainly by the combined effects of bulk molecular diffusion (as in the free fluid) and Knudsen flow, with generally smaller contributions from other mechanisms such as surface diffusion and Poiseuille flow. Knudsen flow, which has the characteristics of a diffusive process, occurs because molecules striking the pore wall are instantaneously adsorbed and re-emitted in a random direction. The relative importance of bulk and Knudsen diffusion depends on the relative frequency of molecule-molecule and molecule-wall collisions, which in turn depends on the ratio of the mean free path to pore diameter. Thus Knudsen flow becomes dominant in small pores at low pressures, while in larger pores and at higher pressures diffusion occurs mainly by the molecular mechanism. Since the mechanism of diffusion may well be different at different pressures, one must be cautious about extrapolating from experimental diffusivity data, obtained at low pressures, to the high pressures commonly employed in industrial processes. 

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