Resonant Excitation Double Autoionization Research Articles

Theoretical investigations on electron-impact single-ionization cross section of Sn 11+

The total cross-section of electron-impact single ionization for the ground configuration 3d 104s 24p 64d 3 and excited configuration 3d 104s 24p 64d 24f of Sn 11+ is determined from the ionization threshold to 1000 eV. The contributions of direct ionization, excitation auto-ionization, and resonant excitation double auto-ionization to the total electron-impact single ionization cross-section are systematically demonstrated. The cross-section of direct ionization and excited auto-ionization are determined using the level-to-level method, while the cross-section of partially resonant excited double auto-ionization are determined through the configuration averaged method. To obtain convergence, excitation channels with the maximum principal quantum number up to n = 25 are considered. A comparison of the present results with the experimental data [Borovik et al. J. Phys. B 46, 175 201 (2013)] reveal considerably improved agreement when including the resonant excitation double auto-ionization in the calculation.

Prominent role of indirect processes in electron-impact ionization of Xe24+ ions

SynopsisThe cross section for electron-impact single ionization of Xe24+ has been studied both experimentally and theoretically. The cross section exhibits significant features from resonant ionization processes such as resonant-excitation double autoionization. These are well described by the theoretical calculations if the latter take into account excitations to atomic shells with principal quantum numbers of up to n = 39.

Electron-impact excitation and single- and multiple-ionization cross sections of heavy ions: Sn13+as an example

The cross sections of electron impact with Sn${}^{13+}$ resulting in the production of Sn${}^{13+}$-Sn${}^{16+}$ are investigated theoretically by using the fine-structure-level distorted-wave approximation from the threshold to 4000 eV. The electron-impact excitation, ionization, and resonant excitation processes are included in the cross sections. Contributions from different processes are determined by independent processes and isolated resonance approximations. The decay pathways and branching ratios for single and sequentially multiple autoionization for the final autoionized states are determined by detailed calculations. The cross section forming Sn${}^{13+}$ is the largest and next are those for Sn${}^{14+}$ and Sn${}^{15+}$. The double-ionization cross section forming Sn${}^{15+}$ is smaller than that forming Sn${}^{14+}$, yet the relative magnitude is larger at higher incident electron energy. A comparison with a recent measurement for the single-ionization cross section [A. Borovik, Jr. et al., J. Phys. B 46, 175201 (2013)] showed that accurate determination of the branching ratios for single and double autoionization and including contributions of the resonant excitation double autoionization are crucial to obtain accurate results.

ELECTRON-IMPACT IONIZATION OF P-LIKE IONS FORMING Si-LIKE IONS

We have calculated electron-impact ionization (EII) for P-like systems from P to Zn15 + forming Si-like ions. The work was performed using the flexible atomic code (FAC) which is based on a distorted-wave approximation. All 3l → nl' (n = 3-35) excitation-autoionization (EA) channels near the 3p direct ionization threshold and 2l → nl' (n = 3-10) EA channels at the higher energies are included. Close attention has been paid to the detailed branching ratios. Our calculated total EII cross sections are compared both with previous FAC calculations, which omitted many of these EA channels, and with the available experimental results. Moreover, for Fe11 +, we find that part of the remaining discrepancies between our calculations and recent measurements can be accounted for by the inclusion of the resonant excitation double autoionization process. Lastly, at the temperatures where each ion is predicted to peak in abundances in collisional ionization equilibrium, the Maxwellian rate coefficients derived from our calculations differ by 50%-7% from the previous FAC rate coefficients, with the difference decreasing with increasing charge.

Relativistic effects on resonant interactions between electrons and highly charged ions

We report measurements of resonant processes in electron collisions with very highly charged heavy ions made using an electron beam ion trap. By measuring the ion abundance ratio in the trap at the equilibrium condition as a function of electron energy, we have observed resonant processes such as dielectronic recombination and resonant excitation double autoionization very clearly. Remarkable relativistic effects due to the generalized Breit interaction have been clearly shown in dielectronic recombination for highly charged heavy ions.

Activities at the Tokyo-EBIT 2005

A report is given on recent activities at the Tokyo-EBIT (electron beam ion trap), which include mainly two subjects; (1) interaction of highly charged ions with surface, and (2) electron - highly charged ion collisions. For the former subject, highly charged ions extracted from the EBIT are used to impact samples, such as H-terminated Si. Incident charge dependence of sputtering ion yield is measured by means of time-of-flight secondary ion mass spectrometry (TOF-SIMS). For the latter subject, we have constructed the second beam line for extracted highly charged ions, where ions with several charge states are monitored at the same time with a position sensitive detector. From the electron beam energy dependence of the charge state distribution at the equilibrium, resonant processes, such as dielectronic recombination and resonant excitation double autoionization, are studied.

Investigation of resonant inner-shell processes with an electron beam ion trap

The collision processes of highly charged ions with electrons have been studied with an electron beam ion trap. Resonant inner-shell processes such as dielectronic recombination and resonant excitation double autoionization were investigated by observing the number ratio of extracted ions with adjacent charge states.

