Excitation Autoionization Research Articles

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 ( ).

Excitation autoionization rates from ground and excited levels in Li-likeAr15+to S-likeAr2+

The rate coefficients for excitation autoionization (EA) from the levels of the ground and first excited configuration in nearly all argon ions (Li-like Ar${}^{15+}$ to S-like Ar${}^{2+})$ have been computed. Collisional excitation cross sections are computed in the distorted-wave approximation and integrated over a Maxwellian distribution of free-electron energies. Radiative and autoionizing transition probabilities are computed using the fully relativistic, intermediate coupling code RELAC. For each ion, the rates of EA are compared to semi-empirical calculations of the direct ionization rate. For charge states with few $M$-shell electrons in the ground configuration (Na-, Mg-, and Al-like), EA can significantly enhance the direct ionization rate coefficients. In the first excited configuration of each ion considered, the EA rate coefficient enhances the direct ionization rate coefficient only at low temperatures.

Electron impact ionization of and photoionization of Be at the K edge

We present new R-matrix calculations for ionization processes involving the beryllium K-shell, namely electron impact ionization of and single and double photoionization of Be. These are modelled near inner-shell thresholds in a unified way, coupling direct and indirect processes, enabling comparison of the respective autoionization features. Below the K edge, we predict that direct double photoionization contributes 4% to the total cross section background for photoionizing Be; the onset of K-shell excitation - autoionization (EA), an indirect mechanism, increases the background by a factor 20. By contrast, we find that EA in ionization increases its background by only 4%; this is less than previous estimates, but in line with experiment. We also study high-energy behaviour for photoionizing excited states, e.g. , where we find a background dominated by ionization. This process is normally omitted in R-matrix schemes used in opacity and recombination calculations, leading to orders-of-magnitude underestimation at high energies of excited-state photoionization, which also exhibits a large core resonance above the K edge.

Electron-impact ionization of tungsten ions in the configuration-average distorted-wave approximation

Single-ionization cross sections for ${\mathrm{W}}^{4+},$ ${\mathrm{W}}^{5+},$ and ${\mathrm{W}}^{6+}$ are calculated in the configuration-average distorted-wave approximation. Contributions from excitation autoionization are found to be strong for each ion. When comparing theory with the recent crossed-beams measurements of Stenke et al. [J. Phys. B 28, 2711 (1995)], ionization from the ground configuration dominates for ${\mathrm{W}}^{4+}$ and ${\mathrm{W}}^{5+},$ while ionization from metastables of excited configurations dominates for ${\mathrm{W}}^{6+}.$

Low-Energy (5−40 eV) Electron-Stimulated Desorption of Atomic Hydrogen and Metastable Emission from Amorphous Ice

Low-energy (5−40 eV) electron-stimulated desorption (ESD) of D (1 2S) from amorphous D2O water films has been studied using laser resonance-enhanced multiphoton ionization spectroscopy. The D (1 2S) desorption product has a ∼6.5 ± 0.3 eV threshold energy, relative to the vacuum level, and a low velocity distribution. ESD of electronically excited D (n ≥ 2 2S) was not detected using photoionization schemes. A small metastable (n ≥ 2 2P) desorption yield and/or emission of ultraviolet (UV) photons (hν ≥ ∼6 eV) was detected at incident electron energies >25 eV, using a multichannel plate detection technique. We attribute the ground state D desorption to exciton decay and associate the small metastable/UV photon emission yield with dissociation of doubly excited states and efficient autoionization.

Dielectronic recombination and excitation autoionization rate coefficients for potassiumlike Mo23+ to fluorinelike Mo33+

Fully relativistic, ab initio calculations of the rate of dielectronic recombination (DR) have been performed for fluorinelike ${\mathrm{Mo}}^{33+}$ to magnesiumlike ${\mathrm{Mo}}^{30+}$ and chlorinelike ${\mathrm{Mo}}^{25+}$ and argonlike ${\mathrm{Mo}}^{24+}$. Calculations of the rate of excitation autoionization (EA) have been performed for neonlike ${\mathrm{Mo}}^{32+}$ to aluminumlike ${\mathrm{Mo}}^{29+}$ and argonlike ${\mathrm{Mo}}^{24+}$ and potassiumlike ${\mathrm{Mo}}^{23+}$. The detailed calculations of the atomic structure and rate coefficients for charge states in these groups allow interpolation of the DR and EA rates for the more complex ions having 3${\mathit{s}}^{2}$3${\mathit{p}}^{\mathit{k}}$ (k=1--4 for DR and k=2--5 for EA) ground states. The calculations for DR are broken up by different classes of excitation channels; simple, analytic formulas are then fit to the calculations. The effects of configuration interaction on the rates of DR and EA have been studied in detail and are found to have a slight effect on only a small class of the Auger rates needed for the present work. Radiative transitions between energy levels in the continuum are investigated and found to have a moderate effect on the DR rates and a small effect on the EA rates of the ions in this paper. \textcopyright{} 1996 The American Physical Society.

