Determination of Sticking Coefficients

Measuring the uptake of a gas by a surface as a function of the dose to which the surface is exposed is the most straightforward way to determine a sticking coefficient. In such experiments, great care should be taken to ensure that gas and surface are in thermal equilibrium. In addition, we need to determine the coverage, either by surface sensitive methods (XPS, AES, IR) or by thermal desorption and ensure that adsorption is not accompanied by desorption. 

For example, consider the dissociative adsorption of methane on a Ni(lOO) surface. If the experiment is performed above 350 K, methane dissociates into carbon atoms and hydrogen that desorbs instantaneously. Consequently, one determines the uptake by measuring (e.g. with Auger electron spectroscopy) how much carbon is deposited after exposure of the surface to a certain amount of methane. A plot of the resulting carbon coverage against the methane exposure represents the uptake curve. 

In general, if an adsorbing molecule A occupies a single adsorption site, the adsorption process follows first-order kinetics  

This differential equation is readily solved by separation of variables, leading to 

The gas dose or exposure is normally expressed in the units of time multiplied by the pressure. Of course, it is more meaningful to give the exposure D in terms of a number of molecules hitting a site during the experiment  

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Determination of the re-erosion yields of the various types of deposited layers and the determination of sticking coefficients for ions and radicals under various conditions... 

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