Systems with variable desorption energies

In many desorption experiments, the desorption activation energy depends on coverage and examples of the kinds of variation observed are shown in Fig. 32. Desorption of the type shown in curves D and E can also be produced by the presence of two states on the surface. There are several sources of desorption energy variation but the two main causes are (i) lateral interactions and (ii) surface inhomogeneity. The latter has been treated for the case of small uniform patches [270] if they are of equal surface area cL4, then 

The slope between 8X and 02 (in an order plot of In Rd versus In 0) can be written as 

McCabe and Schmidt [275] have used a revised form of the Redhead equation, eqn. (116), including coverage variations of Ed, viz. 

If the coverage variations are significant, cd/RT 1 but cd usually is small compared with Ed, then, at least approximately 

Like the Edwards line shape analysis, the mathematics involved is fairly complex and for brevity is not repeated in detail here. They began by making the general desorption equation dimensionless, viz. 

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When calculating adsorption-desorption probabilities one has to use models, that allow one to couple the energetic variables in the gas phase with those in the adsorbed layer. Let us divide the translational motion of an adsorbed molecule into normal and tangential components. We shall denote the energies of the molecular motion along x, y, z eixes as Ey, E respectively. The momentum of the projectile molecule in the same coordinate system can be presented as... 

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