Ionization and autoionization of rubidium atoms by electron impact


Background. The autoionization makes a significant contribution to the total ionization cross section of atoms. In recent years, the autoionization cross sections have been experimentally investigated for the subvalent shells of alkali metal atoms. These studies have shown that for heavy rubidium and cesium atoms the autoionization contribution is more than 30% of the total ionization cross section. The availability of such experimental data is the basis for developing new or improving existing theoretical models.
Methods. Calculations of the excitation cross sections were carried out in the relativistic distorted wave approximation with the inclusion of the correlation corrections. Ionization cross sections were calculated in the relativistic binary approximation. All calculations were carried out as part of the universal software package Flexible Atomic Code. Radial orbitals for constructing basic relativistic wave functions were obtained by solving the Dirac-Fock-Slater equation. The calculations were carried out in jj coupling. The theoretical excitation cross section for the 4p6 shell was obtained as the sum of the cross sections for 55 autoionizing states from 4p55s2, 4p55s6s, 4p54d5s, 4p55s5d configurations. Similarly, the excitation cross section of the 4s2 shell is the sum of calculated excitation cross sections for 37 autoionizing states from 4s4p65s2, 4s4p65s5p, 4s4p65s6s, 4s4p65s4d, 4s4p66s6p, 4s4p65s7s, 4s4p65s5d configurations.
Results. The calculated results were compared with the experimental autoionization cross section and the cross section obtained in Born approximation. It was shown that the (4p6 + 4s2) cross section satisfactorily describes the experimental cross section. The ionization cross sections for 5s, 4p6, 4s2, and 3d10 shells in binary approximation were obtained. Comparison of obtained data showed that the excitation of the 4p6 and 4s2 shells contributes up to 30% and 4%, respectively, to the total single ionization cross section. The contributions from direct ionization of the 4p6, 4s2 and 3d10 shells are up to 24%, 1.8% and 2.4%, respectively.
Conclusions. The paper presents a theoretical study of the process of autoionization in excitation of the 4p6 and 4s2 subvalent shells of rubidium atoms. Calculations of the cross sections in the relativistic distorted-wave approximation with configuration interaction corrections showed satisfactory agreement with the experimental data. This confirms the need to include correlation and relativistic corrections when calculating the excitation cross sections for a heavy rubidium atom. A comparative analysis of the total theoretical ionization and excitation cross sections with an experimental total single ionization cross section showed that binary approximation is an efficient method for calculating the direct ionization cross sections of atoms

Keywords: atom, autoionization, ionization, excitation, rubidium