Autoionizing Levels Research Articles

THEORETICAL WAVELENGTHS OF Fe XVI L-SHELL TRANSITIONS AND COMPARISON WITH LABORATORY MEASUREMENTS ANDCHANDRAOBSERVATIONS OF CAPELLA

We have used the relativistic multi-reference Moller-Plesset perturbation theory to calculate the energy levels of Fe XVI, including those of the autoionizing levels with a hole state in the L shell. Comparison of the resulting L-shell transition wavelengths with those from recent laboratory measurements shows remarkable agreement, i.e., agreement within the experimental uncertainties. Our calculation allows us to predict the wavelength of the second strongest 2p-3d Fe XVI line, which has not yet been directly observed in the laboratory, to be 15.266 A. This wavelength is within 0.0042 A of the strong Fe XVII line commonly labeled 3D. Relying on the high accuracy of our calculations, we have reassigned two previously identified lines and predict a different location than previously thought for the strongest Fe XVI magnetic quadrupole transition. Inspection of the spectra of Capella recorded with the transmission grating spectrometers on the Chandra X-ray Observatory yields features corresponding to the predicted location of the innershell excited Fe XVI lines. These features have not been identified before. Our analysis shows that these features are most likely from Fe XVI.

Photoelectron ionization spectra in a system interacting with a neighbor atom

We study long-time photoelectron ionization spectra of a system interacting with a neighbor two-level atom under the influence of a laser field. The system has at least a ground state and a continuum of states that are free with respect to one electron. In a weak light field, a Fano resonance is revealed. Also we consider an atomic autoionization level of the system, which leads to two Fano resonances under the influence of a weak field.

Radiative rates and electron impact excitation rate coefficients for Ne-like selenium, Se XXV

In this article we report calculations of energy levels, radiative rates, electron impact collision strengths, and effective collision strengths for transitions among the 241 fine-structure levels arising from 2 l 8 and 2 l 7 n ′ l ′ ( n ′ ≤ 6 and l ′ ≤ n ′ − 1 ) configurations of Ne-like Se XXV using the Flexible Atomic Code. Energy levels and radiative rates are calculated within the relativistic configuration-interaction method. Direct excitation collision strengths are calculated using the relativistic distorted-wave approximation and high-energy collision strengths are obtained in the relativistic plane-wave approximation. Resonance contributions through the relevant Na-like doubly-excited configurations 2 l 7 n ′ l ′ n ″ l ″ ( 3 ≤ n ′ ≤ 7 , l ′ ≤ n ′ − 1 , n ′ ≤ n ″ ≤ 50 , and l ″ ≤ 8 ) are explicitly taken into account via the independent-process and isolated-resonance approximation using distorted waves. Resonant stabilizing transitions and possibly important radiative decays from the resonances toward low-lying autoionizing levels are considered. In addition, the resonance contributions from Na-like 2 l 6 3 l ′ 3 l ″ n ‴ l ‴ ( n ‴ = 3 − 6 ) configurations are included and found to be predominant for many transitions among the singly-excited states in Ne-like Se XXV. We present the radiative rates, oscillator strengths, and line strengths for all electric dipole, magnetic dipole, electric quadrupole, magnetic quadrupole, electric octopole, and magnetic octopole transitions among the 241 levels. The effective collision strengths are reported for all 28920 transitions among the 241 levels over a wide temperature range up to 10 keV. To assess the reliability and accuracy of the present collisional data, we have performed a 27-state close-coupling calculation, employing the Dirac R-matrix theory. The results from the close-coupling calculation and the independent-process calculation for the identical target states are found to be in good agreement.

Radiative transitions in the system of autoionizing levels of cesium atoms excited by electron impact

We study the energy dependences of electron excitation cross sections of the autoionizing levels (5p56s2)2P3/2 and (5p55d(3P)6s)2P3/2, as well as the autoionization cross sections of cesium atoms in the impact energy range from the excitation threshold to 600 eV. It is established that an increase in the excitation cross section of the 2P3/2 levels (to 60% of the maximum value) and the deep minimum in the autoionization cross section observed in the energy region of 17–27 eV are caused by the predominance of the radiative decay channel for autoionizing levels with excitation thresholds above 17 eV. The high efficiency of the process is mainly related to the resonance excitation of these levels in the near-threshold energy region.

