Publications (199)390.13 Total impact

Article: Rmatrix with pseudostates study of single photon double ionization of endohedral Be and Mg atoms
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ABSTRACT: We perform Rmatrix with pseudostates calculations to investigate single photon double ionization of Be@C60 and Mg@C60. We contrast the present results to our previous studies of He@C60. We find the presence of the confinement resonances in the double photoionization cross sections of endohedral Be and Mg. Keeping the same C60 spherical shell potential in each case for these systems, we explore the change in magnitude and the energy dependence of the resonance features relative to the background cross sections.Journal of Physics B Atomic Molecular and Optical Physics 03/2015; 48(6). DOI:10.1088/09534075/48/6/065201 · 1.92 Impact Factor 
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ABSTRACT: 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 higherrow elements. This introduces an uncertainty that is especially acute for photoionized clouds, where the low temperatures mean that DR occurs energetically through very lowlying autoionizing states. This paper discusses S$^{2+} \rightarrow$ S$^+$ 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 secondrow 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$^{2+} \rightarrow$ S$^+$ DR rate coefficient at $\sim 10^{4}$~K. We present new calculations of the DR rate coefficient for S$^{2+} \rightarrow$ S$^+$ 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. 
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ABSTRACT: We present an Xray absorption model for the interstellar medium, to be referred to as ISMabs, that takes into account both neutral and ionized species of cosmically abundant elements, and includes the most accurate atomic data available. Using highresolution spectra from eight Xray binaries obtained with the Chandra High Energy Transmission Grating Spectrometer, we proceed to benchmark the atomic data in the model particularly in the neon Kedge region. Compared with previous photoabsorption models, which solely rely on neutral species, the inclusion of ions leads to improved spectral fits. Fit parameters comprise the column densities of abundant contributors that allow direct estimates of the ionization states. ISMabs is provided in the appropriate format to be implemented in widely used Xray spectral fitting packages such as XSPEC, ISIS and SHERPAThe Astrophysical Journal 02/2015; 800(1):29. DOI:10.1088/0004637X/800/1/29 · 6.28 Impact Factor 
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ABSTRACT: The Rmatrix with pseudostates (RMPS) and timedependent closecoupling (TDCC) methods are used to calculate photoionizationexcitation and double photoionization cross sections of He and He@C60.Journal of Physics Conference Series 04/2014; 488(2):022038. DOI:10.1088/17426596/488/2/022038 
Article: KShell Photoionization of Oxygen
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ABSTRACT: Rmatrix calculations of the 1s photoionization in oxygen are reported. Proper treatment of spectator Auger broadening, relaxation, pseudoresonace elimination, and shakeup/shakeoff are addressed.Journal of Physics Conference Series 04/2014; 488(2):022037. DOI:10.1088/17426596/488/2/022037 
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ABSTRACT: We report BreitPauli and Dirac Rmatrix cross sections for the photoionization of endofullerene Xe@C60, showing strong confinement resonances, in excellent agreement with a recent experiment.Journal of Physics Conference Series 04/2014; 488(2):022030. DOI:10.1088/17426596/488/2/022030 
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ABSTRACT: Chandra highresolution spectra toward eight lowmass Galactic binaries have been analyzed with a photoionization model that is capable of determining the physical state of the interstellar medium. Particular attention is given to the accuracy of the atomic data. Hydrogen column densities are derived with a broadband fit that takes into account pileup effects, and in general are in good agreement with previous results. The dominant features in the oxygenedge region are O I and O II K$\alpha$ absorption lines whose simultaneous fits lead to average values of the ionization parameter of $\log\xi=2.90$ and oxygen abundance of $A_{\rm O}=0.70$. The latter is relative to the standard by Grevesse & Sauval (1998), but a rescaling with the revision by Asplund et al. (2009) would lead to an average abundance value fairly close to solar. The low average oxygen column density ($N_{\rm O}=9.2 \times 10^{17}$~cm$^{2}$) suggests a correlation with the low ionization parameters, the latter also being in evidence in the column density ratios OII/OI and OIII/OI that are estimated to be less than 0.1. We do not find conclusive evidence for absorption by any other compound but atomic oxygen.The Astrophysical Journal 03/2014; 790(2). DOI:10.1088/0004637X/790/2/131 · 6.28 Impact Factor 

