Excitation-autoionization Contributions Research Articles

Excitation-autoionization contribution to single ionization of Sr by electron impact

The 4p6 excitation-autoionization (EA) cross-section of Sr atoms has been measured at electron-impact energies from the threshold to 600 eV. The cross-section has a pronounced resonance character in the near-threshold region, reaching its maximum value of (2.5 ± 0.8) × 10−16 cm2 at 24.5 eV. The shape and absolute value of the cross-section are determined by the excitation dynamics and the decay mode of autoionizing states from 4p54d5s2, 4p54d2nl and 4p54d5snl configurations. A comparative analysis of the measured EA cross-section, the calculated 5s2 ionization cross-section and the available ionization data showed that (i) direct ionization of the 5s2 shell and EA of the 4p6 shell adequately describe the single ionization of Sr atoms; (ii) 4p6 EA contribution reaches 25% of the total single ionization cross-section of Sr atoms.

Electron-impact ionization ofW5+

Electron-impact single-ionization cross sections for the ${\mathrm{W}}^{5+}$ ion have been studied experimentally and theoretically. Measurements of a detailed ionization spectrum and of absolute cross sections were performed employing the crossed-beams method in the energy range from the ionization onset up to 1000 eV. The experimental data show a prominent contribution of ${\mathrm{W}}^{5+}$ ions in metastable states. The theoretical analysis includes level-to-level calculations from the $4{f}^{14}5{s}^{2}5{p}^{6}5d$ ground configuration and long-lived levels of the $6s$, $5{p}^{5}5{d}^{2}$, $4{f}^{13}5{d}^{2}$, and $4{f}^{13}5d\phantom{\rule{0.16em}{0ex}}6s$ configurations. Direct-ionization and excitation-autoionization contributions to the total single-ionization cross sections were calculated employing a distorted-wave approximation. Radiative damping was taken into account. It is shown that correlation effects play an important role and lead to substantial reduction of cross sections. Theory and experiment are in quite good agreement when, within a statistical model, a ($85\ifmmode\pm\else\textpm\fi{}9$)% fraction of parent ions in the ground configuration is assumed. The strongest contributions of metastable parent ions arise from the $5{p}^{5}5{d}^{2}$ and $4{f}^{13}5{d}^{2}$ configurations.

Electron-impact ionization of the Kr atom

Electron-impact ionization cross sections are calculated for the ground configuration of the Kr atom. Time-dependent close-coupling cross sections for the direct ionization of the 4s and 4p subshells leading to single ionization are found to be a factor of 2 smaller than distorted-wave calculations due to the presence of strong continuum correlation effects. With the inclusion of relatively small excitation-autoionization contributions, the total single ionization cross sections for Kr are found to be slightly larger than crossed-beams experimental measurements.

STORAGE RING CROSS SECTION MEASUREMENTS FOR ELECTRON IMPACT IONIZATION OF Fe7+

We have measured electron impact ionization (EII) for Fe 7+ from the ionization threshold up to 1200 eV. The measurements were performed using the TSR heavy ion storage ring. The ions were stored long enough prior to measurement to remove most metastables, resulting in a beam of 94% ground state ions. Comparing with the previously recommended atomic data, we find that the Arnaud & Raymond (1992) cross section is up to about 40\% larger than our measurement, with the largest discrepancies below about 400~eV. The cross section of Dere (2007) agrees to within 10%, which is about the magnitude of the experimental uncertainties. The remaining discrepancies between measurement and the most recent theory are likely due to shortcomings in the theoretical treatment of the excitation-autoionization contribution.

Electron-Impact Ionization of Se16+ and Se17+ Ions

SynopsisElectron-impact ionization cross sections for Se16+ and Se17+ ions have been calculated using semirelativistic configuration-average distorted-wave (CADW) method for both the direct and indirect ionization contributions. The excitation-autoionization contributions originating from the excitations of 2s and 2p subshels were calculated using a semirelativistic level to level distorted-wave (LLDW) method. Direct ionization cross section calculations involving 2s,2p,3s, and 3p subshells were also calculated using the LANL collisional atomic code and comparisons are made.

