Near-threshold positron impact ionization of hydrogen

The hyperspherical hidden crossing method (HHCM) is used to investigate positron impact ionization of hydrogen near threshold. An important feature of this method is that it can provide valuable insight into scattering processes. In the calculation of positron–hydrogen ionization, the adiabatic Hamiltonian is expanded about the Wannier saddle point; anharmonic corrections are treated perturbatively. The S-wave results are consistent with the Wannier threshold law and with the extended threshold law that was previously derived using the HHCM. We have extended the previous HHCM calculation to higher angular momenta L and have calculated the absolute ionization cross-section for L = 0, 1 and 2. The HHCM calculation confirms that the S-wave ionization cross-section is small and provides the reason why it is small. The HHCM ionization cross-section (summed over the lowest partial waves) is compared with a convergent close-coupling calculation, a 33-state close-coupling calculation and experimental data.