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ABSTRACT: Double photoionization accompanied by loss of n C atoms (n = 0, 2, 4, 6) was investigated by merging beams of Xe@C60+ ions and synchrotron radiation and measuring the yields of product ions. The giant 4d dipole resonance of the caged Xe atom has a prominent signature in the cross section for these product channels, which together account for 6.2 ± 1.4 of the total Xe 4d oscillator strength of 10. Compared to that for a free Xe atom, the oscillator strength is redistributed in photon energy due to multipath interference of outgoing Xe 4d photoelectron waves that may be transmitted or reflected by the spherical C60+ molecular cage, yielding socalled confinement resonances. The data are compared with an earlier measurement and with theoretical predictions for this singlemolecule photoelectron interferometer system. Relativistic Rmatrix calculations for the Xe atom in a spherical potential shell representing the fullerene cage show the sensitivity of the interference pattern to the molecular geometry.Physical Review A 11/2013; 88:053402. DOI:10.1103/PhysRevA.88.053402 · 2.99 Impact Factor 
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ABSTRACT: Relative partial cross sections have been measured following photoexcitation of atomic chlorine near the Cl 2p and Cl 1s ionization thresholds. In addition, BreitPauli Rmatrix calculations have been carried out in the region of the 2p thresholds, and the results are compared with experiment. Owing to angularmomentum considerations, it was found that the resonances associated with the higher 2p1 thresholds should be significantly wider than the lower ones, and this is borne out in both the experimental and the theoretical results. It is shown that a large number of resonance series contribute to the cross section, which make it difficult to untangle, and suggestions for further work to better understand the spectra are presented.Physical Review A 11/2013; 88(5):53425. DOI:10.1103/PhysRevA.88.053425 · 2.99 Impact Factor 
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ABSTRACT: An analytical formula is developed to represent accurately the photoabsorption cross section of O I for all energies of interest in Xray spectral modeling. In the vicinity of the Kedge, a Rydberg series expression is used to fit Rmatrix results, including important orbital relaxation effects, that accurately predict the absorption oscillator strengths below threshold and merge consistently and continuously to the abovethreshold cross section. Further minor adjustments are made to the threshold energies in order to reliably align the atomic Rydberg resonances after consideration of both experimental and observed line positions. At energies far below or above the Kedge region, the formulation is based on both outer and innershell direct photoionization, including significant shakeup and shakeoff processes that result in photoionizationexcitation and double photoionization contributions to the total cross section. The ultimate purpose for developing a definitive model for oxygen absorption is to resolve standing discrepancies between the astronomically observed and laboratory measured line positions, and between the inferred atomic and molecular oxygen abundances in the interstellar medium from XSTAR and SPEX spectral models.The Astrophysical Journal 10/2013; 779(1). DOI:10.1088/0004637X/779/1/78 · 6.28 Impact Factor 
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ABSTRACT: The Rmatrix with pseudostates method is applied to the case of twoelectron excitation and ionization processes in He@C60. A finite sphericalwell potential, which simulates the effect of the fullerene enclosing cage, is incorporated into the Hamiltonian using a modified Rmatrix method, as recently applied to similar single photoionization studies of Xe@C60 and Ca@C60. Comparison is made with timedependent closecoupling results for twoelectron processes in He and He@C60, and excellent agreement is found in the respective descriptions of confinement resonances. Earlier findings are confirmed that the cage potential causes a redistribution between the n = 2 and n = 3 photoionization–excitation cross sections. The Rmatrix with pseudostates method, in particular, is wellsuited for treating atomic Feshbach resonances and studying multielectron systems.Journal of Physics B Atomic Molecular and Optical Physics 09/2013; 46(19):195201. DOI:10.1088/09534075/46/19/195201 · 1.92 Impact Factor 



Conference Paper: Suppression of Dielectronic Recombination at Finite Densities
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ABSTRACT: Synopsis: Densitydependent effective dielectronic recombination rate coeffcients are determined in order to explore finitedensity effects on the ionization balance of plasmas.XXVIII International Conference on Photonic, Electronic and Atomic Collisions; 07/2013 

Conference Paper: ORAL PRESENTATION: Dielectronic Recombination in Finite Density Plasmas
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ABSTRACT: (WITHDRAWNDUETOBUDGETCUTS)Congress of Canadian Association of Physicists; 05/2013 
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ABSTRACT: We have developed a general model for determining densitydependent effective dielectronic recombination (DR) rate coefficients in order to explore finitedensity effects on the ionization balance of plasmas. Our model consists of multiplying by a suppression factor those highlyaccurate total zerodensity DR rate coefficients which have been produced from stateoftheart theoretical calculations and which have been benchmarked by experiment. The suppression factor is based upon earlier detailed collisionradiative 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 densitydependent 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 e = 1 cm–3) and finite (n e = 1010 cm–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 collisionalradiative calculations which utilize stateoftheart 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.The Astrophysical Journal 04/2013; 768(1):82. DOI:10.1088/0004637X/768/1/82 · 6.28 Impact Factor 
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ABSTRACT: Xray photoabsorption cross sections have been computed for all magnesium ions using the $R$matrix method. A comparison with the other available data for Mg IIMg X shows good qualitative agreement in the resultant resonance shapes. However, for the lower ionization stages, and for singlyionized Mg II in particular, the previous $R$matrix results (Witthoeft et al.2009; Witthoeft et al. 2011) overestimate the Kedge position due to the neglect of important orbital relaxation effects, and a global shift downward in photon energy of those cross sections is therefore warranted. We have found that the cross sections for Mg I and Mg II are further complicated by the Mshell ($n=3$) occupancy. As a result, the treatment of spectator Auger decay of $1s\rightarrow np$ resonances using a method based on multichannel quantum defect theory and an optical potential becomes problematic, making it necessary to implement an alternative, approximate treatment of Auger decay for neutral Mg. The new cross sections are used to fit the Mg K edge in XMMNewton spectra of the lowmass Xray binary GS 1826238, where most of the interstellar Mg is found to be in ionized form.The Astrophysical Journal Supplement Series 03/2013; 214(1). DOI:10.1088/00670049/214/1/8 · 14.14 Impact Factor
Publication Stats
2k  Citations  
390.13  Total Impact Points  
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Institutions

1925–2014

Western Michigan University
 Department of Physics
Kalamazoo, Michigan, United States


2013

University of Alberta
 Department of Mechanical Engineering
Edmonton, Alberta, Canada


2009

Denison University
 Department of Physics and Astronomy
Granville, Ohio, United States


1996–2006

University of Strathclyde
 Department of Physics
Glasgow, Scotland, United Kingdom


2000

Sapienza University of Rome
 Department of Chemistry
Roma, Latium, Italy


1993–1998

Auburn University
 Department of Physics
Auburn, AL, United States