Observation of resonant-excitation double autoionization in electron-I50+collisions

We present a clear observation of a resonant contribution to electron impact ionization of Li-like highly charged ions (I{sup 50+}). Li-like ions are the simplest in which resonant-excitation double autoionization (REDA) can occur. REDA was observed by measuring the ratio of the numbers of trapped ions of two neighbring charge states as a function of electron energy under equilibrium conditions using an electron beam ion trap. The resonant strength was determined by normalizing the experimental data to theoretical ionization cross sections at the nonresonant interaction energies. Good agreement was found between the observed and calculated resonant strengths for this observation of REDA in Li-like highly charged ions.

Electron-impact single and multiple ionization of Mg+ ions

Absolute cross sections for single, double and triple electron-impact ionization of Mg+ ions have been measured by employing a crossed-beams experimental set-up. Contributions of indirect resonant processes to the total cross section for single ionization such as resonant-excitation–double-autoionization and resonant-excitation–auto-double-ionization have been observed utilizing a high-resolution energy scan technique with statistical uncertainties as low as 0.04%. The experimental cross sections are compared with results of recent R-matrix calculations showing agreement in the overall features but pronounced differences in the details of indirect ionization contributions. Double ionization was found to be dominated by the direct removal of a 2p-electron with subsequent autoionization. There are also indications of indirect processes in the triple ionization of Mg+ but no clear evidence could be established.

Resonant contributions to the electron-impact ionization of sodiumlike chromium

Electron-impact-ionization cross sections for Na-like Cr 1 3 + have been calculated using semirelativistic distorted-wave Born-exchange approximation. We have included direct ionization, excitation autoionization, and resonant-excitation double-autoionization (REDA) processes in our calculation. The overall agreement between experimental data and our theoretical values is very good. Strong resonant peaks from REDA processes are found, and the REDA processes contribute about 50% and 25% to the average total ionization cross section between 570 and 650 eV and between 650 and 860 eV, respectively. A detailed investigation about the effects of radiative damping and loss channels on the REDA cross section has been carried out. The ionization-rate coefficients of Cr 1 3 + are also presented. Besides, we have compared our calculated results of Fe 1 5 + to experimental data and other theoretical values for examining the reliability of our calculations.

Contributions of resonant excitation double autoionization tothe electron-impact ionization of Ti11+

Electron-impact ionization cross sections for sodium-liketitanium have been calculated using the semirelativisticdistorted-wave Born exchange approximation method. Wehave performed a comprehensive calculation for the resonantexcitation double autoionization (REDA) processes. Our calculation confirms the prediction by Gregory et al (1990 Phys. Rev. A41 6512) concerningthe contribution of REDA processes in the electron energyregion 550⩽E⩽625 eV. However, strong peaks fromREDA are also found between 450 and 525 eV. The REDA processescontribute about 30% to the average total ionization crosssection in the resonant region and enhance the total rate coefficients by as much as a factor of 1-2 in the electrontemperature range 40-500 eV. The reduction of REDAcontributions due to the effects of radiative decay and losschannels is also studied in detail.

Electron-impact single ionization of low-charged titanium ions

The electron-impact single ionization of low-charged Ti ions was studied both experimentally and theoretically. Absolute cross sections for ions in charge states q = 1 to 6 except 3 were measured using a crossed-beams technique as well as calculated with the configuration-average distorted-wave method, including both direct ionization and excitation-autoionization contributions. For charge states q = 4, 5 and 6, the measurements are in good agreement with the calculations considering that large components of the incident ion beam were in metastable configurations. The cross sections for single ionization of and were determined in part with a high-resolution energy-scan method. Besides contributions of excitation-autoionization mechanisms, a remarkable number of structures arising from resonant-excitation double-autoionization (REDA) processes could be observed.

Distorted-wave cross sections for electron-impact ionization of Ar7+

Distorted-wave methods have been used to calculate cross sections for electron-impact ionization of sodiumlike ${\mathrm{Ar}}^{7+}$ for collision energies ranging from near threshold to 1 keV. We included contributions due to excitation-autoionization and resonant excitation--double autoionization (REDA), as well as the contributions from direct ionization of the 3s electron and the n=2 inner-shell electrons. We find that the effects of radiative damping and loss channels reduce the REDA contributions by a factor of 4. We compare our calculated results to experimental results and to theoretical cross sections obtained in a close-coupling approximation. \textcopyright{} 1996 The American Physical Society.

Resonant effects on electron-impact ionization of sodiumlike selenium.

We have calculated the resonant-excitation-double-autoionization (REDA) cross sections for sodiumlike selenium. The REDA process is treated as a three-step process (i.e., resonant electron capture followed by sequential double Auger emission). The necessary atomic data are computed using the multiconfiguration Dirac-Fock model. The effects of radiative damping can reduce the calculated REDA cross sections by a factor of 6, and the loss channels can further reduce these cross sections by a factor of 2. The REDA processes produce many strong narrow resonances and contribute 10--20 % to the average total ionization cross sections.

