Knudsen Layer Problem with Adsorbate

Gaseous media neeir a surface is perturbed by the scattering and adsorption-desorption processes, as well as the surface chemical reactions (see Figs. 1,2). This perturbation may be taken into account via a kinetic boundary condition (KBC) at z = Zt h (in the scale of the gas phase kinetic parameters the value z, can be set to zero) for the distribution function Qc- The kinetic boundsuy problem (Knudsen layer problem) for the distribution function 9c(b,r,t) is formulated in the following way 

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 

Adsorbed pai ticle states aie chaiacterized by negative energies Ek- Then non-reactive eidsorption-desorption processes may be treated as the transitions between the energetic states of translational motion 

To simplify the calculations they used to suppose that all the processes occur in one plane. 

Taking into ficcount the dependence of the right hand side of Eq. (6.1.11) on suid that of Rc c, Dc c on fc, allows one to conclude that the problems of adsorbed and Knudsen layers fire closely connected. This connection may be important for the problems of the film growth and for the problems of gas-surface interaction as well (e.g. for the calculation of the slip and exchange coefficients, heat and mass exchange at the boimdary, and so on). 

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