Dielectronic Recombination Cross Sections Research Articles

Dielectronic-recombination cross sections of hydrogenlike argon.

Relative dielectronic-recombination cross sections for hydrogenlike argon are presented. The contributions of the KLL, KLM, KLN, KLO, and KLP groups of resonances are compared to theoretical calculations. The experimental method consists of the formation and interaction of ions with electrons in an ion trap followed by an analysis of the extracted ions to determine relative yields. Comparison with theory shows that the total cross sections agree within \ifmmode\pm\else\textpm\fi{}6%.

Dielectronic recombination of light Be-like and B-like ions

Dielectronic recombination cross sections from the ground and metastable states of the Be-like ions C2+, O4+ and F5+, and the B-like ions O3+ and F4+, have been calculated and compared with experimental results obtained by a merged-beams technique. The comparison of theory and experiment is complicated by the presence of weak electric fields in the interaction region. However, the authors qualitatively explain the main features of their experimental results. In particular, for Be-like ions they find evidence of a large contribution from the initial 2s2p 3P metastable state recombining into an Auger metastable final state.

Dielectronic recombination on and electron-impact excitation of heliumlike argon.

We have measured cross sections for \ensuremath{\Delta}n=1 dielectronic recombination (DR) on He-like argon and found good agreement with theoretical calculations based on the Hartree-Fock atomic model. Experimental absolute cross sections were obtained by using the electron-energy dependence of yields of He-like and Li-like argon ions from the Kansas State University electron-beam ion source to measure the ratio of the DR cross section on He-like argon to the electron-impact-ionization cross section of Li-like argon and normalizing to the latter. The K x-ray emission spectra due to \ensuremath{\Delta}n=1,2 DR and n=1\ensuremath{\rightarrow}2 electron-impact excitation of He-like argon were also observed with a Si(Li) detector placed at 0\ifmmode^\circ\else\textdegree\fi{} relative to the electron-beam direction. By normalizing to the theoretical KLL integrated DR differential cross section, we obtained differential and partial differential DR cross sections and differential electron-impact excitation cross sections. We found good agreement with Hartree-Fock calculations for DR and with distorted-wave calculations for electron-impact excitation.

Physics of and recent results from the Lawrence Livermore EBIT source

The electron beam ion trap at Lawrence Livermore National Laboratory is used to produce and trap very highly charged ions for the study of electron-ion interactions. Measurements of X-ray transitions in neonlike thorium (Th 80+), ionization cross sections of lithiumlike barium (Ba 53+), and of level-specific dielectronic recombination cross sections of helium-like iron (Fe 24+) are presented.

Relativistic Auger rates for dielectronic recombination with highly-charged few-electron ions

Dielectronic recombination (DR) cross sections are calculated for high-Z, two-electron ions using a distorted wave theory. We show DR cross sections for KLL dielectronic recombination of hydrogen-likeU ions and the corresponding Auger rates. The Auger rates are calculated with correlated relativistic wave functions and include the Breit interaction. The dependence of the Auger rates on the nuclear chargeZ is also investigated.

Validity of the independent-processes approximation for resonance structures in electron-ion scattering cross sections.

The total inelastic cross section for electron-ion scattering may be found in the independent-processes approximation by adding the resonant cross section to the nonresonant background cross section. We study the validity of this approximation for electron excitation of multiply charged ions. The resonant-excitation cross section is calculated independently using distorted waves for various Li-like and Na-like ions using (N+1)-electron atomic-structure methods previously developed for the calculation of dielectronic-recombination cross sections. To check the effects of interference between the two scattering processes, we also carry out detailed close-coupling calculations for the same atomic ions using the R-matrix method. For low ionization stages, interference effects manifest themselves sometimes as strong window features in the close-coupling cross section, which are not present in the independent-processes cross section. For higher ionization stages, however, the resonance features found in the independent-processes approximation are found to be in good agreement with the close-coupling results.

Dielectronic recombination of Bi80+ and Bi82+

We have calculated dielectronic recombination (DR) cross sections for \ensuremath{\Delta}n=0 transitions in Li-like Bi using the semirelativistic version of the autostructure code. We present results for the full Rydberg series associated with the 2s\ensuremath{\rightarrow}2p\ifmmode\bar\else\textasciimacron\fi{} (j=1/2) core excitation, which opens up at n=20 and spans only 16 Ry, compared to the series associated with the 2s\ensuremath{\rightarrow}2p (j=3/2) excitation, which opens up at n=6 and spans 200 Ry. We have also calculated fully relativistic and semirelativistic KLL DR cross sections for H-like Bi and find the effect of the Breit interaction to be important.

Dielectronic capture processes in highly charged uranium ions.