Indirect contributions to electron-impact ionization of Kr24+, Kr25+, and Xe43+

Electron-impact ionization cross sections have been calculated for ${\mathrm{Kr}}^{24+}$ and ${\mathrm{Kr}}^{25+}$. The contributions of excitation-autoionization and resonant excitation double autoionization (REDA) were included in addition to the cross sections for direct ionization of a 3s electron. For ${\mathrm{Xe}}^{43+}$ the REDA contributions were extended to two times the ionization threshold. The calculations were performed using the relativistic distorted-wave and the multiconfiguration Dirac-Fock methods. For ${\mathrm{Kr}}^{25+}$ the indirect processes enhance the ionization cross section by a factor of 4. For ${\mathrm{Kr}}^{24+}$ the total cross section is about 3.5 times the direct-ionization cross section. The REDA process produces numerous strong resonances and contributes about 20% to the average ionization cross section for ${\mathrm{Kr}}^{24+}$ and ${\mathrm{Kr}}^{25+}$ and 15% to the average ionization cross section for ${\mathrm{Xe}}^{43+}$.

Distorted-wave calculations of cross sections for electron-impact ionization of lithiumlike ions

We used distorted-wave and multiconfigurational Dirac-Fock methods to calculate direct and indirect contributions to cross sections for electron-impact ionization of ${\mathrm{C}}^{3+}$, ${\mathrm{N}}^{4+}$, ${\mathrm{O}}^{5+}$, and ${\mathrm{F}}^{6+}$. The features due to excitation autoionization and resonant excitation double autoionization observed in recent cross-beam measurements [G. Hofmann et al., Z. Phys. D 16, 113 (1990)] are well reproduced in our results. Our cross sections are also in good agreement with reported results of nonrelativistic close-coupling calculations [S. S. Tayal and R. J. W. Henry, Phys, Rev. A 42, 1831 (1990); 44, 2955 (1991)] over most of the energy range studied. We examine the effects of radiative decay on the intermediate states, and we show that neglect of certain channels in the close-coupling calculation results in overestimation of the cross sections in the region of the 1s2s2pnl resonances.

Indirect processes in electron-impact ionization of Xe43+

Relativistic-distorted-wave methods have been used to calculate electron-impact-ionization cross sections for ${\mathrm{Xe}}^{43+}$. Cross sections were calculated for direct ionization of the 3s electron and the n=2 inner-shell electrons. Electron-impact-excitation cross sections were calculated for transitions from the n=2 subshells to the n=3 and 4 levels of the ion. Detailed Auger and radiative rates were calculated for the intermediate excited states and these were used to determine the excitation-autoionization and resonant-excitation double-autoionization contributions to the ionization cross section. Our results are in excellent agreement with reported experimental results for incident electron energies between 8 and 15 keV. Recent measurements using the electron-beam ion trap are in qualitative agreement with our results for energies between 4 and 8 keV.

Contributions of resonant excitation double autoionization to the electron-impact ionization of Fe15+

Electron-impact ionization cross sections for ${\mathrm{Fe}}^{15+}$ have been calculated using relativitsic distorted-wave methods. We have performed the most comprehensive calculation to date by including more than 10 000 autoionizing levels for resonant-excitation double-autoionization (REDA) processes. The total cross sections exhibit many strong narrow resonances. The REDA process contributes about 30% to the average total cross sections. General good agreement between theory and experiment is attained, but new experimental investigations are needed to search for the predicted resonances.

HIGH-RESOLUTION MEASUREMENTS OF ELECTRON-IMPACT IONIZATION CROSS SECTIONS FOR LI-LIKE IONS

For crossed electron and ion beams we have developed an experimental technique which allows to measure cross sections for electron impact ionization of ions with relative uncertainties less than 0.1 %. At this level of accuracy it is possible to do ion spectroscopy of intermediate excited states contributing to net ionization via single and multiple autoionization processes. For ions B2+, C3+, N4+ and O5+ along the Li isoelectronic sequence we have performed quantitative measurements of state-resolved contributions from excitation-autoionization (EA), resonant-excitation-double autoionization (REDA) and resonant-excitation auto-double-ionization (READI).

Cross sections for inner-shell excitation of Na-like ions

The contributions of excitation autoionization to electron-impact ionization in the Nalike ${\mathrm{Al}}^{2+}$ and ${\mathrm{Si}}^{3+}$ are calculated using a two-state close-coupling approximation. Inner-shell excitations of the type $2{p}^{6}3s\ensuremath{\rightarrow}2{p}^{5}3s3l(l=0,1,2)$ are included. The contributions of resonant-excitation double autoionization are estimated from a three-state close-coupling approximation with the aid of quantum-defect-theory analysis. These contributions explain the absence of the largest predicted excitation-autoionization step in the measurements of Crandall et al. Predictions for ${\mathrm{Mg}}^{+}$ also lead to similar agreement with experiment. We conclude that the experiments of Crandall et al. for the total ionization cross sections for ${\mathrm{Mg}}^{+}$, ${\mathrm{Al}}^{2+}$, and ${\mathrm{Si}}^{3+}$ by electron impact include measurements of the physical processes of direct-ionization, inner-shell excitation autoionization, and inner-shell resonant-excitation double autoionization.