Theoretical study of low-energy electron and positron scattering on Be, Mg and Ca

Static exchange and static exchange plus polarisation calculations are carried out for elastic electron and positron scattering with Be, Mg and Ca. All three atoms are found to have a low-lying 2 P resonance in the electron scattering case. Magnesium and calcium are also predicted to have higher lying 2 D shape resonances near 2.0 and 0.7 eV, respectively. The 2 D resonance in Mg is very weak and broad ( Gamma >3.0 eV) while that in Ca is much more pronounced and is much narrower ( Gamma approximately 0.6 eV). These findings are in agreement with recent experiments. In addition a very broad feature appears in e - -Be d-wave scattering near 5 eV. While maxima occur in the p-wave cross sections for positron scattering, these do not appear to be due to resonances. However, evidence is provided for exceptionally broad resonances in d-wave positron scattering and for a virtual (or antibound) state in s-wave scattering.