Absolute Total and Partial Cross Sections for Ionization of Free Lanthanide Atoms by Electron Impact

Abstract

Absolute values of the total-apparent, total-counting, and charge separated partial cross sections have been measured for electron impact ionization of free atoms of Ce, Nd, Sm, Gd, Dy, Er and Yb. The electron energies have ranged from the single ionization threshold to 900 eV. Absolute total cross sections were determined with a crossed electron-atom beam assembly combined with time-of-flight velocity and oscillating crystal sensor accumulation measurements of target atoms. The relative partial cross sections were obtained separately with a magnetic deflection mass selector, and they were normalized to the total cross sections. The absolute cross sections have been determined with accuracy 23-33% depending on the target atom. Some of the single ionization cross section curves show complicated resonance-like structure in the region of 4f ionization thresholds. The 4d ionization exhibits a peak of shape resonance feature in the Ce³⁺ and Nd³⁺ curves, but the peak becomes less prominent with the increase in the atomic number Z and disappears beyond the Eu atom; this may be attributed to the lowering of the centrifugal barrier potential for f electrons in higher Z elements. Evidences of shake-up and/or shake-off processes associated with 5p or 5s electrons are observed in the quadruple ionization.