Particles coming off the surface

The energy and momentum content of particles coming off the surface reflect the exchange of energy between the different degrees of freedom during interaction with the surface. The simplest case with full thermal accommodation is found in thermal desorption. Without interactions between the adsorbed particles, the rate follows first-order kinetics 

The conclusion that chemisorbed particles are in thermal equilibrium with the sohd while coupled to the surface may, however, no longer be valid if the molecules are released into 

FIGURE 3.8. The mean residence time t of NO molecules adsorbed on a Pt(l 11) surface as a function of temperature T [34]. 

This effect of cooling in desorption is quite common [28] and extends also to other degrees of freedom. For example. Fig. 3.9 demonstrates that the mean rotational temperature Trot of NO molecules desorbing from a Pt(l 11) surface equals the surface temperature only up to about 400K and then levels off because of incomplete excitation of the rotational motion during the desorption process [36]. As a consequence, this effect also means that rapidly rotating molecules exhibit a lower sticking coefficient. 

FIGURE 3.9. Variation of the rotational temperature Trot of NO molecules 

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It had been known since 1941 that red blood-cells are agglutinated by fluids containing influenza virus. In 1942, G. K. Hirst proved that purified influenza virus is quickly and quantitatively adsorbed on red cells, but that, after the elapse of several hours, the virus particle, apparently intact, has come off the surface of the red cells. After this treatment, cells were unable to be agglutinated by influenza virus or to adsorb virus particles. The problem of the interaction of influenza virus with red cells was attacked in a comprehensive manner by Sir Macfarlane Burnet and his coworkers in Melbourne in the period of... 

Characterization techniques become surface sensitive if the particles or radiation to be detected come from the outer layers of the sample. Low energy electrons, ions and neutrals can only travel over distances between one and ten interatomic spacings in the solid state, implying that such particles coming off a catalyst reveal surface-specific information. The inherent disadvantage of the small mean free path is that measurements need to be carried out in vacuum, which conflicts with the wish to investigate catalysts under reaction conditions. 

The mechanisms of particle removal have been studied in the past few years. Reports show that the particles adhere to a surface primarily by van der Waals forces, electrostatic attraction, or capillary action.2 The cleaning is by hydrodynamic lubrication. The thickness of the hydrodynamic fluid layer, as estimated, was around 3.7 pm.1 On the contrary, numerical analysis concluded that the lift force in the hydrodynamic boundary layer of fluid was too small to lift particles off the surface.3 The possible removal force comes likely from the drag force between the brush and the wafer surface. Major... 

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