Ionization cross sections and rate coefficients for the ground state of AsI-like ions

Detailed level-by-level calculations of cross sections and rate coefficients for electron impact direct ionization and excitation-autoionization of AsI-like ions in the 3d 104s 24p 3 (4S3/2) ground state have been performed. The cross sections are presented in the energy range near the threshold for the five ions Mo9+, Xe21+, Pr26+, Dy33+ and W41+. The rate coefficients are given for all the ions from Sr5+ to U59+ in the AsI sequence at the seven electron temperatures (k T e = 0.1E I ,0.3E I ,0.5E I ,0.7E I ,E I ,2E I and 10E I , where E I is the first ionization energy). The calculations include the contribution of direct ionization (DI) calculated using the Lotz formula approximation and the contributions of excitation-autoionization (EA) computed in the framework of the Distorted Wave (DW) approximation for the 4s − nl, 3d − nl and 3p − nl resonant inner-shell excitations. The ionization enhancement due to the EA channels is presented as a function of Z along the AsI isoelectronic sequence. The present results show the great importance of the EA processes; an ionization enhancement factor of up to 6.5 is predicted for instance for Dy33+ (Z = 66) at electron temperature of coronal equilibrium maximum abundance.

Electron-impact ionization of Al

Perturbative distorted-wave and non-perturbative close-coupling methods are used to calculate electron-impact ionization cross sections for the ground state of the neutral Al atom. Configuration-average distorted-wave calculations are made for both direct ionization and excitation-autoionization contributions. The total perturbative results are found to be almost a factor of 2 higher than experiment over a wide energy range. On the other hand, the R-matrix with pseudo-states results for total ionization are found to be in good agreement with experiment. Comparison of time-dependent close-coupling calculations for the direct ionization with the R-matrix with pseudo-state calculations for total ionization reveals that both the direct ionization and excitation-autoionization contributions are strongly affected by correlation effects.

Electron-impact ionization of C+excited states

Nonperturbative close-coupling and perturbative distorted-wave methods are used to calculate electron-impact ionization cross sections for excited states of C${}^{+}$. Both the time-dependent close-coupling and the time-independent distorted-wave methods are used to calculate direct ionization cross sections for the $1{s}^{2}2{s}^{2}3l$ configurations. An $R$ matrix with pseudostates method is used to calculate total ionization cross sections for the $1{s}^{2}2{s}^{2}3l$ configurations, including both direct and indirect excitation-autoionization contributions. Comparison of the results shows both a substantial reduction of the direct ionization cross sections due to nonperturbative effects and a large contribution to the total ionization cross sections due to indirect contributions coming from $2s\ensuremath{\rightarrow}2p$ excitations. The ionization cross-section results will permit the generation of more accurate generalized collisional-radiative ionization coefficients needed for the modeling of moderately dense carbon plasmas.

Excitation-autoionization cross sections and rate coefficients for Ga-like ions

Results of cross-section and rate coefficient calculations for electron impact direct and indirect ionization of ions belonging to the GeI isoelectronic sequence (ground 3d104s24p2) are presented. The cross-sections are given at near threshold energies for the five ions Kr4+, Mo10+, Xe22+, Pr27+ and Dy34+. The rate coefficients are computed for all the ions from Kr4+ to U60+ in the GeI sequence at seven electron temperatures (kTe = 0.1EI, 0.3EI, 0.5EI, 0.7EI, EI, 2EI and 10EI, where EI is the first ionization energy). The calculations include the contribution of direct ionization (DI) calculated with the Lotz formula approximation and the contributions of excitation-autoionization (EA) calculated in the framework of the distorted wave (DW) approximation for the 4s-nl, 3d-nl and 3p-nl resonant inner-shell excitations. The ionization enhancement due to the EA channels is shown as a function of Z along the GeI isoelectronic sequence.

Electron-impact ionization of the boron atom

Accurate knowledge of the electron-impact ionization of the B atom is urgently needed in current fusion plasma experiments to help design ITER wall components. Since no atomic measurements exist, nonperturbative time-dependent close-coupling (TDCC) calculations are carried out to accurately determine the direct ionization cross sections of the outer two subshells of B. Perturbative distorted-wave and semiempirical binary encounter calculations are found to yield cross sections from 26% lower to an order of magnitude higher than the current TDCC results. Unlike almost all neutral atoms, large excitation-autoionization contributions are found for the B atom. Nonperturbative $R$ matrix with pseudostates (RMPS) calculations are also carried out to accurately determine the total ionization cross section of B. Previous 60 LS-term RMPS calculations are found to yield cross sections up to 40% higher than the current more extensive 476 LS-term RMPS results.