The emission line near 1319 Å in solar and stellar spectra

An emission line near 1319 A is one of the strongest unidentified lines in the ultraviolet spectra of cool dwarf stars. In most line lists it is identified as a transition in N I, although its intensity would then be anomalous and the observed wavelength does not fit precisely that expected for N I. The line is also observed in cool giant stars. The measured wavelength of the line in stellar spectra is 1318.94 (+,- 0.01) A. Observations of giant stars provide further evidence that this line is not due to N I. It is proposed that this line is a decay from a previously unknown level in S I, which lies above the first ionization limit. This is identified with the 3d singlet D (odd parity) term. The previous tentative assignment of this term to the S I line at 1309.3 A then needs to be revised. The 1309.3 A line has been identified here for the first time in an astrophysical source. The singlet D (odd parity) level could, in principle, be populated by collisions from nearby autoionizing levels that have large number-densities, through population by di-electronic capture. Spin-orbit interaction with the autoionizing triplet D (odd parity) term might also lead to di-electronic capture into the singlet D (odd parity) level. A line at 1309.87 A observed in cool giant stars is identified as a transition in P II, pumped by the O I resonance lines.

Postcollision interaction upon electron excitation of 4sns 1 S 0, 3 S 1 and 4snd 1 D 2, 3 D J levels of zinc atom

The optical excitation functions (OEFs) for four series of spectral lines of the Zn atom at the transitions 4sn1s1S0 → 4s4p1P1∘ (n1 = 6–10), 4sn2s3S1 → 4s4p3P2∘ (n2 = 6−8), 4sn3d1D2 → 4s4p1P1∘ (n3 = 4−6), 4sn4d3D1, 2, 3 → 4s4p3P2∘ (n4 = 4−7) excited by a monoenergetic (0.04–0.08 eV) electron beam with energies from the excitation threshold to 18–19 eV are measured. In the energy dependences of the excitation cross sections, the effect of post-collision interaction of slow (scattered) and fast (emitted after the autoionization state decay) electrons is observed and described. This interaction leads to the capture of a slow electron into the listed discrete levels and the appearance of maxima in the excitation functions. The energies of the maxima are used to determine the energies and widths of single autoionization states or their groups by approximate formulas of the classical approach. The autoionization levels of the zinc atom responsible for the maxima observed are determined. Possible transfer of the orbital angular momentum between electrons is analyzed.

Out-of-plane (e,2e) angular distributions and energy spectra of heliumL=0,1,2autoionizing levels

Angular distribution and spectral ($e$,$2$$e$) measurements are reported for the helium autoionizing levels $(2{s}^{2}){}^{1}S$, $(2{p}^{2}){}^{1}D$, and $(2s2p){}^{1}P$. A special out-of-plane geometry is used where the ejected electrons are emitted in a plane perpendicular to the scattered electron direction. The kinematics are chosen so that this plane contains the momentum-transfer direction. While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the autoionizing levels and exhibits a characteristic shape for each orbital angular momentum $L=0,1,2$. A second-order model in the projectile-target interaction correctly reproduces the observed magnitudes of the recoil peaks, but is a factor of 2 too large in the central out-of-plane region. Observed ($e$,2$e$) energy spectra for the three resonances over the full angular range are well reproduced by the second-order calculation. Calculations using a first-order model fail to reproduce both the magnitudes of the recoil peaks and the spectral line profiles.

Detailed level-by-level study of dielectronic recombination for Ne-like gold

Ab initio calculation of the dielectronic recombination (DR) rate coefficient for Ne-like gold in the ground state 2s22p6 is performed. The value of DR rate coefficient has been calculated over a wide temperature range from 1 to 105 eV. The present level-by-level calculations include the contributions of all the levels belonging to the following Na-like doubly excited configuration complexes, namely, 2s22p53ln′l′, 2s12p63ln′l′ (n′⩽12, l′⩽8), 2s22p54ln′l′, 2s12p64ln′l′ (n′=4, 5) and 1s12s22p63l3l′. There are 18 853 doubly excited levels in all these complexes. The contributions of 2s22p53ln′l′ and 2s12p63ln′l′ for n′>12 are evaluated by applying the complex-by-complex n′−3 extrapolation method, which are found to represent at most 3 and 0.04% of the total DR rate coefficient, respectively. The contribution of complex series 2s22p53ln′l′ is a little more than 90% to the total DR rate coefficient. The contributions of 2s12p63ln′l′, 2s22p54ln′l′ and 2s12p64ln′l′ complexes are about 2, 6 and 1%, respectively. The DR contributions of 2s22p53ln′l′ and 2s12p63ln′l′ complexes with l′>6 are found to be negligible (about 0.6 and 0.8%). For complex series 2s22p53ln′l′, the dominant contribution comes from the lowest complex 2s22p53l3l′ at all temperatures. For complex series 2s12p63ln′l′, the dominant contribution comes from the 2s12p63l3l′ complex at low and medium electron temperature (kTe<10 keV), while at higher electron temperature (kTe>10 keV) the contribution of 2s12p63l4l′ complex becomes larger. For complex series 2s22p54ln′l′ and 2s12p64ln′l′, the contribution from n′=5 complex exceeds the contribution from n′=4 complex at about 2000 eV. The effects of radiative decays to autoionizing levels possibly followed by radiative cascades (DAC) are checked for the (2s2p)73l3l′ and (2s2p)73l4l′ complexes and found to be negligible. The total DR rate coefficient is fitted to an empirical formula.