ELECTRON–ION COLLISIONAL IONIZATION CROSS SECTIONS FOR THE Li ISOELECTRONIC SEQUENCE

In this work, we calculate the electron impact ionization cross sections for Li-like isoelectronic-sequence ions in the range 6 ≤Z≤ 29 by means of a distorted-wave Born exchange approximation method including a relativistic correction. The total cross sections include the contributions from direct ionization and excitation autoionization. For excitation autoionization the branching ratio and the configuration interaction are considered. A package of computer codes has been developed and used to calculate these cross sections. Systematic study of the dependence of the cross sections on impact energies and nuclear charges is carried out. The results of the calculations are fitted by empirical formulas to meet the requirements of applications. The fitted values are in good agreement with the calculated results.

Evidence of charge transfer from Na adatoms to the Cs substrate: A collisionally excited autoionization electron-emission study.

Na adsorption on a bulk Cs substrate at 20 K has been studied with collisionally excited autoionization electron spectroscopy using 1 keV ${\mathrm{Ne}}^{+}$ ion impact. Our results show a slight decrease in the surface work function and a quite small Ne 2${\mathit{p}}^{4}$3${\mathit{s}}^{2}$ $^{1}$D-to${\mathrm{\ensuremath{-}}}^{3}$P conversion rate at very low coverages, suggesting that, in contrast to the expectation based on the electronegativity consideration, a partial charge transfer occurs from Na adatoms to the more electropositive Cs substrate. We attribute this to the shift and broadening of the valence level of Na adatoms to overlap with the substrate conduction band. \textcopyright{} 1996 The American Physical Society.

Effect of configuration interaction on excitation autoionization cross sections

The effect of configuration interaction (CI) on the theoretical calculation of autoionization cross section for Li-like ions are studied in detail by employing the distorted wave Born exchange approximation. We consider the 1s22s → 1s2s2p and 1s22s → 1s2s3p transitions for C3+ and Al10+ ions. These two transitions account for the largest excitation autoionization cross section. It is found that the CI has a significant effect on the calculated results. The effect becomes larger with increasing atomic number.

Inner-shell excitation energy and autoionization rates for Li-, Be-, B-like ions with Z = 6–54

Absolute energies of the autoionization states 1s2s22pn [LSJ] with n = 0–2, 1s2s2pn [LSJ] with n = 1–3 and 1s2pn [LSJ] with n = 2–4 and non-autoionizing states 1s22s22pn [LSJ] with n = 0–1, 1s22s2pn [LSJ] with n = 0–2 and 1s22pn [LSJ] with n = 1–3 were calculated for ions with nuclear charge Z = 6–54 by using the 1/Z perturbation theory method. Relativistic corrections were taken into account in the frame of the Breit operators. The autoionization rates for the above-mentioned autoionization states for ions with Z = 6–54 were computed as well. These data are very important for modelling the satellite spectra produced by high-temperature plasma with thermal and non-thermal electrons.

Adsorbate-adsorbate interactions in Na covered Mg surfaces

We present a study on alkali cluster formation induced by ion bombardment, the information being obtained by Collisionally Excited Autoionization Electron Spectroscopy (CEAES). We show that a uniform alkali film on a metal surface can be easily converted in large patches by low energy ion bombardment. The observed phenomenon is coverage dependent, in fact, for alkali density below a critical value the adlayer is uniformly removed. Beyond this critical value a rearrangement mechanism leads to the formation of extended adatom aggregates revealed by the splitting in two components of both Ne and Na autoionization lines. A Mg atomic structure is also revealed in the spectrum, and, according to our interpretation model, no splitting is observed for this peak due to slow excited atoms.

