Sticking probability
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The sticking probability is the probability that
molecules A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
are trapped on surfaces and adsorb chemically. From Langmuir's adsorption isotherm,
molecules A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
cannot adsorb on surfaces when the adsorption sites are already occupied by other molecules, so the sticking probability can be expressed as follows: S=S_0(1-\theta) where S_0 is the initial sticking probability and \theta is the surface coverage fraction ranging from 0 to 1. Similarly, when molecules adsorb on surfaces dissociatively, the sticking probability is S=S_0(1-\theta)^2 The square is owing to the fact that a disassociation of 1 molecule into 2 parts requires 2 adsorption sites. These equations are simple and can be easily understood but cannot explain experimental results. In 1958, P. Kisliuk presented an equation for the sticking probability that can explain experimental results. In his theory, molecules are trapped in precursor states of physisorption before chemisorption. Then the molecules meet adsorption sites that molecules can adsorb to chemically, so the molecules behave as follows. If these sites are not occupied, molecules do the following (with probability in parentheses): # adsorb on the surface chemically (P_a) # desorb from the surface (P_b) # move to the next precursor state (P_c) and if these sites are occupied, they # desorb from the surface (P_b') # move to the next precursor state (P_c') Note that an occupied site is defined as one where there is a chemically bonded adsorbate so by definition it would be P_a'=0. Then the sticking probability is, according to equation (6) of the reference, S=S_0\left(1+\frac K \right)^ = S_0 \frac K = \frac + P_b When K=1, this equation is identical in result to Langmuir's adsorption isotherm.


Notes


References

* ''The constitution and fundamental properties of solids and liquids. part i. solids.'' Irving Langmuir;
J. Am. Chem. Soc. The ''Journal of the American Chemical Society'' is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the ''Journal of Analytic ...
38, 2221-95 1916 {{DEFAULTSORT:Sticking Probability Physical chemistry Materials science