Electron-impact ionization of atomic ions in the Sn and Xe isonuclear sequences

Electron-impact ionization cross-sections for all atomic ions in the Sn and Xe isonuclear sequences are calculated using a semi-relativistic configuration-average distorted-wave method. Contributions to the cross-sections are included from both direct ionization and indirect excitation-autoionization. Branching ratio calculations are carried out for Cu-like Sn 21+and Xe 25+ and Na-like Sn 39+ and Xe 43+ to determine when best to ignore indirect excitation-autoionization contributions due to radiation damping along the isonuclear sequences. Although experiments exist for Xe through to Xe 6+, Xe 8+, and Xe 43+ the configuration-average distorted-wave results only agree well with the Xe 8+ measurements, once metastable states are taken into account and with the Xe 43+ measurements once radiation damping of the excitation-autoionization contribution is accounted for. Temperature-dependent rate coefficients are calculated for each Sn and Xe ionization stage and then put in a collisional-radiative format that should prove useful in modelling astrophysical and laboratory plasmas.

Electron-impact ionization ofPbq+ions forq=1–10

Theoretical calculations and experimental crossed-beam measurements are compared for electron-impact single ionization of ${\mathrm{Pb}}^{q+}$ ions for $q=1--10$. We compare with two main theoretical methods. First, we check against configuration-average distorted-wave calculations, which include both direct-ionization and indirect excitation-autoionization contributions. Second, for ion stages ${\mathrm{Pb}}^{+}$ through to ${\mathrm{Pb}}^{5+}$, we calculate the dominant excitation-autoionization channels using level-resolved distorted-wave theory to evaluate the excitation cross sections. We find that for ion stages ${\mathrm{Pb}}^{+}$, ${\mathrm{Pb}}^{2+}$, and ${\mathrm{Pb}}^{3+}$, distorted-wave theory significantly overestimates the total-ionization cross section, due to an overestimation of the direct-ionization cross section from the $5d$ subshell. For ion stages ${\mathrm{Pb}}^{4+}$ through to ${\mathrm{Pb}}^{10+}$ there is good agreement between theory and experiment. We find evidence for significant metastable fraction in the ion beam of the experiment for ion stages ${\mathrm{Pb}}^{2+}$, ${\mathrm{Pb}}^{3+}$, ${\mathrm{Pb}}^{4+}$, ${\mathrm{Pb}}^{5+}$, and ${\mathrm{Pb}}^{6+}$. For ion stage ${\mathrm{Pb}}^{3+}$ we find that the level-resolved distorted-wave calculation of the excitation autoionization results in a slight reduction of the configuration-average theoretical results, due to splitting of levels within the autoionizing configurations. We also investigate two semiempirical methods of calculating the direct-ionization cross sections: namely, the Lotz method and the binary encounter Bethe method. We find that both methods provide results which are significantly lower than the distorted-wave method for the $5d$-subshell direct ionization of ${\mathrm{Pb}}^{+}$, ${\mathrm{Pb}}^{2+}$, and ${\mathrm{Pb}}^{3+}$. For the higher ion stages, both methods are lower than the distorted-wave direct-ionization cross-section results, trending towards the distorted-wave results as the ion stage increases.

Electron-impact ionization ofBiq+forq=1–10

Theoretical calculations and experimental crossed-beam measurements are compared for electron-impact single ionization of ${\mathrm{Bi}}^{q+}$ for $q=1--10$. The configuration-average distorted-wave calculations include both direct ionization and indirect excitation-autoionization contributions. For ${\mathrm{Bi}}^{2+}$ it is necessary to account for levels within an excited configuration that straddle the ionization threshold. This was included via level-resolved atomic structure calculations, from which statistical partitions of the configuration-averaged cross sections could then be made. Evidence is found for ionization contributions from an excited configuration in ${\mathrm{Bi}}^{3+}$ and ${\mathrm{Bi}}^{5+}$. Good agreement is found between theory and experiment for all the ionization stages of Bi studied.

Argon ionization cross sections for charge state distribution modeling in electron cyclotron resonance ion source

An updated and more accurate database for single- and double-ionization cross sections for almost all argon ions has been developed for the modeling of the charge state distribution (CSD) within an electron cyclotron resonance ion source. When the highly non-Maxwellian anisotropic electron-distribution function, is modeled by a Fokker–Planck code, one has to use the ionization cross sections instead of the Maxwellian rate coefficients. Most of the fitting coefficients used within the well-established semi-empirical formulas for direct ionization and double ionization have been recalculated using more accurate crossed-beam experimental data available. The shift of the CSD to higher-charge states due to the contribution of excitation autoionization and double ionization is presented by comparing the GEM code modeling using the Lotz formula and the cross sections with updated fitting coefficients.

