Abstract
Between an ion and a hydrogen (H) atom in an excited state, there exists an effectively isotropic interaction potential which behaves as 1/R2 at large separations R. The abnormal long-range part, stemming from the energy degeneracy peculiar to the H system, is expected to have a drastic influence on the low-energy collision processes. The present paper addresses this issue by considering protonium (pp) formation in antiproton (p) collisions with excited H atoms. The calculation of the pp formation process is carried out by means of a semiclassical (quantum-classical hybrid) approach. At low collision energies E, the formation cross section is found to be far above the so-called Langevin cross section and reveals the E-1 dependence. This result is in clear contrast to that for the H atoms in the ground state with no degeneracy.
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