Regular Article

Dipole enhancement of the protonium formation cross section in antiproton collisions with excited hydrogen atoms^★

Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai 3-1-1, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan

^a e-mail: sakimoto@isas.jaxa.jp

Received: 12 June 2017
Received in final form: 23 October 2017
Published online: 18 January 2017

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 (p̄p) formation in antiproton (p̄) collisions with excited H atoms. The calculation of the p̄p 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.