[en] Electron impact ionization and recombination define charge state distribution in collisional plasma. Line intensities in the spectra from plasma strongly depend on the population of ions. The aim of this work is to study electron impact ionization from the ground configuration levels of N⁺ and N²⁺ ions. The study includes both direct and indirect ionization processes. The indirect process is investigated as excitation to autoionizing states, followed by Auger transitions. The Flexible Atomic Code (FAC) is used to calculate energy levels, radiative and Auger transition probabilities, electron-impact ionization, and excitation cross sections. The ionization and excitation cross sections are investigated in the distorted wave (DW) approximation. The DW cross sections are estimated in the potential of the ionizing ion.

[en] Energy levels and emission spectra of W²⁵⁺ ion have been studied by performing the large-scale relativistic configuration interaction calculations. Configuration interaction strength is used to determine the configurations exhibiting the largest influence on the 4f³, 4d⁹4f⁴, 4f²5s, 4f²5p, 4f²5d, 4f²5f, 4f²5g, and 4f²6g configuration energies. It is shown that correlation effects are crucial for 4f²5s→4f³ transition which in a single-configuration approach occurs due to the weak electric octupole transitions. As well, the correlation effects affect 4f²5d→4f³ transitions by increasing transition probabilities by an order. The corona model has been used to estimate the contribution of various transitions to the emission in a low-density electron beam ion trap (EBIT) plasma. Modeling in 10–30 nm wavelength range produces lines which do not form emission bands and can be observed in EBIT plasma. - Highlights: • Energy levels and radiative transition probabilities are studied for the W²⁵⁺ ion. • Correlation effects are investigated for main configurations of the ion. • The corona model is used to find lines in spectra

[en] Electron-impact ionization cross sections for the ground level of the W²⁵⁺ ion have been investigated by performing level-to-level calculations and using the Dirac–Fock–Slater method in the single-configuration approach. The main attention has been focused on the influence of the increasing principal and orbital quantum numbers on the excitation-autoionization (EA) process and its contribution to the total ionization cross sections. The obtained results demonstrate that excitations to the high-nl shells ($n⩾<!--\; ???\; -->9$) increase cross sections of the indirect ionization process by about 60% compared to the excitations to the lower shells ($n⩽<!--\; ???\; -->8$). It was established that excitations to the shells with the orbital quantum number l = 4 give the greatest contribution to EA. Maxwellian rate coefficients derived from the cross sections for the ground state are compared with the previously obtained values from the configuration-average distorted-wave (CADW) approximation. The rate coefficients for direct ionization (DI) are smaller than the corresponding CADW values, while the EA rate coefficients are larger than the ones from the CADW calculations. The total DI+EA rate coefficients are about 20% larger than the CADW rate coefficients. (paper)

[en] Electron-impact single ionization is investigated for the levels of the ground $3d^5$ and long-lived $3d^{4}4s$ configurations of the Fe³ ion. In addition, the Maxwellian rate coefficients are tabulated in the study. The difference among the cross sections reaches $\sim$11% and $\sim$17% for the levels of the ground and long-lived configurations, respectively. The excitation autoionization (EA) provides the larger variation of the cross sections compared to the direct ionization (DI). It is demonstrated that the EA cross sections for the levels of the ground configuration vary more strongly compared to the values calculated for the excited configuration. On the other hand, the DI cross sections evaluated for the levels of the ground configuration agree within $\sim$4% while the difference of $\sim$9% is obtained for the levels of the long-lived configuration.

[en] The radiative and Auger cascades following the creation of the 2s shell vacancy in the Fe²⁺ ion are investigated by performing level-to-level calculations. The branching ratios of the cascades are analyzed and the main decay mechanisms are identified for all levels of the Fe³⁺ 2s 3d⁶ configuration. The study shows that the Fe⁵⁺ ion is mainly populated in the cascade process. It is demonstrated that the ion yield strongly depends on the level of the initial configuration. The ion yield is presented for all levels of the Fe³⁺ 2s 3d⁶ configuration. A study of the time dependence of the population for configurations obtained in cascades shows that it takes ∼10⁻¹⁵ s to reach configurations of the Fe⁶⁺ ion. We demonstrate that previous calculations for the decay of the 2s shell vacancy produced in Fe²⁺ provide an overestimated ion yield for higher ionization stages. (paper)

[en] Electron-impact double and triple ionization (TI) is investigated for the Se³⁺ ion by performing level-to-level calculations. It is demonstrated that ionization-autoionization, excitation double-autoionization, direct double ionization (DDI), and resonant-excitation triple-autoionization contribute to the total double ionization process. The ionization–ionization (II) branch plays the main role in the DDI process. The excitation-II part of DDI with subsequent autoionization dominates in the formation of the Se⁶⁺ ions. The contribution to the TI cross sections from the II process is not observed. The correlation effects considered for the 3p shell ionization open the way for the TI. (paper)

[en] Energy levels, radiative transition wavelengths and probabilities have been studied for the ${\mathrm{W}}^{26+}$ ion using multiconfiguration Dirac–Fock and Dirac–Fock–Slater methods. Corona and collisional-radiative models have been applied to determine lines and corresponding configurations in a low-density electron beam ion trap (EBIT) plasma. Correlation effects for the 4f², $4{\mathrm{d}}^{9}4{\mathrm{f}}^3$, $4\mathrm{f}5l$ ($l=0,\dots <!--\; ???\; -->,4$), $4\mathrm{f}n\mathrm{g}$ ($n=5,6,7$) configurations have been estimated by presenting configuration interaction strengths. It was determined that correlation effects are important for the $4\mathrm{f}5\mathrm{s}\to <!--\; ???\; -->4{\mathrm{f}}^2$ transitions corresponding to weak electric octupole transitions in a single-configuration approach. Correlation effects influence the $4\mathrm{f}5\mathrm{d}\to <!--\; ???\; -->4{\mathrm{f}}^2$ transitions by increasing transition probabilities by an order of magnitude. Identification of some lines observed in fusion plasma has been proposed. Spectra modeling shows strong increase of lines originating from the $4\mathrm{f}5\mathrm{s}\to <!--\; ???\; -->4{\mathrm{f}}^2$ transitions. Other transitions from the 10–30 nm region can be of interest for the EBIT plasma. (paper)

[en] Excitation-autoionization (EA) and direct ionization (DI) Maxwellian rate coefficients (MRC) are presented for the levels of the ground configuration of the W ²⁶⁺ ion. The study of EA and DI cross sections is performed by applying the Dirac–Fock–Slater method within the level-to-level distorted-wave approximation including radiative damping. Excitations to the high-nl shells (n ≤ 25) are taken into account for the EA process. It is shown that electron-impact ionization from the different levels of the ground configuration produces a significant variation of MRC. Excitations to configurations with energy levels that straddle the ionization threshold play the main role in enhancing MRC for the highest level of the ground configuration. The difference of 40% among the EA MRC is determined for the levels of the ground configuration. (paper)