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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Ion Storage Ring Measurements of Low Temperature Dielectronic Recombination Rate Coefficients for Modeling X-Ray Photoionized Cosmic Plasmas

Low temperature dielectronic recombination (DR) is the dominant recombination mechanism for most ions in X-ray photoionized cosmic plasmas. Reliably modeling and interpreting spectra from these plasmas requires accurate low temperature DR rate Coefficients. Of particular importance are the DR rate coefficients for the iron L-shell ions (Fe XVII-Fe XXIV). These ions are predicted to play an important role in determining the thermal structure and line emission of X-ray photoionized plasmas, which form in the media surrounding accretion powered sources such as X-ray binaries (XRBs), active galactic nuclei (AGN), and cataclysmic variables (Savin et al., 2000). The need for reliable DR data of iron L-shell ions has become particularly urgent after the launches of Chandra and XMM-Newton. These satellites are now providing high-resolution X-ray spectra from a wide range of X-ray photoionized sources. Interpreting the spectra from these sources requires reliable DR rate coefficients. However, at the temperatures relevant, for X-ray photoionized plasmas, existing theoretical DR rate coefficients can differ from one another by factors of two to orders of magnitudes.

Savin, D. W.↗
DOE OSTI2013

SUPPRESSION OF DIELECTRONIC RECOMBINATION DUE TO FINITE DENSITY EFFECTS

We have developed a general model for determining density-dependent effective dielectronic recombination (DR) rate coefficients in order to explore finite-density effects on the ionization balance of plasmas. Our model consists of multiplying by a suppression factor those highly-accurate total zero-density DR rate coefficients which have been produced from state-of-the-art theoretical calculations and which have been benchmarked by experiment. The suppression factor is based upon earlier detailed collision-radiative calculations which were made for a wide range of ions at various densities and temperatures, but used a simplified treatment of DR. A general suppression formula is then developed as a function of isoelectronic sequence, charge, density, and temperature. These density-dependent effective DR rate coefficients are then used in the plasma simulation code Cloudy to compute ionization balance curves for both collisionally ionized and photoionized plasmas at very low (n{sub e} = 1 cm{sup -3}) and finite (n{sub e} = 10{sup 10} cm{sup -3}) densities. We find that the denser case is significantly more ionized due to suppression of DR, warranting further studies of density effects on DR by detailed collisional-radiative calculations which utilize state-of-the-art partial DR rate coefficients. This is expected to impact the predictions of the ionization balance in denser cosmic gases such as those found in nova and supernova shells, accretion disks, and the broad emission line regions in active galactic nuclei.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Theoretical investigation of dielectronic recombination of Sn{sup 12+} ions

Theoretical calculations have been made for the dielectronic recombination (DR) rate coefficients of Sn{sup 12+} ion using a relativistic flexible atomic code with configuration interaction. Comparison of the rate coefficients for 4s, 4p, and 4d subshell excitation shows that while the 4p subshell excitation dominates over the whole temperature region, 4d subshell excitation at low temperature and 4s subshell excitation at high temperature cannot be neglected. In order to facilitate simple applications, the calculated DR rate coefficients are fitted to an empirical formula. The total DR rate coefficient is greater than either the radiative recombination or three-body recombination coefficients for electron temperatures greater than 1 eV. Therefore, DR can strongly influence the ionization balance of laser-produced tin plasmas.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

EFFECTS OF CONFIGURATION INTERACTION FOR DIELECTRONIC RECOMBINATION OF Na-LIKE IONS FORMING Mg-LIKE IONS

Theoretical dielectronic recombination (DR) rate coefficient calculations can be sensitive to configuration interaction (CI) between resonances with different captured electron principle quantum numbers n. Here we explore the importance of this multi-n CI process for DR via 2l {yields} 3l' core excitations and its effect on the total DR rate coefficient. Results are presented for selected Na-like ions from Ca{sup 9+} to Zn{sup 19+}. We find that including this multi-n CI can reduce the DR rate coefficient by up to {approx}10% at temperatures where an ion is predicted to form in collisional ionization equilibrium and up to {approx}15% at higher temperatures. To a first approximation, this will translate into a corresponding increase in the ion abundance. Charge state distributions calculation seeking to be accurate to better than 10% will thus need to take this effect into account. We also present simple fits to the calculated rate coefficients for ease of incorporation into plasma models.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS2000