Out-of-plane (e,2e) experiments on helium L = 0, 1, 2 autoionizing levels

We present out-of-plane (e, 2e) measurements (using an incident electron energy of 488 eV) and calculations for the helium L = 0, 1, and 2 autoionizing levels and for direct ionization [1]. While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the autoionizing levels and exhibits a characteristic shape for each orbital angular momentum L = 0, 1, 2. These findings can qualitatively be explained by an L-dependent addition to the ionization amplitude, but only a second-order model in the projectile–target interaction can quantitatively reproduce the observed magnitudes of the recoil peaks. We present the data as both angular distributions and energy spectra for the resonances. Preliminary experimental out-of-plane (e, 2e) (using incident electron of 150 eV) results will also be shown. The experimental part of this work is being carried out using an existing (e,2e) apparatus modified to allow the electron gun (see figure 2) to move on the surface of a (mathematical) cone [2]. This permits the measurement of out-of-plane (e,2e) angular distributions, for a full 360°, using a special geometry that allows out-of-plane conditions to be combined with the binary peak in a single measurement [2].

Resonance excitation process for Ni-like gold

A detailed and large-scale calculation on the resonant excitation rate coefficients from the ground state to the 106 fine-structure levels belonging to 3 l 17 4 l ′ ( l = 0 , 1 , 2 ; l ′ = 0 , 1 , 2 , 3 ) configurations of Ni-like gold have been performed using the relativistic distorted-wave approximation. The contributions through all possible Cu-like doubly excited states 3 l 17 4 l ′ n ″ l ″ and 3 l 17 5 l ′ n ″ l ″ ( n ″ ≤ 15 , l ″ ≤ 8 ) are calculated explicitly. The radiative damping effects on resonant excitation rates are studied. Significant effects arising from decays to autoionizing levels are also investigated. The n −3scaling law is investigated and is used to obtain the contributions from high- n configurations. Contributions from resonant excitation are found to be as important as direct excitation processes for most transitions. In some cases, resonant excitation can enhance the excitation rate coefficients by an order of magnitude. In addition, the total dielectronic recombination rate coefficients of Ni-like gold are also presented and compared with other works.

Low-lying dielectronic resonances of Fe XXII for accurate energy determination of autoionising Rydberg levels on boronlike cores

We present high resolution measurements of dielectronic recombination (DR) of Fe XXII forming doubly excited Fe XXI. These measurements were performed at the Test Storage Ring (TSR) of the Max-Planck-Institute of Nuclear Physics (MPIK). Low-lying DR resonances allow precise energy determination of the doubly excited autoionising levels in Fe XXI and, with suitable future calculations of Rydberg binding energies (n ≥ 7), of 2s22p to 2s2p2 excitation energies of the boronlike Fe XXII core.

Recent out-of-plane (e,2e) experiments on autoionizing levels of helium

Out-of-plane (e, 2e) measurements and calculations are reported for the helium autoionizing levels (2s2)1S, (2p2)1D, (2s2p)1P, and for direct ionization [1]. While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the autoionizing levels and exhibits a characteristic shape for each orbital angular momentum L = 0, 1, 2. These findings can qualitatively be explained by an L-dependent addition to the ionization amplitude, but only a second-order model in the projectile-target interaction can quantitatively reproduce the observed magnitudes of the recoil peaks. We present the data as both angular distributions and energy spectra for the resonances.The experimental part of this work is being carried out using an existing (e,2e) apparatus modied to allow the electron gun (see figure 1) to move on the surface of a (mathematical) cone [2]. This permits the measurement of out-of-plane (e,2e) angular distributions, for a full 360, using a special geometry that allows out-of-plane conditions to be combined with the binary peak in a single measurement. Details of the apparatus will be presented.