Experimental and theoretical study of electron-impact ionization of atomic ions in the Sm isonuclear sequence

Experimental measurements and theoretical calculations of absolute cross sections are carried out for the electron-impact ionization of atomic ions in the Sm rare-earth-metal isonuclear sequence. Absolute cross sections for ${\mathrm{Sm}}^{q+}$ ions for $q=1\ensuremath{-}12$ were measured using the electron-ion crossed-beam technique and calculated with the configuration-average distorted-wave method for the charge states $q=4\ensuremath{-}12.$ The theory includes both direct ionization and excitation-autoionization contributions, and includes transitions from both ground and metastable levels. We present the cross sections in an electron-impact energy range from threshold up to 1000 eV for all Sm ions up to ${\mathrm{Sm}}^{12+}.$ These systems are extremally complex, but the configuration-average method, when combined with statistical averaging of the cross sections over the many levels, provides results that are in overall good agreement with the experimental measurements. The single-ionization cross sections are dominated by contributions from indirect mechanisms of excitation-autoionization in the low-energy region.

Electron-impact ionization of multiply charged manganese ions

Distorted-wave calculations are carried out for the electron-impact ionization of atomic ions in the Mn isonuclear sequence. Detailed level to level calculations are performed for the dominant excitation-autoionization contributions. The validity of the configuration-average approach is studied and used to calculate direct ionization and higher n excitation-autoionization, when the level to level approach proves computationally too demanding. A combination of the two methods is used to analyze the recent crossed-beams experimental measurements of ${\mathrm{Mn}}^{5+},$ ${\mathrm{Mn}}^{6+},$ ${\mathrm{Mn}}^{7+},$ and ${\mathrm{Mn}}^{8+}$ by Rejoub and Phaneuf [Phys. Rev. A 61, 032706 (2000)]. The comparison between theory and experiment indicates that a large fraction of the initial population of each measured ion is in metastable states of excited configurations.

Electron-impact single ionization of Kr10+ and Kr11+ ions

Cross sections for electron-impact single ionization of ground state Kr10+ and Kr11+ ions have been calculated in the distorted-wave approximation including both direct ionization and excitation-autoionization processes. The direct ionization cross sections are calculated for the ejection of 3s, 3p and 3d subshell electrons and the excitation-autoionization cross sections are calculated for the inner-shell excitations of the type 3p nl (n = 4,5,6;l = 0,1,2,3) and 3s n´l´ (n´ = 4,5;l´ = 0,1,2,3). In addition, the inner-shell excitation of the type 2p3d, 4l (l = 0,1,2,3) is included for Kr11+. The radiative decay effect has been found to be negligible by carrying out explicit calculations of the radiative rates and autoionizing rates. The present results show that the direct process dominates the total ionization cross section for the two ions and that the contributions of excitation-autoionization are about 15% and 13% of the total cross sections for Kr10+ and Kr11+ ions, respectively. The present results are in good agreement with the experimental measurement of Oualim et al.

Distorted-wave calculations for electron-impact ionization of Si4+ and Si5+

Cross sections for electron-impact ionization of and ions have been calculated in the distorted-wave approximation. In the present calculations both direct-ionization and excitation-autoionization processes are included. For ions, the cross sections are calculated for ionization from both the ground configuration and the excited configuration. Excitation-autoionization substantially contributes to the ionization of the excited configuration and is approximately a factor of four above direct ionization at energies below the ground-state threshold. For ionization from the ground configurations of both and ions, the direct process dominates the total ionization cross section, and contributions of excitation-autoionization are very small. The present results are in reasonable agreement with the experimental measurements of Thompson and Gregory (Thompson J S and Gregory D C 1994 A 50 1377). Remaining discrepancies between the experiment and theory are discussed.

Electron-impact ionization of Fe 14+ and other atomic ions in the Mg isoelectronic sequence

Distorted-wave calculations are carried out for the electron-impact ionization of Fe 14+ in both the ground 3 s 2 1 S 0 and metastable 3 s3 p 3 P 0.2 levels. The cross sections and Maxwellian-average rate coefficients include direct ionization of the 2 s, 2 p, 3 s, and 3 p subshells, as well as inner-shell excitations from the 2 s and 2 p subshells leading to autoionization. The excitation—autoionization contributions are found to be large for both the ground the metastable levels. To assist modeling efforts of moderately dense plasmas, the direct and indirect rate coefficients are resolved as to the final level of the ionization process and assembled in a standard database file. A complete database for selected metallic ions in the Mg isoelectronic sequence will reside in electronic form at the Controlled Fusion Atomic Data Center at ORNL ( ).

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.