Time-of-flight study of medium energy range collisions of metastable helium atoms with atomic or diatomic molecular targets

Abstract

Collisions of He(23S) on atomic (He, Ar) or molecular (D2, O2) targets are experimentally investigated at low keV energies (0.1-1.5 keV) by a time-of-flight technique. This method yields an overall picture of the various inelastic processes and their relative probabilities. Direct experimental evidence of the importance of the He(23P) and He(n'l') n'>or=3 excitations of the projectile with He and D2 targets is reported. With an O2 target differential cross sections for He+ production measured at various energies reveal a rainbow feature. With the Ar target, the predominance of Penning ionisation and excitation transfer processes is confirmed. The experimental results concerning the He*-He system have been compared with the predictions of available theoretical models: the important He(23P) production is qualitatively in agreement with calculations based on the c3 Sigma g+-b'3 Pi g rotational coupling. The covalent-ionic transition in the He2 molecule is proposed as a primary mechanism which can account for the Rydberg He(nl) population. For the other targets the possibility of connecting those Rydberg excitations with the existence of a transient ionic complex (He+-T-) is examined.