Stable Bound States of Yb and Pr Negative Ions

Andersen {\it et al} [1] concluded, through careful experimental investigation, that the electron affinity (EA) of Yb should be less than 3 meV and the accuracy of the theoretical calculations was deemed insufficient to provide a definitive answer to whether a stable bound state of the negative Yb ion exists. Such a small EA value for Yb is suitable for quenching Rydberg states, going through the formation of a temporary negative ion by ground state atoms with low EA's [2]. Our result obtained using the recent Regge- pole methodology [3] in which a Thomas-Fermi potential incorporates the important core- polarization potential, contradicts the conclusion in [1] by predicting a binding energy of 28~meV for the Yb$^-$ ion with a d-orbital electron attachment, including a Ramsauer-Townsend minimum at 20 meV and an s-wave Wigner threshold behavior of the total elastic cross section. Results for the e$^-$-Pr scattering will also be presented and contrasted with those for e$^-$-Yb scattering. \\$[1]$ H. H. Andersen, T. Andersen and U. V. Petersen, J. Phys. B {\bf 31}, 2239 (1998)\\$[2]$ I. I. Fabrikant and V. S. Lebedev, J. Phys. B {\bf 33}, 1521 (2000)\\$[3]$ D.Sokolovski {\it et al}, Phys. Rev. A {\bf 76}, 012705 (2007)