Systematic study on ionization cross sections of electron - ion collisions for the Li-like isoelectronic sequence

In this work, we calculated the ionization cross sections of electron impact for the Li-like isoelectronic sequence. The excitation autoionization is also included in the calculation. The results of the calculations are fitted by empirical formulae to meet the requirements of the applications. The fitted results are in good agreement with the calculated results.

Contribution of excitation autoionization to the electron-impact ionization of Ar7+

We report a detailed calculation of the contribution of excitation autoionization to the electron-impact ionization of Ar{sup 7+} by a distorted-wave Born approximation with exchange. Inner-shell excitations of the type 2{ital s}{sup 2}2{ital p}{sup 6}3{ital s}{r_arrow}2{ital s}{sup 2}2{ital p}{sup 5}3{ital snl} ({ital n}=3,4,5; {ital l}=0,1,2,3) and 2{ital s}{sup 2}2{ital p}{sup 6}3{ital s}{r_arrow}2{ital s}2{ital p}{sup 6}3{ital snl} ({ital n}=3,4; {ital l}=0,1,2,3) are included. The level-to-level excitation cross sections and autoionization branching ratios are calculated in detail. The present results demonstrate that the use of unit branching ratios will overestimate the excitation-autoionization cross section by about 20{endash}40{percent}. Furthermore, the calculation shows the largest contribution is from 2{ital p}{r_arrow}3{ital p} and 2{ital p}{r_arrow}3{ital d} excitations and the latter dominates the contribution of excitation autoionization to electron-impact ionization. The calculated results are also compared with recent experimental measurements and theoretical calculations. Being different from the results of Griffin, Bottcher, and Pindzola [Phys. Rev. A {bold 45}, 2929 (1992); {bold 24}, 2273 (1981)], our results for the branching ratio are closer to the measurement of Rachafi {ital et} {ital al}. [J. Phys. B {bold 24}, 1037 (1991)] and our values are smaller than the experimental one of Zhang {ital et} {ital al}. [Phys. Rev. Amore » {bold 45}, 2929 (1992)], while the results for the unit branching ratio are very close to the calculation of Tayal [Phys. Rev. A {bold 49}, 2561 (1994)]. In addition, we find that the configuration interaction changes the autoionization branching ratios of individual levels, but has little effect on the total excitation-autoionization cross sections. {copyright} {ital 1996 The American Physical Society.}« less

Evidence of alkali adatom island formation induced by ion bombardment on a Na covered Pt surface

We report on adatom island formation induced by Ne + ion bombardment on a Na covered Pt surface. By using the collisionally excited autoionization electron spectroscopy (CEAES) we observe that, for large initial coverages, ion impact causes the Na adatom aggregation creating large surface areas which are relatively Na-rich and Na-depleted. Decay of excited atoms above these alkali islands and above the more or less clean Pt substrate zone with very different work functions results in the splitting of each atomic transition line in two well separated components.

Excitation-autoionization cross sections and rate coefficients of Cu-like ions.

Detailed level-by-level calculations of the excitation-autoionization (EA) cross sections and rate coefficients were performed using the relativistic distorted-wave method along the Cu isoelectronic sequence for all the elements with 34\ensuremath{\le}Z\ensuremath{\le}92. While in a previous work only the 3d-4l inner-shell collisional excitations had been taken into account, the present extensive calculations include the 3${\mathit{d}}^{10}$4s-3${\mathit{d}}^{9}$4snl (n=4--7 and l=0--n-1) and the 3${\mathit{p}}^{6}$3${\mathit{d}}^{10}$4s-3${\mathit{p}}^{5}$3${\mathit{d}}^{10}$4snl (n=4,5 and l=0--n-1) excitations. An extrapolation method is used to evaluate the contribution for higher principal quantum numbers. Configuration mixing and secondary autoionization processes following radiative decay from autoionizing levels are also included. The results show that the EA processes give a dominant contribution compared to direct ionization, up to a factor of about 15 at Z=43. The additional inner-shell excitations for Z55 produce an increase in the EA effect varying from 20% to a factor of 2, with respect to the previously predicted 3d-4l EA rates. The additional excitations are the most significant for heavy elements with Z\ensuremath{\gtrsim}55, since they open EA channels, resulting in an EA rate varying typically along the sequence from 3 to 1 times the direct ionization rate. \textcopyright{} 1996 The American Physical Society.

