FORMATION AND DESTRUCTION OF FAST METASTABLE HELIUM ATOMS IN CHARGE-CHANGING COLLISIONS

Collisions involving one-electron capture and loss during the passage of 60-450 kev helium beams through hydrogen have been investigated by a technique which allows the fast metastable atom population of the beam to be determined. One-electron loss cross sections σ0*1 and σ01 for metastable and ground-state helium atoms have been separately determined. σ0*1 is found to be between 13 and 3 times greater than σ01 in the present energy range. It is shown that at energies between 80 and 250 kev, where the formation of He2+ is relatively unlikely, a satisfactory description of the charge equilibration of fast helium beams can be obtained in terms of charge-changing collisions involving He+ ions and ground-state and metastable helium atoms. Cross sections σ10 and σ10* for one-electron capture into metastable and ground states of helium have been estimated in the range 75-250 kev where it is found that σ10* is between 30% and 50% of σ10.