Autoionization of Helium Induced by Heavy Ion Impact

Abstract

The electron spectra of autoionizing levels in helium excited by photons, electrons and light ions has been the subject of extensive studies¹. Due to Wigner’s spin conservation rule no appreciable formation of triplet levels in doubly excited HeI** has been found by proton impact on He target gas. However, triplet states can be formed via electron exchange², using bombardment of structured ions such as He⁺. For sufficiently low-velocity ions autoionization may be influenced by the proximity of the projectile during the decay of the helium excited states. Decay of excited states of the quasi-molecule formed during slow collisions has been observed to produce shifts in the energy of excited states³ as well as Stark mixing of magnetic substates which decay in the electric field of the projectile. The latter effect influences the angular distributions of ejected electrons.⁴ Little is known regarding the excitation of helium autoionization states by heavy multiply charged ions. Excitation by fast 30 MeV 0⁵⁺ resulted in strong excitation of He(2s2p)¹D series with essentially no triplet state excitation⁵. The present measurements extend the range of velocity and charge states of the incident ions to provide greater detail on the relative importance of collision parameters on the excitation and de-excitation process.