Formation of antiprotonic helium (p)over-barHe+ and ionization in low-energy collisions of (p)over-bar with He in the ground 1 1S and metastable 2 3S and 2 1S states

Calculations of low-energy collisions of antiprotons (p) over bar with helium atoms in the ground (1 S-1) and metastable (2 S-3 and 2 S-1) states are carried out by using a semiclassical method and by further assuming that one of the two electrons is passive. Antiproton capture to form antiprotonic helium (-> (p) over bar He+ + e) and breakup ionization (-> (p) over bar + He+ + e) are investigated. The energy dependence of the capture and ionization cross sections and the distributions of the product states are presented and compared with previous theoretical calculations. For the metastable helium targets, most of the (p) over bar He+ atoms produced in the capture have long lifetimes against subsequent Auger decays and can be observed experimentally. For the ground-state targets, however, the formation of such long-lived states is rather a rare event at low collision energies.