Modeling X-Ray Photoionized Plasmas: Ion Storage Ring Measurements of Low Temperature Dielectronic Recombination Rate Coefficients for L-Shell Iron

Iron L-shell ions (Fe XVII to Fe XXIV) play an important role in determining the line emission and thermal and ionization structures of photoionized gases. Existing uncertainties in the theoretical low temperature dielectronic recombination (DR) rate coefficients for these ions significantly affects our ability to model and interpret observations of photoionized plasmas. To help address this issue, we have initiated a laboratory program to produce reliable low temperature DR rates. Here, we present some of our recent results and discuss some of their astrophysical implications.

Savin, D. W.↗
DOE OSTI2018

Iso-nuclear tungsten dielectronic recombination rates for use in magnetically-confined fusion plasmas

Under the auspices of the IAEA Atomic and Molecular Data Center and the Korean Atomic Energy Research Institute, our assembled group of authors has reviewed the current state of dielectronic recombination (DR) rate coefficients for various ion stages of tungsten (W). Subsequent recommendations were based upon available experimental data, first-principle calculations carried out in support of this paper and from available recombination data within existing atomic databases. If a recommendation was possible, data were compiled, evaluated and fitted to a functional form with associated uncertainty information retained, where available. This paper also considers the variation of the W fractional abundance due to the underlying atomic data when employing different data sets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1992

Dielectronic recombination as a function of electric field strength

Dielectronic recombination (DR) is the dominant recombination mechanism at coronal temperatures and densities. We present a procedure for calculating DR rate coefficients as a function of electric field strength and apply this method to carbon ions. We focus on the competing effects of enhancement by plasma microfields and rate decrease through collisional excitation and ionization. We find that, in the case of C(3+), a significant rate enhancement results, leading to a reinterpretation of C IV emission-line intensities in the sun and late-type stars. We further consider how macroscopic electric fields, in particular motional electric fields, can affect DR rate coefficients, demonstrating dramatic rate increases for a number of the carbon ions.

Reisenfeld, Daniel B.↗
NASA NTRS1998

Theoretical Studies of Important Processes in Planetary and Comet Atmospheres

Using theoretical quantum chemical calculations, I have successfully described the dissociative recombination (DR) of O2(+) leading to the excited S-1 state of atomic oxygen, the upper state of the well known green line emission. The process is described by O2(+) + e(-) yields O(S-1)) + O(D-1) (1) where e(-) is an electron and the product oxygen atoms are both excited. This process is important in the atmospheres of Venus, Mars and Earth. I have shown in prior calculations that only one repulsive potential curve of O2, f(sup l)Sigma(sub u, sup +) can generate O(S-1) from DR of the lowest vibrational levels of O2(+). However, in the prior results, the calculated quantum yield (i.e. the number of O(S-1) atoms produced for every two product atoms) from the O2(+) v = 0 level was smaller than the laboratory and atmospheric measurements by more than an order of magnitude. Including only direct recombination, the calculated quantum yield for O(S-1) is only 0.0016. In a calculation that accounts for both direct and indirect recombination, the quantum yield is 0.0012. The range of experimentally determined quantum yields is between 0.01 and 0.23. Because of this large difference between the theoretical and experimental quantum yields, it was thought for some time that the ionospheric and laboratory O2(+) must be vibrationally excited since for excited levels, theory gave quantum yields that are similar to experimental yields. It was also suggested that some other process was generating O(S-1) but none could be identified. Under current NASA support, I have found that reaction (1) proceeds via an unusual mechanism. The f(sup 1)Sigma(sub u, sup,+) state does not cross the ion between the turning points of the v = 0 level of the O2(+) ground state. The lack of a favorable crossing leads to a very small calculated DR rate coefficient. However, this mechanism assumes that initial electron capture must occur into the repulsive state that leads to )(S-1). Instead, I have found that initial electron capture occurs mostly into the B(sup 3)Sigma(sup u, sub -) state which crosses the ion between the turning points of the v = 0 ion level and has a large DR rate coefficient. The B state dissociates to O(D-1) and )(P-3). After capture, some of the flux is transferred to the f(sup 1)Sigma(sup u, sub +) state via symmetry mixed intermediate Rydberg states. The neutral Rydberg states are a mixture of Sigma(sub u, sup +)-1 and Sigma(sub u, sup -)-3 symmetry.

