Near-Threshold S-wave Positron-Hydrogen Ionization

We have used the hyperspherical hidden crossing method to compute the S-wave positron-hydrogen ionization cross section near threshold. For total angular momentum zero, we expanded the adiabatic Hamiltonian about the saddle point associated with the Wannier ridge. Our results are consistent with previous values of the Wannier exponent [1] and second order correction terms to the threshold law [2,3]. By calculating the transition probability within the reaction zone we were able to determine the absolute S- wave positron-hydrogen ionization cross section near threshold. S.J. W. and J.S. acknowledge support from NSF, under a collaborative grant (PHYS- 0440714, PHYS-0440565). [1] H. Klar, J. Phys. B 14, 4165 (1981). [2] W. Ihra, J. H. Macek, F. Mota-Furado and P.F. O'Mahony, Phys. Rev. Lett. 78, 4027 (1997). [3] James Sternberg, S. J.Ward, J. H. Macek and J. Shertzer, Bull. Am. Phys. Soc. 49, 52 (2004).