The electrical cleanup of helium on molybdenum surfaces has been investigated. In this case, the phenomenon can be described by a trapping of ions upon impact with the surface. Measurements of the trapping efficiency &eegr; (atoms removed/ions collected) have been made over a range of ion energies from 150 to 2600 ev. The re‐emission of trapped helium atoms was investigated in detail. The contribution of re‐emission to the pumping process is described by a phenomenological theory. The results indicate that the trapping efficiency is lowered by a short time re‐emission of trapped atoms. Through the long‐time re‐emission of atoms, the theory accounts quantitatively for the observed saturation of the cleanup process.