By use of a classical model, the phenomenon of trapping of a gas particle at the surface of a cold solid is investigated. The equation of motion is solved by the adoption of a semirepulsive potential of interaction and the equations determining the critical kinetic energy for trapping which is essential for the evaluation of the capture coefficient are obtained. These equations are simplified for the case that a gas particle collides on its own crystalline phase on account of the long “regression time” and the critical kinetic energy is thereby obtained. The regression time is the total time during which an initially free gas particle becomes trapped by the crystal surface before it is reevaporated again. It is long as compared to the period of lattice vibration.
The cryopumping of condensable nonpolar gases is investigated. The existence of an adsorbed, mobile phase is taken into account in the derivation. The capture coefficients obtained for N2 and CO2 based on the geometric area of the cryosurface are quite satisfactory as compared with those derived from experimental data. Pressures as functions of flow rates were measured for CO2 at a cryosurface temperature of 77.4 °K. The results are in good agreement with that calculated from the present simple theory.