MOLECULAR SURFACE PUMPING: CRYOPUMPING

Weak van der Waals attractive forces may provide molecular gas pumping on surfaces cooled at sufficiently low temperatures. Cryopumps make use of a cold surface to adsorb gas molecules. They are usually classed into two families, depending whether they rely on the binding forces between gas molecules and solid surfaces (cryosorption pumps) or on the even weaker forces which bind condensed gas molecules together (condensation pumps). The design features and the possible applications of both types of cryopumps are presented. The peculiar behaviour of condensed hydrogen, i.e. its desorption induced by infrared thermal radiation, is described and discussed. 1. GENERALITIES 1.1 Gas density and pressure at T ≠ 300 K A gas pressure is defined only if gas molecules present an isotropic distribution of directions and a maxwellian distribution of speeds. Under UHV conditions, pressure gauges do not measure pressure but gas density. If two communicating vessels V1 and V2 are kept at different temperatures (T1 and T2) the gas densities (d1 and d2) and pressures (p1 and p2) are given by 2211 p /TT p = 2121 d /TT d = In other words, in the colder vessel the gas density is higher but the pressure is lower. /TT 21 is the thermomolecular transpiration factor. 1.2 Gas condensation When a surface is "cold enough", the sojourn time of the molecules adsorbed on the surface, as defined by the Frenkel equatio