One of the crucial factors in determining how long a surface can be maintained clean (or, alternatively, how long it takes to build-up a certain surface concentration of adsorbed species) is the number of gas molecules impacting on the surface from the gas phase.

The incident flux is the number of incident molecules per unit time per unit area of surface.

(Note - the flux takes no account of the angle of incidence, it is merely a summation of all the arriving molecules over all possible incident angles)

For a given set of conditions (P,T etc.) the flux is readily calculated using a combination of the ideas of statistical physics, the ideal gas equation and the Maxwell-Boltzmann gas velocity distribution.

Hertz-Knudsen formula for the incident flux:

F = P / (2 pi m k T)^1/2 [ molecules m-2 s-1 ]

Thus, the molecular flux is directly proportional to the pressure.

The sticking coefficient, S , is a measure of the fraction of incident molecules which adsorb upon the surface i.e. it is a probability and lies in the range 0 - 1 , where the limits correspond to no adsorption and complete adsorption of all incident molecules respectively. In general, S depends upon many variables i.e.

S = f ( surface coverage , temperature, crystal face .... )

The surface coverage of an adsorbed species is usually specified as the number of adsorbed species per unit area of surface (e.g. in molecules cm-2).

A monolayer (1 ML) of adsorbate is taken to correspond to the maximum attainable surface concentration of adsorbed species bound to the substrate.