The effect of configuration complex on dielectronic recombination process in highly ionized plasmas

This paper analyses the effect of configuration complex on dielectronic recombination (DR) process in highly ionized plasmas (Xe26+, Dy38+, W46+) by using the multiconfiguration relativistic Hartree–Fock method. Resonant and nonresonant radiative stabilizing transitions and decays to autoionizing levels followed by radiative cascades are included. Collisional transitions following electron capture are neglected. The remarkable difference between the isoelectronic trend of the rate coefficients for DR through 3d94l4l′ and through 3d94l5l′ is emphasized. The trend of DR through 3d94l4l′ shows irregularities at relatively low temperature due to the progressive closing of DR channels as atomic number Z increases.

Total dielectronic recombination rate coefficient for Co-like tungsten

Ab initio calculation of the total dielectronic recombination (DR) rate coefficient from the ground state 3 s 23 p 63 d 9( J=5/2) of Co-like tungsten is performed employing the relativistic distorted-wave approximation with configuration-interaction. The DR contributions mainly come from complex series 3 d 84 ln′ l′. The complex series 3 p 53 d 10 n′ l′, 3 p 53 d 94 ln′ l′ and 3 d 85 ln′ l′ also contribute significantly to the total DR rates at relatively high electron temperatures. The l′ and n′ dependences of the partial rate coefficient are investigated. The inclusion of decays into autoionizing levels followed by radiative cascades (DAC) enlarges the total DR rate coefficients by a factor of about 10%. The level-by-level extrapolation method is developed to include DAC effects. The total DR rate coefficients are fitted to an empirical formula. It is shown that at temperatures above 2.5 keV the Burgess–Merts (BM) semiempirical formula can provide DR results with an accuracy of about 15%, whereas at electron temperatures below 100 eV it underestimates the DR rate coefficients by up to a few orders of magnitude and its temperature dependence is completely inadequate. The comparison of the results for Ni-like and Co-like tungsten shows that these two sets of DR rate coefficients are very close in magnitude at relatively high electron temperatures.

Out-of-Plane (e,2e) Experiments on HeliumL=0, 1, 2 Autoionizing Levels

Out-of-plane (e, 2e) measurements are reported for the helium autoionizing levels (2s2)1S, (2p2)1D, (2s2p)1P, and for direct ionization. While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the autoionizing levels and exhibits a characteristic shape for each orbital angular momentum L=0, 1, 2. A second-order model in the projectile-target interaction correctly reproduces the observed magnitudes of the recoil peaks, but is a factor of 2 too large in the central out-of-plane region.

Effects of Configuration Interaction on Dielectronic Recombination of Cu-Like Gold Ion through 3d + e → 4l'4l'' Capture Process

Effects of configuration interaction on dielectronic recombination of Cu-like gold ions through the 3d + e → 4l'4l'' capture process are studied in the models of isolated configurations approximation, configuration mixing within 3d9 4s4l'4l'', and configuration mixing within 3d9 4l4l'4l'' (l ≥ 0), employing a relativistic distorted-wave approximation. Nonresonant raditive stabilizing transitions and decays to autoionizing levels followed by radiative cascades, as well as resonant stabilizing transitions from the resonant levels, are taken into account. The additional mixing with 3d94l4l'4l'' (0 < l ≤ l' ≤ l'' ≤ 3) shifts slightly the peaks of cross section towards low energy. The rate coefficient at low temperature is dramatically changed. The rate coefficient at temperature above 100 eV is enhanced by a factor between 26% and 13%.

