Complex angular momentum analysis of low-energy electron elastic scattering from lanthanide atoms

Abstract

Electron attachment to the lanthanide and Hf atoms resulting in the formation of stable excited lanthanide and Hf anions as Regge resonances is explored in the near-threshold electron impact energy region, $E<1.0$ eV. The investigation uses the recent Regge-pole methodology wherein is embedded the electron-electron correlations together with a Thomas-Fermi--type model potential incorporating the crucial core-polarization interaction. The near-threshold electron elastic total cross sections (TCSs) for the lanthanide and Hf atoms are found to be characterized by extremely narrow resonances whose energy positions are identified with the binding energies (BEs) of the resultant anions formed during the collision as Regge resonances. The extracted BEs for excited lanthanide anions are contrasted with those of the most recently calculated electron affinities (ground state BEs). We conclude that the BEs for the ${\mathrm{Pr}}^{\ensuremath{-}}$, ${\mathrm{Tb}}^{\ensuremath{-}}$, ${\mathrm{Dy}}^{\ensuremath{-}}$, ${\mathrm{Ho}}^{\ensuremath{-}}$, ${\mathrm{Er}}^{\ensuremath{-}}$, and ${\mathrm{Tm}}^{\ensuremath{-}}$ anions of O'Malley and Beck [Phys. Rev. A 79, 012511 (2009)] are not identifiable with the electron affinities as claimed. Formation of bound excited anions is identified in the elastic TCSs of all the lanthanide atoms including Hf, except Eu and Gd. The imaginary part of the complex angular momentum $L$, $\mathrm{Im}L$ is used to distinguish between the shape resonances and the bound excited negative ions. These results challenge both experimentalists and theoreticians alike since the excited anions are very weakly bound, but mostly tenuously bound ($\mathrm{BEs}<0.1$ eV). Shape resonances and Ramsauer-Townsend minima are also presented.