Calculations of low-energy collisions of antiprotons $\overline{p}$ with helium atoms in the ground $(1{}^1\mathit{S})$ and metastable $(2{}^3\mathit{S}$ and $2{}^1\mathit{S})$ 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 $\left(\to \overline{p}{\mathrm{He}}^{+}+e\right)$ and breakup ionization $\left(\to \overline{p}+{\mathrm{He}}^{+}+e\right)$ 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 $\overline{p}{\mathrm{He}}^{+}$ 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.

• Received 17 February 2015

DOI:https://doi.org/10.1103/PhysRevA.91.042502