Dielectronic recombination of Co-like gold ions

Ab initio calculation of the total dielectronic recombination (DR) rate coefficient from the ground and the first excited states of Co-like gold is performed employing the relativistic distorted-wave approximation with configuration interaction. The DR contributions are explicitly taken into account from the relevant complexes of a Ni-like ion: 3d33/2 3d65/2n′l′, 3p5 3d10n′l′, 3s 3p63d10n′l′, 3d8 4ln′l′, 3p5 3d9 4ln′l′ and 3s 3p6 3d9 4ln′l′ with n′ ⩽ 25, and 3d8 5ln′l′ with n′ ⩽ 9. The contributions from a higher n′ complex are evaluated by an extrapolation procedure. The DR contributions mainly come from complex series 3d8 4ln′l′ and 3p5 3d9 4ln′l′. The complex series 3p5 3d10n′l′ and 3d8 5ln′l′ also contribute significantly to the total DR rates at low and high electron temperatures, respectively. The l′ and n′ dependences of the partial rate coefficient are investigated. The possible important decays into autoionizing levels followed by radiative cascade (DAC) from the resonant levels are taken into account, as well as the resonant stabilizing and non-resonant stabilizing transitions. The inclusion of DAC transitions enlarges the total DR rate coefficients by a factor of about 10% and may break down the usual n′−3 scaling law of the partial DR rates along some complex series. To evaluate the high n′ contributions from these complex series, the level-by-level extrapolation method is developed to include DAC effects. The total DR rate coefficients are fitted to an empirical formula. The present results are compared with those from the semiempirical Burgess–Merts approximation. The DR rate coefficients of Ni-like gold are also presented and compared to those of Co-like gold. In addition, some comments on the published DR data for the NiI isoelectronic sequence are drawn from the present calculation.

Resonance-excitation process for Ni-like tantalum

A detailed large-scale calculation on the resonant excitation rate coefficients from the ground state to the 106 fine-structure levels belonging to $3{l}^{17}4{l}^{\ensuremath{'}}$ ($l=0,1,2$; ${l}^{\ensuremath{'}}=0,1,2,3$) configurations of Ni-like tantalum have been performed using the relativistic distorted-wave approximation. The contributions through all possible Cu-like doubly excited states $3{l}^{17}4{l}^{\ensuremath{'}}{n}^{\ensuremath{''}}{l}^{\ensuremath{''}}$ and $3{l}^{17}5{l}^{\ensuremath{'}}{n}^{\ensuremath{''}}{l}^{\ensuremath{''}}$ (${n}^{\ensuremath{''}}\ensuremath{\leqslant}15$, ${l}^{\ensuremath{''}}\ensuremath{\leqslant}8$) are calculated. The validity of ${n}^{\ensuremath{''}\ensuremath{-}3}$ scaling law is investigated. The radiative damping effects on resonant excitation rates are studied. The significant effects arising from decays to autoionizing levels are also investigated. The contributions from resonant excitation are found to be as important as direct excitation processes for most transitions. In some cases, the resonant excitation can enhance the excitation rate coefficients by an order of magnitude. Large discrepancies between the present resonant excitation rate coefficients with previously published values are found, and the present results should be more reliable and accurate.

Resonance excitation rate coefficients of Ni-like tungsten

A detailed and large-scale calculation on the resonant excitation rate coefficients from the ground state to the 106 fine-structure levels belonging to 3l174l′(l = 0, 1, 2; l′ = 0, 1, 2, 3) configurations of Ni-like tungsten has been performed using the relativistic distorted-wave method. The contributions through all possible Cu-like doubly excited states 3l174l′nl″ and 3l175l′nl″(n ⩽ 15, l″ ⩽ 8) are calculated explicitly. The n−3 scaling law is investigated and is used to obtain the contributions from high-n configurations. The radiative damping effects on resonant excitation rates are studied. The effects arising from the inclusion of decays to autoionizing levels are also investigated. The contributions from resonant excitation are found to be as important as the direct-excitation processes for most transitions. In some cases, the resonant excitation can enhance the excitation rate coefficients by a factor of an order of magnitude. Because all possible resonances associated with the ▵n = 1 and ▵n = 2 core excitations and the effects of decays into autoionizing levels from the resonant states are taken into account, large discrepancies between the present excitation rate coefficients and R-matrix values are found.

Triple-resonance autoionization of uranium optimized for diode laser excitation

The photoionization of uranium via three-step excitation has been optimized for isotope selective trace analysis. A search found 13 new J = 6, 7, and 8 odd-parity states in the 36,850–37,200 cm − 1 region that allow blue–red–red three-photon excitation at wavelengths favorable for commercial diode laser systems. From each of these 13 states, near-threshold autoionization spectra were recorded at a resolution of 3 × 10 − 4 cm − 1 . Some 30 even-parity autoionizing levels with J = 5 to 9 have also been observed and characterized. Comparison of the spectra allows J assignment for the new intermediate levels as well as selection of an optimized path for trace analysis. We show that, for a chosen scheme using a narrow J = 9 autoionizing resonance at 49,972 cm − 1 , all of the excitation steps can be saturated with powers available from single-mode cw diode laser systems when exciting an atomic beam in perpendicular geometry.