Contribution of high-nlshells to electron-impact ionization processes

Abstract

The contribution to electron-impact ionization cross sections from excitations to high-nl shells and a consequent autoionization is investigated. We perform relativistic subconfiguration-average and detailed level-to-level calculations for this process. Ionization cross sections for the W27+ ion are presented to illustrate the large influence of the high shells (n ^ 9) and orbitals (/ > 4) in the excitation-autoionization process. The obtained results show that the excitations to the high shells (n ^ 9) increase cross sections of the indirect ionization process by a factor of 2 compared to the excitations to the lower shells (n < 8). The excitations to the shells with orbital quantum number l = 4 give the largest contribution compared with the other orbital quantum numbers /. Radiative damping reduces the cross sections of the indirect process approximately twofold in the case of the level-to-level calculations. Determined data show that the excitation-autoionization process contributes approximately 40 % to the total ionization cross sections.

Influence of excitations to high-nlshells for the ionization process in theW26+ion

“…The subconfigurations of 4d 9 4f 2 7d and 4d 9 4f 2 7f are below the ionization threshold. All presented configurations correspond to the same single-electron promotions as for the W 27+ ion [11], except for a few cases. Configurations produced by the 4p → 5f and 4d → 7d promotions are below the ionization threshold for the W 27+ ion.…”

“…Cross sections are studied in a distorted wave (DW) approximation which provides the reliable data for the higher ionization stages [8][9][10][11]. The local central potentials for the ground configurations are used to generate the necessary bound and continuum wave functions for each ion.…”

“…The electron-impact ionization of tungsten ions as intrinsic impurity in the fusion plasmas has been widely studied experimentally [2][3][4][5] and theoretically [6][7][8][9][10]. Previous investigation which employed Dirac-Fock-Slater (DFS) approximation for the W 27+ ion demonstrated very large contribution to the cross sections of indirect process of the ionization from excitations to the higher shells (n 9) [11]. It was shown that the excitations to the higher shells increase the cross sections for the indirect process by a factor of 2.…”

“…Higher principal and orbital quantum numbers were omitted in their calculations since good agreement with the experimental measurements was obtained at the low ionization stages [7,8]. For the W 27+ ion, good agreement was found between subconfiguration-average distorted-wave (SCADW) and level-to-level distorted-wave (LLDW) calculations when autoionization branching ratios were equal to 1 [11]. However, radiative damping played a crucial role for the ionization cross sections.…”

Fusion-Related Ionization and Recombination Data for Tungsten Ions in Low to Moderately High Charge States

“…The necessity for inclusion of high-nl states in DW calculations has also been emphasized recently by Jonauskas et al [58] for single ionization of W 27+ ions.…”