Guberman, Steven L.↗
DOE OSTI2015

Bootstrapping dielectronic recombination from second-row elements and the orion nebula

Dielectronic recombination (DR) is the dominant recombination process for most heavy elements in photoionized clouds. Accurate DR rates for a species can be predicted when the positions of autoionizing states are known. Unfortunately such data are not available for most third- and higher-row elements. This introduces an uncertainty that is especially acute for photoionized clouds, where the low temperatures mean that DR occurs energetically through very low-lying autoionizing states. This paper discusses S{sup 2+} → S{sup +} DR, the process that is largely responsible for establishing the [S iii]/[S ii] ratio in nebulae. We derive an empirical rate coefficient using a novel method for second-row ions, which do have accurate data. Photoionization models are used to reproduce the [O iii]/[O ii]/[O i]/[Ne iii] intensity ratios in central regions of the Orion Nebula. O and Ne have accurate atomic data and can be used to derive an empirical S{sup 2+} → S{sup +} DR rate coefficient at ∼10{sup 4} K. We present new calculations of the DR rate coefficient for S{sup 2+} → S{sup +} and quantify how uncertainties in the autoionizing level positions affect it. The empirical and theoretical results are combined and we derive a simple fit to the resulting rate coefficient at all temperatures for incorporation into spectral synthesis codes. This method can be used to derive empirical DR rates for other ions, provided that good observations of several stages of ionization of O and Ne are available.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2011

Atomic Data for Neutron-capture Elements I. Photoionization and Recombination Properties of Low-charge Selenium Ions

We present multi-configuration Breit-Pauli AUTOSTRUCTURE calculations of distorted-wave photoionization (PI) cross sections. and total and partial final-state resolved radiative recombination (RR) and dielectronic recombination (DR) rate coefficients for the first six ions of the trans-iron element Se. These calculations were motivated by the recent detection of Se emission lines in a large number of planetary nebulae. Se is a potentially useful tracer of neutron-capture nucleosynthesis. but accurate determinations of its abundance in photoionized nebulae have been hindered by the lack of atomic data governing its ionization balance. Our calculations were carried out in intermediate coupling with semi re1ativistic radial wavefunctions. PI and recombination data were determined for levels within the ground configuration of each ion, and experimental PI cross-section measurements were used to benchmark our results. For DR, we allowed (Delta)n = 0 core excitations, which are important at photoionized plasma temperatures. We find that DR is the dominant recombination process for each of these Se ions at temperatures representative of photoionized nebulae (approx.10(exp 4) K). In order to estimate the uncertainties of these data, we compared results from three different configuration-interaction expansions for each ion, and also tested the sensitivity of the results to the radial scaling factors in the structure calculations. We find that the internal uncertainties are typically 30-50% for the direct PI cross sections and approx.10% for the computed RR rate coefficients, while those for low-temperature DR can be considerably larger (from 15-30% up to two orders of magnitude) due to the unknown energies of near-threshold autoionization resonances. These data are available at the CDS, and fitting coefficients to the total RR and DR rate coefficients are presented. The results are suitable for incorporation into photoionization codes used to numerically simulate astrophysical nebulae, and will enable robust determinations of nebular Se abundances.

Sterling, N. C.↗
DOE OSTI2017

Statistical dielectronic recombination rates for multielectron ions in plasma

We describe the general analytic derivation of the dielectronic recombination (DR) rate coefficient for multielectron ions in a plasma based on the statistical theory of an atom in terms of the spatial distribution of the atomic electron density. The dielectronic recombination rates for complex multielectron tungsten ions are calculated numerically in a wide range of variation of the plasma temperature, which is important for modern nuclear fusion studies. The results of statistical theory are compared with the data obtained using level-by-level codes ADPAK, FAC, HULLAC, and experimental results. We consider different statistical DR models based on the Thomas–Fermi distribution, viz., integral and differential with respect to the orbital angular momenta of the ion core and the trapped electron, as well as the Rost model, which is an analog of the Frank–Condon model as applied to atomic structures. In view of its universality and relative simplicity, the statistical approach can be used for obtaining express estimates of the dielectronic recombination rate coefficients in complex calculations of the parameters of the thermonuclear plasmas. The application of statistical methods also provides information for the dielectronic recombination rates with much smaller computer time expenditures as compared to available level-by-level codes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Dissociative recombination and mutual neutralization of heavier molecular ions: C{sub 10}H{sub 8}{sup +}, WF{sub 5}{sup +}, and C{sub n}F{sub m}{sup +}

