Phenomenological Approximations for Boundary Operators

Here we shall review the most populai phenomenological approximations of KBC, that, though having no strict physical basis, have certain importance for applications. Such approximations contain fitting parameters and can be used for parametri-sations of experimental results for realistic surfaces. 

The most simple form of the scattering operator for stmctureless particles is given by the Maxwell kernel (Niven 1972) (it is common to use velocity variables (v, v ) instead of momentum ones (p,p ) for nonreactive scattering kernels) 

In the case of incomplete accomodation the tempereture of diffusively desorbing particles T would differ firom that of the surface T,. Energy accomodation coefficient as for such conditions is assumed to be of the form 

The Maxwell scattering kernel contains two dimensionless fitting parameters aio and y = mr/ l2kBTz, that can depend on velocity t and surface features. Expression (14.3.1) meets both condition (14.2.1) and microscopic detsdled balance. In (Epstein 1967) the following approximation for the local adsoi-ption coefficient ai(v ) (that can be used instead of aio in (14.3.1)) has been proposed 

Another generalization of the model (14.3.1) is based on the introduction of the beam distribution indicatrix instead of specular part. The Maxwell or Gauss forms 

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