Penning and associative ionisations of argon atoms by collisions with metastable helium atoms He(2¹S) and He(2³S) of thermal energies 0.03-0.40eV

This work, devoted to the 2¹S and 2³S metastable states of the helium atom, is mainly experimental. It centres on observation of the non-bound states Ar⁺ and bound states HeAr⁺ created during collisions of these metastable atoms with argon atoms in the ground state. The Penning and associative ionisation cross-sections corresponding to these two processes were obtained as a function of the relative speed of approach of the He*+Ar particles in the thermal region 1200-4500m/s (0.3-0.40eV). The experimental time of flight method used in this experiment is original in its application to the measurement of reaction products such as Ar⁺ and HeAr⁺ ions. The results obtained testify to the efficiency of this method since the variations in the Penning ionisation cross-section sigmasub(IP)(v) and associative ionisation cross-section sigmasub(IA)(v) against speed have not been observed in other laboratories. They are given here for the couples He(2³S)+Ar and He(2¹S) + Ar. The theoretical interpretation attempted for the cross-sections sigmasub(IP)(v) and sigmasub(IA)(v) is based on a very recent model due to the American school of Miller and the Japanese school of Nakamura. This model, common to all interpretations, can lead to different results according to whether quantum effects are taken into account.