Dissociative recombination (DR) rate coefficients for the naphthalene cation, C{sub 10}H{sub 8}{sup +}, and WF{sub 5}{sup +}, and mutual neutralization (MN) rate coefficients for these species and five C{sub n}F{sub m}{sup +} ions, were determined at 300 K using variable electron and neutral density attachment mass spectrometry (VENDAMS). DR proceeds at 9 ± 3 × 10{sup −7} cm{sup 3} s{sup −1} for C{sub 10}H{sub 8}{sup +} and at 6.1 ± 1.4 × 10{sup −7} cm{sup 3} s{sup −1} for WF{sub 5}{sup +}. Consistent with previous results, MN for the polyatomic cations with the halide anions Cl{sup −}, Br{sup −}, and I{sup −} exhibits an approximate μ{sup −1/2} reduced mass dependence of the reactant partners, demonstrating that ion collision velocities influence the rate coefficients. This work is an extension of VENDAMS to systems, where low reactant concentrations are necessary to avoid significant reaction of product ions with the neutral precursor, i.e., conditions not suitable for traditional flowing afterglow measurements, as well as to ions of masses > ∼ 100 Da, which are not amenable to the study of DR in magnetic storage rings. Our results expand the sparse literature on DR and MN of heavier ions.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS2003

Effect of Viscosity on the Crystallization of Undercooled Liquids

There have been numerous studies of glasses indicating that low-gravity processing enhances glass formation. NASA PI s are investigating the effect of low-g processing on the nucleation and crystal growth rates. Dr. Ethridge is investigating a potential mechanism for glass crystallization involving shear thinning of liquids in 1-g. For shear thinning liquids, low-g (low convection) processing will enhance glass formation. The study of the viscosity of glass forming substances at low shear rates is important to understand these new crystallization mechanisms. The temperature dependence of the viscosity of undercooled liquids is also very important for NASA s containerless processing studies. In general, the viscosity of undercooled liquids is not known, yet knowledge of viscosity is required for crystallization calculations. Many researchers have used the Turnbull equation in error. Subsequent nucleation and crystallization calculations can be in error by many orders of magnitude. This demonstrates the requirement for better methods for interpolating and extrapolating the viscosity of undercooled liquids. This is also true for undercooled water. Since amorphous water ice is the predominant form of water in the universe, astrophysicists have modeled the crystallization of amorphous water ice with viscosity relations that may be in error by five orders-of-magnitude.

Source record↗
NASA NTRS1991

Dissociative recombination of the ground state of N2(+)

Large-scale calculations of the dissociative recombination cross sections and rates for the v = 0 level of the N2(+) ground state are reported, and the important role played by vibrationally excited Rydberg states lying both below and above the v = 0 level of the ion is demonstrated. The large-scale electronic wave function calculations were done using triple zeta plus polarization nuclear-centered-valence Gaussian basis sets. The electronic widths were obtained using smaller wave functions, and the cross sections were calculated on the basis of the multichannel quantum defect theory. The DR rate is calculated at 1.6 x 10 to the -7th x (Te/300) to the -0.37 cu cm/sec for Te in the range of 100 to 1000 K, and is found to be in excellent agreement with prior microwave afterglow experiments but in disagreement with recent merged beam results. It is inferred that the dominant mechanism for DR imparts sufficient energy to the product atoms to allow for escape from the Martian atmosphere.

Guberman, Steven L.↗
NASA NTRS2006

Collisional Ionization Equilibrium for Optically Thin Plasmas

Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have utilized state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He to Zn. We have also utilized state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using our data and the recommended electron impact ionization data of Mazzotta et al. (1998), we have calculated improved collisional ionization equilibrium calculations. We compare our calculated fractional ionic abundances using these data with those presented by Mazzotta et al. (1998) for all elements from H to Ni, and with the fractional abundances derived from the modern DR and RR calculations of Gu (2003a,b, 2004) for Mg, Si, S, Ar, Ca, Fe, and Ni.

