The capture of slow antiprotons in noble gases

The capture of slow antiprotons (energies less than 1.0 au, i.e. roughly 27 eV) by the rare gas atoms helium, neon and argon is considered. Appropriate to this low velocity, the capture cross sections are calculated using the adiabatic `hidden-crossing' theory in which the collision complex is viewed as a transient diatomic molecule with the positively-charged atomic ion and antiproton as nuclei. In addition to the total capture cross section, estimates of the percentage population of long-lived `circular' states is given. Our calculations suggest that a few per cent of captured antiprotons occupy these states in helium but in the case of argon or neon the probability of primary capture into such long-lived states is negligible. These results are at variance with previous calculations of antiproton capture cross sections.