Pumping at solid surfaces

If experimental conditions in an evacuated apparatus are arranged so that the average time of sojourn of molecules or ions on solid surfaces which they strike is long compared with the flight time through the space, then pumping at the solid surfaces may be said to have occurred. Since flight times are not widely variable, the primary factors of interest for increasing pumping at solid surfaces are those which influence the time of sojourn. Sojourn times can be controlled over a wide range of magnitudes by appropriate design, and this paper will review physical variables determining the sojourn time. Dominant among these are the temperature of the solid surface, the chemical nature of the bombarding and bombarded species, the quantity of the bombarding species already pumped, the kinetic energy of the bombarding species and the physical nature of the bombarded surface. In general these factors are contained in the wider fields of study of physical adsorption, chemical adsorption and positive ion impact on surfaces. The paper is a selection from these fields of those results bearing directly on the practical problem of pumping at solid surfaces.