Evaluation of antiproton-impact ionization of He atoms below 40 keV

We have performed extensive close-coupling calculations to obtain total impact ionization cross sections of He atoms by antiprotons at energies from 1 keV to 300 keV by expanding the time-dependent two-electron wave functions in terms of helium eigenstates. The stability of the calculated ionization probabilities and total ionization cross sections with respect to the choice of basis functions has been examined. Our total ionization cross sections are only slightly higher than the results from the forced-impulse approximation of Reading et al. [J. Phys. B 30, L189 (1997)] but they disagree with the existing experimental data below 40 keV [Hvelplund et al., J. Phys. B 27, 925 (1994)]. We conclude that new measurements for the ionization of He by antiprotons are needed in the low-energy region.