Electron-impact double ionization of tungsten atoms and ions at low ionization stages

Abstract

The double ionization of neutral tungsten atoms and W 2+ , W 4+ , and W 6+ ions by electron impact is investigated. The calculations are performed taking into account the Auger decay of single ionization states with inner vacancies in the 4d, 4f, 5s, 5p, and 5d shells as well as the direct double ionization using sudden perturbation model. The direct single ionization crosssection is obtained in relativistic distorted wave or binary-encounter-dipole approximations; the contribution from the indirect ionization by excitation-autoionization process is included too. It is shown that both processes-Auger decay of inner vacancy states and direct double ionization-play an important role in the double ionization process. The calculated cross-sections are compared with the experimental data.

Contribution of high-nlshells to electron-impact ionization processes

“…These data were supplemented by the electron-impact single-, double-, and triple-ionization measurements of W9+ ions in the charge states <7 = 1 -10, q = 1-6, and q = 1-4, respectively [6,7], The configuration-average distortedwave (C ADW) calculations provided good agreement with ex perimental measurements for higher ionization stages (q > 4) [8], The distorted-wave (DW) approach succeeded in getting a very good agreement with experiment for the ionization stages up to the W9+ ion and therefore it was applied to all isonuclear sequences [9]. Further level-to-level studies [10] included the W atom and W2+ ion in the binary-encounter-dipole model as well as W4+ and W6+ ions in the DW approach. For the tung sten isonuclear sequence, cross sections were calculated using semirelativistic and relativistic DW methods for configurations and subconfigurations, respectively [9].…”

“…The DW approach succeeded in getting a very good agreement with experiment for the ionization stages up to W 9+ ion and therefore it was applied to all isonuclear sequence [9]. Further level-to-level studies [10] included W atom and W 2+ ion in the binaryencounter-dipole model as well as W 4+ and W 6+ ions in the DW approach. For the tungsten isonuclear sequence, cross sections were calculated using semirelativistic and relativistic DW methods for configurations and subconfigurations, respectively [9].…”

Influence of excitations to high-nlshells for the ionization process in theW26+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.…”

“…Such processes have to be well understood for predicting possible plasma scenarios. 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].…”

Electron-impact single ionizaiton for W 4+ and W 5+

“…[13] In theoretical aspects, the calculations for the EII cross section of tungsten ions are challenging due to its pronounced manyelectron, relativistic effects, and the presence of open d or f shells. Therefore, some semi-empirical methods, such as the widely used Lotz expression, [10] binary-encounter Bethe (BEB) model, [14,15] and configuration-averaged distortedwave (CADW) method, [16][17][18][19] were often used for prediction. For example, Kwon et al reported calculated EII cross sections of the neutral W atom and W + ion using the BEB model.…”