Sticking probability
Encyclopedia
The sticking probability is the probability that molecules are trapped on surfaces and adsorb chemically. From Langmuir's adsorption isotherm
, molecules cannot adsorb on surfaces when the adsorption sites
are already occupied by other molecules, so the sticking probability can be expressed as follows:
where s0 is the initial sticking probability and c is the coverage.
Similarly, when molecules adsorb on surfaces dissociatively, the sticking probability is
Although these equations are simple and can be easily understood, they cannot explain experimental results. Their simple explanation is not enough.
In 1958, P. Kisliuk presented an equation that can explain experimental results. In his theory, molecules are trapped in precursor states (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
and if these sites are occupied, they
Then the sticking probability is
)
When K=1, this equation equals Langmuir's adsorption isotherm
.
Langmuir equation
The Langmuir equation relates the coverage or adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature. The equation was developed by Irving Langmuir in 1916...
, molecules cannot adsorb on surfaces when the adsorption sites
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
are already occupied by other molecules, so the sticking probability can be expressed as follows:
- s=s0(1-c)
where s0 is the initial sticking probability and c is the coverage.
Similarly, when molecules adsorb on surfaces dissociatively, the sticking probability is
- s=s0(1-c)2
Although these equations are simple and can be easily understood, they cannot explain experimental results. Their simple explanation is not enough.
In 1958, P. Kisliuk presented an equation that can explain experimental results. In his theory, molecules are trapped in precursor states (physisorption
Physisorption
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption...
) before chemisorption
Chemisorption
Chemisorption is a sub-class of adsorption, driven by a chemical reaction occurring at the exposed surface. A new chemical species is generated at the adsorbant surface...
. 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
- desorb from the surface (pd: probability)
- move to the next precursor state (pm: probability)
- adsorb on the surface chemically (pa: probability)
and if these sites are occupied, they
- desorb from the surface (pd': probability)
- move to the next precursor state (pm': probability)
Then the sticking probability is
- s=s0(1+cK/(pa+pd)-1
)
When K=1, this equation equals Langmuir's adsorption isotherm
Langmuir equation
The Langmuir equation relates the coverage or adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature. The equation was developed by Irving Langmuir in 1916...
.