Collisional-radiative modeling of the L-shell emission of Mo30+ to Mo33+ emitted from a high-temperature-low-density tokamak plasma.

The x-ray spectra of several highly stripped molybdenum ions have been recorded between 0.6 and 5.5 A in the Frascati tokamak upgrade with a rotating crystal spectrometer. Detailed, quasi-steady-state collisional-radiative models have been used to interpret emission features from inner shell, electron impact excitations in molybdenum ions near the neonlike charge state and to characterize the charge state distribution in the plasma. Processes such as resonant excitation, excitation autoionization, and dielectronic recombination have been included in the models of the molybdenum ions{close_quote} emission features. Introducing the excitation-autoionization process into ionization equilibrium calculations brings agreement between observations and calculations of the relative ionization equilibrium fractions of highly stripped molybdenum ions. Absolutely calibrated spectra and detailed models for the excitation processes in these molybdenum ions allow us to calculate crucial plasma parameters, such as the concentration of impurity ions in the plasma and the amount of power lost from the plasma through impurity line radiation. {copyright} {ital 1996 The American Physical Society.}

THE EXCITATION-AUTOIONIZATION OF Na-LIKE Cu ION

The excitation autoionization of Na-like Cu ion origination from inner-shell excitations of 1s22s22p６3s→1s22s2２ｐ5３ｓ３ｌ（ｌ＝０，１，２） was investigated in some detail. The inner-shell excitation cross sections were calculated in the distorted Columob-Born exchange approximation and the target ionic states were obtained in a single configuraton approximation. The autoionizing branch ratios were calculated by using both single configuration and configuration interaction wavefunctions. The results showed that, at an incident electron energy of 1090 eV, the correlation enhancement of excitation autoionization cross sections was mainly due to 1s22s22p53s3d configuration,in which the correlation effects increasd the excitation autoionization cross sections nearly to a factor of 2. The excitation autoionization cross sections for 1s22s22p53s2 were reduced about 36% when compared with the results of single configuration approximation. Howecer, The configuration interaction had only a slight effect on the autoionization cross sections for excitation to 1s22s22p53s3p configuration.

Absolute cross sections for the electron-impact single ionization of Mo4+ and Mo5+ ions.

Absolute total cross sections for the electron-impact single ionization of ${\mathrm{Mo}}^{4+}$ and ${\mathrm{Mo}}^{5+}$ ions have been measured using a crossed-beams technique from below the ground-state ionization threshold to 500 eV with typical total uncertainties of 9% near the peak of the cross sections. Molybdenum ion production in the source was facilitated by a mini-oven sublimating ${\mathrm{MoO}}_{3}$. The measured cross sections are in good agreement with distorted-wave calculations and are dominated by contributions from excitation autoionization. Nonzero cross sections below the threshold for ionization of ${\mathrm{Mo}}^{5+}$(4${\mathit{p}}^{6}$4d) ground-state ions indicate that metastable ions were present in the beam extracted from an electron-cyclotron-resonance ion source. No evidence of metastable ions was found in the case of the ${\mathrm{Mo}}^{4+}$ measurements. Ionization rate coefficients and fitting parameters are presented for the experimental data.

Electron impact ionization of Ba, Ba+ and Ba2+

The binary encounter approximation has been used to investigate the effect of charge state of the target atom (ion) on the electron impact excitation autoionization cross-sections. Roothan-Hartree-Fock (RHF) velocity distribution for the bound electrons has been used through-out the calculations. The present results give a good account of the experimental observations.

Photoionization of the Be isoelectronic sequence from the ground and the 1S bound excited states.

Theoretical photoionization spectra below the 2p threshold of the Be isoelectronic sequence from the ground and a few selected $^{1}$S bound excited states are calculated using a B-spline-based configuration-interaction approach. The resonant structures of the doubly excited autoionization series following photoionization from different initial states are analyzed in detail in terms of various initial- and final-state multielectron interactions. In addition, our calculation has shown that the resonant widths vary smoothly as functions of the effective principal quantum number \ensuremath{\nu} along the autoionization series and approach approximately an expected ${\ensuremath{\nu}}^{\mathrm{\ensuremath{-}}3}$ dependence as \ensuremath{\nu} increases.