Dielectronic capture processes in highly charged uranium ions are studied in the isolated-resonance, distorted-wave approximation. A fully relativistic Dirac-Fock method is employed to calculate resonant transfer and excitation cross sections for collisions of ${\mathrm{U}}^{90+}$ and ${\mathrm{U}}^{89+}$ with ${\mathrm{H}}_{2}$ and dielectronic recombination cross sections associated with the 2p\ensuremath{\rightarrow}2s stabilizing transition of ${\mathrm{U}}^{89+}$. A semirelativistic method based on the previously developed autostructure code is found to give cross sections in good agreement with the Dirac-Fock method in all cases. When the Dirac-Fock method is further extended to include the effect of the two-body Breit interaction, substantial changes are found in certain low-energy cross sections associated with K-shell resonant transfer and excitation, but little change is found in the \ensuremath{\Delta}n=0 dielectronic recombination cross sections.

Resonant electron capture by the metastable C V and O VII ions

Dielectronic recombination rates and cross sections for the metastable C V and O VII ions are calculated and found to be extremely large, with cross sections of the order of 10−15 cm2 and rates of the order of 10−9 cm3 s−1 for individual resonances. Electric field enhancement and composition of the metastable 1s2s ion beams in terms of (1S) and (3S) are used to reproduce the recent experimental data of Andersen et al. [Phys. Rev. Lett. 62, 2656 (1989)] for O VII and unpublished data for C V. The overall features are well explained in the case of carbon, while the features for oxygen are well explained except for the broad prominent peaks at 2.5 and 6.8 eV, which are only partially explained.

Auger rates for dielectronic recombination cross sections with highly charged relativistic heavy ions

Relativistic wave functions and corrections to the Coulomb interaction between two electrons are used to calculate Auger rates and the cross section for dielectronic recombination (DR). Results are given for hydrogen-like Pb ions. The cross section of the DR is compared with an estimate of the cross section of the direct radiative electron capture.

Dielectronic recombination cross sections for H-like ions.

Dielectronic recombination cross sections for several H-like atomic ions are calculated in an isolated-resonance, distorted-wave approximation. Fine-structure and configuration-interaction effects are examined in detail for the ${\mathrm{O}}^{7+}$ cross section. Hartree-Fock, intermediate-coupled, multiconfiguration dielectronic recombination cross sections for ${\mathrm{O}}^{7+}$ are then compared with the recent experimental measurements obtained with the Test Storage Ring in Heidelberg. The cross-section spectra line up well in energy and the shape of the main resonance structures are comparable. The experimental integrated cross sections differ by up to 20% from theory, but this may be due in part to uncertainties in the electron distribution function.

Dielectronic recombination of hydrogenlike oxygen in a heavy-ion storage ring.

State-resolved cross sections for the dielectronic recombination of hydrogenlike oxygen ions with free electrons have been measured for the first time using the electron cooling device in the heavy-ion Test Storage Ring in Heidelberg. Energies and cross sections for individual terms of the configuration 2l2l' were determined. Two-electron resonances 2lnl' with n\ensuremath{\ge}3 contribute 90% to the total dielectronic-recombination cross section. Energies and cross sections are in reasonable agreement with available calculations.

Dielectronic recombination on heliumlike argon.

We have used the electron-energy dependence of yields of heliumlike and lithiumlike argon ions from the Kansas State University electron-beam ions source (EBIS) to measure the ratio of the cross section for \ensuremath{\Delta} n=1 dielectronic recombination of heliumlike argon to that for electron ionization of lithiumlike argon. By normalizing to the latter cross section we obtain absolute dielectronic recombination cross sections and find good agreement with theoretical calculations for the lower-energy resonances.

Dielectronic recombination for C VI, and O VII,VII.

Dielectronic recombination (DR) cross sections and rate coefficients for the H-like and He-like C and O ions are calculated in the zero-field, zero-density limit. The peak rates are approximately (1.2--1.6){times}10{sup {minus}12} cm{sup 3}/sec for all four ions. For the H-like ions, the contribution to total rates from high Rydberg states ({ital n}{ge}6) is found large, accounting for nearly one-half of the total rates. Agreements with earlier estimates are at a {plus minus}5% level. For the He-like ions, the high-Rydberg-state contribution is strongly suppressed because of the extra Auger channel which opens up at {ital n}=5--7. Agreement with previous calculations is again at a 5% level. The DR cross sections for the initial metastable states of the He-like ions are also estimated for a few low-lying resonance states near the DR threshold. The cross section for the 2{ital s} {sup 1}S{r arrow}1s2p {sup 1}P+nl excitation capture is found to be very large.

Distorted-wave calculations of dielectronic recombination for C3+ and O5+ in small electric fields.