Bryans, P.↗
NASA NTRS2000

Photoionization and Recombination

Theoretically self-consistent calculations for photoionization and (e + ion) recombination are described. The same eigenfunction expansion for the ion is employed in coupled channel calculations for both processes, thus ensuring consistency between cross sections and rates. The theoretical treatment of (e + ion) recombination subsumes both the non-resonant recombination ("radiative recombination"), and the resonant recombination ("di-electronic recombination") processes in a unified scheme. In addition to the total, unified recombination rates, level-specific recombination rates and photoionization cross sections are obtained for a large number of atomic levels. Both relativistic Breit-Pauli, and non-relativistic LS coupling, calculations are carried out in the close coupling approximation using the R-matrix method. Although the calculations are computationally intensive, they yield nearly all photoionization and recombination parameters needed for astrophysical photoionization models with higher precision than hitherto possible, estimated at about 10-20% from comparison with experimentally available data (including experimentally derived DR rates). Results are electronically available for over 40 atoms and ions. Photoionization and recombination of He-, and Li-like C and Fe are described for X-ray modeling. The unified method yields total and complete (e+ion) recombination rate coefficients, that can not otherwise be obtained theoretically or experimentally.

Nahar, Sultana N.↗
DOE OSTI2016

Excitation energies, radiative and autoionization rates, dielectronic satellite lines, and dielectronic recombination rates for excited states of Rb-like W from Kr-like W [Relativistic atomic data for Rb-like tungsten]

Energy levels, radiative transition probabilities, and autoionization rates for [Ni]$4{s}^{2}4{p}^{6}{nl}$, [Ni]$4{s}^{2}4{p}^{5}4l^{\prime} {nl}$ ($l^{\prime} =d,f,n$ = 4–7), [Ni]$4s4{p}^{6}4l^{\prime} {nl}$, ($l^{\prime} =d,f,n$ = 4–7), [Ni]$4{s}^{2}4{p}^{5}5l^{\prime} {nl}$ (n = 5–7), and [Ni]$4s4{p}^{6}6l^{\prime} {nl}$ (n = 6–7) states in Rb-like tungsten (W37+) are calculated using the relativistic many-body perturbation theory method (RMBPT code) and the Hartree–Fock-relativistic method (COWAN code). Autoionizing levels above the [Ni]$4{s}^{2}4{p}^{6}$ threshold are considered. It is found that configuration mixing among [Ni]$4{s}^{2}4{p}^{5}4l^{\prime} {nl}$ and [Ni]$4s4{p}^{6}4l^{\prime} {nl}$ plays an important role for all atomic characteristics. Branching ratios relative to the first threshold and intensity factors are calculated for satellite lines, and dielectronic recombination (DR) rate coefficients are determined for the [Ni]$4{s}^{2}4{p}^{6}{nl}$ (n = 4–7) singly excited states, as well as the [Ni]$4{s}^{2}4{p}^{5}4{dnl}$, [Ni]$4{s}^{2}4{p}^{5}4{fnl}$, [Ni]$4s4{p}^{6}4{dnl}$, [Ni]$4{s}^{2}4{p}^{6}4{fnl}$, (n = 4–6), and [Ni]$4{s}^{2}4{p}^{5}5l^{\prime} 5l$ doubly excited nonautoionizing states in Rb-like W37+ ion. Contributions from the [Ni]$4s24{p}^{6}4{fnl}$ (n = 6–7), [Ni]$4{s}^{2}4{p}^{5}5l^{\prime} {nl}$ (n = 5–6), and [Ni]$4{s}^{2}4{p}^{5}6l^{\prime} {nl}$ (n = 6–7) doubly excited autoionizing states are evaluated numerically. The high-n state (with n up to 500) contributions are very important for high temperatures. These contributions are determined by using a scaling procedure. Synthetic dielectronic satellite spectra from Rb-like W are simulated in a broad spectral range from 8 to 70 Å. Here, these calculations provide highly accurate values for a number of W 37+ properties useful for a variety of applications including for fusion applications.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