The Surface Thermal Accommodation Coefficient

Atoms equilibrate efficiently with the temperature of a surface when their kinetic energy is small compared to the heat of adsorption. The thermal accommodation decreases, however, with increasing energy of the atoms. The minimum occurs when the kinetic energy is on the order of the depth of the potential well for adsorption. For higher kinetic energies thermal accommodation increases again to the value for collision with a free atom of the surface material. 

The sticking coefficient, i.e, the probability that a molecule impinging on a surface becomes actually adsorbed, is often smaller than unity. This implies that many molecules rebound from the surface back into the gas phase. The question arises to what extent such molecules exchange energy with the surface. 

The thermal accommodation coefficient reflects the rate at which molecules colliding with a surface lose their energy. It is defined as 

Classical theory on accommodation coefficients predicts that in the limit of very high temperatures 

Accommodation coefficients on bare surfaces appear to depend only on the temperature Tg of the incident molecules, but are insensitive to surface temperature. This insensitivity to surface temperature forms the basis of the following analysis, where we show that the interaction is dominated by the repulsive part of the potential. 

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