The dielectronic recombination cross sections associated with the 2s\ensuremath{\rightarrow}2p transition in Li-like carbon and oxygen ions in the presence of small electric fields have been calculated in the distorted-wave approximation. By convoluting the product of the electron velocity and the theoretical cross section with the experimental velocity distribution, we compare our calculations with recent measurements of high-resolution recombination rates 〈v\ensuremath{\sigma}(v)〉 by Andersen et al. With the exception of the magnitude of the low-energy peak due to the 2p4l resonances, the agreement between experiment and theory is excellent in ${\mathrm{C}}^{3+}$ for a field in the interaction region of just over 3 V/cm. In the case of ${\mathrm{O}}^{5+}$, however, there is good agreement between experiment and theory for the low-Rydberg states, but there appear to be discrepancies between the measured rates and the theoretical rates for the high-Rydberg states, where the effects of fields are important.

Dielectronic recombination of O IV.

Dielectronic recombination (DR) rate coefficients and cross sections are estimated for the O{sup 3+} ion where the 2{ital s} and 2{ital p} electrons of the initial state are excited to higher, {ital n}{ge}3 states ({Delta}{ital n}{sub {ital t}}{ne}0 transitions) during the excitation-capture part of the process. This work completes the calculations of DR parameters for the O{sup 3+} ion, where the {Delta}{ital n}{sub {ital t}}=0 part of this process had been reported recently. We found that the 2{ital p} electron excitation dominates the {Delta}{ital n}{sub {ital t}}{ne}0 process, while the 2{ital s} excitation is suppressed by the cascade corrections that strongly affect the intermediate states 1{ital s}{sup 2}2s2p3snl and 1{ital s}{sup 2}2s2p3dnl. The 2{ital s} contribution is approximately 15% of the {Delta}{ital n}{sub {ital t}}{ne}0 mode, while the total {Delta}{ital n}{sub {ital t}}{ne}0 contribution is roughly 10% of the {Delta}{ital n}{sub {ital t}}=0 cross section.

Dielectronic recombination for metastable O

Dielectronic recombination cross sections for the metastable O vii and C v ions are calculated and found to be extremely large, of the order of ${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$ for the individual resonances. The rates are at the level of ${10}^{\mathrm{\ensuremath{-}}9}$ ${\mathrm{cm}}^{3}$/sec. With some field enhancement and assuming that the initial ion beam is a mixture of metastable 1s2s${(}^{1}$S and $^{3})$ states, the overall feature of the recent experimental data of Andersen et al. [Phys. Rev. Lett. 62, 2656 (1989)] is reproduced, including the broad peaks at the incident-electron kinetic energies of 4.7 and 12.5 eV, and partially also at 2.5 and 6.8 eV. With a slightly different mixture, the C C v data are well reproduced.

Test of predictedΔn≥1L-shell dielectronic-recombination cross sections

The predicted charge-state dependence of dielectronic-recombination cross sections involving \ensuremath{\Delta}n>=1 L-shell excitation has been tested by measurements of resonant transfer and excitation (RTE) for $_{41}\mathrm{Nb}^{\mathrm{q}+}$+${\mathrm{H}}_{2}$ (q=28 to 32) collisions at 3.7 and 4.0 MeV/u. The measured RTE cross sections increase significantly with increasing charge state and thus disagree substantially with theoretical calculations that predict essentially equal cross sections for ${\mathrm{Nb}}^{29+}$, ${\mathrm{Nb}}^{30+}$, and ${\mathrm{Nb}}^{31+}$. This disagreement is contrary to the good agreement previously found between theory and experiment for K-shell RTE.

Storage rings, a new tool for atomic physics

We first introduce the principles of heavy ion storage rings and present the CRYRING as an example. Then we discuss different cooling schemes such as stochastic, electron and laser cooling, and the beam intensity limitations as well as luminosity gain factors. Finally we give some examples for experiments which study the interaction of ions with electrons and photons, utilizing the electron cooler in a storage ring. Among those are the processes of radiative recombination with bare ions, laser induced recombination, and the resonance structure of, the cross section for dielectronic recombination. Also a molecular dynamics simulation of the crystallization of cooled ions stored in CRYRING is reported.

Proposed measurements of dielectronic recombination using the Indiana cooler facility

It is proposed to measure dielectronic recombination (DR) using the IUCF Cooler. This process, which involves the electron-electron interaction, takes place when electron capture is accompanied by simultaneous excitation of the interacting ion resulting in the formation of a doubly-excited intermediate state. The IUCF Cooler, with its high electron beam intensity and high electron-energy resolution, provides a unique opportunity for investigating this fundamental atomic collision process. Specifically, it is proposed to measure DR for collisions of He + ions with electrons in the Cooler at relative energies (in the rest frame of the ion) of about 30–40 eV. The DR cross sections obtained will be used to test theoretical calculations of this correlated two-electron process, which apart from their fundamental interest, have applications to astrophysical studies and to the development of nuclear fusion plasmas.