The ionization and excitation cross sections of atomic hydrogen with antiprotons are calculated using the close-coupling method, where the wave function is expanded by atomic states only on protons. The resulting total cross sections are in good convergence and in agreement with other calculations based on the closecoupling method with considerably large basis set. The angular distribution of ionized electrons is calculated by substituting the obtained wave function in the close-coupling method into the integral form of the transition amplitude. Above 100 keV, the angular distributions of ionized electrons are compared for proton-and antiproton impacts.
Photodetachment of the positronium negative ion, a bound state of one positron and two electrons, has been observed. Development of a method to produce the ions efficiently using a Na coated tungsten surface has enabled the first observation of the photodetachment. The obtained lower limit of the photodetachment cross section for the wavelength of 1064 nm is consistent with the theoretical calculations reported so far. The experimental field developed in the present work gives new opportunities to explore the quantum mechanical three-body problem and to develop energy-tunable positronium beams.
We have applied close-coupling calculations to calculate the single-ionization cross section of helium by antiproton impact at collision energies above 10 keV. The atomic states of helium are expanded by three sets of eigenstate͑s͒ of He ϩ : ͕1s͖, ͕1s,2s,2p͖, and ͕1s,2s,2p,3s,3p,3d͖. The calculations are in good convergence within these models. The present cross sections are in good agreement with the experimental data of Andersen et al. ͓Phys. Rev. A 40, 7366 ͑1989͒; 41, 6359 ͑1990͔͒ and Hvelplund et al. ͓J. Phys. B 27, 925 ͑1994͔͒ at energies above 40 keV. However, our treatment gives larger cross sections than the data of Hvelplund et al. for lower energies and the disagreement is no less than a factor of 2 near 10 keV.
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