Electron capture and ionisation in collisions of slow H⁺ and He²⁺ ions with helium

A crossed-beam technique previously developed and used in this laboratory for studies of ionisation and electron capture in H⁺-He and He²⁺-He collisions down to 64 keV amu^-1 and 50 keV amu^-1 has been adapted to extend measurements down to 9 keV amu^-1 and 6 keV amu^-1 respectively. The method, which incorporates time-of-flight spectroscopy together with electron-ion and ion-ion coincidence counting of the collision products, provides high precision cross sections for individual collision channels. Some serious discrepancies are obtained with previous cross sections for single ionisation measured by Afrosimov et al. (1969, 75) and the important role of transfer ionisation at low energies is confirmed. The need for better theoretical descriptions of low-energy ionisation is demonstrated.