Brendan M. McLaughlin

Interstellar X-ray absorption spectroscopy of oxygen, neon, and iron with the CHANDRA LETGS Spectrum of X0614+091

We —nd resolved interstellar O K, Ne K, and Fe L absorption spectra in the Chandra X-Ray Observatory Low-Energy Transmission Grating Spectrometer (LETGS) spectrum of the low-mass X-ray binary X0614]091. We measure the column densities in O and Ne and —nd direct spectroscopic constraints on the chemical state of the interstellar O. These measurements probably probe a low-density line of sight through the Galaxy, and we discuss the results in the context of our knowledge of the properties of interstellar matter in regions between the spiral arms.

Electron collisional excitation of Ne III: (1s22s22p4 3P2,1,0,1D2,1S0) fine-structure transitions

Effective collision strengths are evaluated from electron-impact excitation collision strengths of doubly ionized atomic neon (Ne III) calculated in the close-coupling approximation using the ab initio R -matrix method. The 56 fine-structure levels arising from the 28 LS target states; 2s2 2p4 , 2s2p5 , 2s2 2p3 3 ( = s,p,d) and 2s2 2p3 4s are retained in the present calculation. The effective collision strengths ( ) for all forbidden transitions within the 1s2 2s2 2p4 3 P2,1,0 , 1 D2 , 1 S0 ground configuration levels are obtained by averaging the electron collision strengths ( ), for a wide range of incident electron energies, over a Maxwellian distribution of velocities. Results are presented for electron temperatures in the range 1000-1 000 000 K, applicable to many laboratory and astrophysical plasmas. We believe that the results are the most accurate to date and so should be used in preference to previous values available in the literature.

Low-energy electron impact excitation of the nitrogen molecule: optically forbidden transitions

Ab initio calculations are performed on the scattering system, at low impact energies, using the R-matrix technique. The ground state and the lowest seven valence excited states, , and , of are included in the close coupling expansion with each state being represented by a configuration interaction wavefunction in a hybrid orbital set. Various approximations for the representation of the correlation between the target and the incident electron are investigated. Integrated and differential scattering cross sections are presented, and compared to experiment, for excitation from the ground level to optically forbidden levels. The results compare favourably with recent experiments in this energy region and remove a number of problems with our previous work.

Electron-impact excitation of the fine-structure levels , ) of singly ionized atomic oxygen

Effective collision strengths for O II ions are evaluated from electron-impact excitation collision strengths of singly ionized atomic oxygen ( O II) calculated in the close-coupling approximation using the ab initio R-matrix method at the Breit-Pauli approximation level. The lowest 21 fine-structure levels arising from the 11 , LS target states are included in the calculation. The effective collision strengths for all forbidden transitions between the ground configuration levels are obtained by averaging the electron collision strengths, for a wide range of incident electron energies, over a Maxwellian distribution of velocities. Results are presented for electron temperatures in the range 2000-1000 000 K, applicable to many laboratory and astrophysical plasmas. We believe that the results are the most accurate to date and so should be used in preference to previous values available in the literature.

K-shell photoionization of ground-state Li-like carbon ions [C3+]: experiment, theory and comparison with time-reversed photorecombination

Absolute cross sections for the K-shell photoionization of ground-state Li-like carbon [C3+(1s22s 2S)] ions were measured by employing the ion?photon merged-beams technique at the Advanced Light Source. The energy ranges 299.8?300.15?eV, 303.29?303.58?eV and 335.61?337.57?eV of the [1s(2s2p)3P]2P, [1s(2s2p)1P]2P and [(1s2s)3S 3p]2P resonances, respectively, were investigated using resolving powers of up to 6000. The autoionization linewidth of the [1s(2s2p)1P]2P resonance was measured to be 27 ? 5?meV and compares favourably with a theoretical result of 26?meV obtained from the intermediate coupling R-matrix method. The present photoionization cross section results are compared with the outcome from photorecombination measurements by employing the principle of detailed balance.

Inner-shell photoexcited resonances in atomic oxygen

Photoionization of atomic oxygen near the 1s-1 threshold is reinvestigated by including a more extensive inner-shell configuration interaction description, and by using an optical potential to account for autoionization to the infinite number of singly excited O+ continua. These new theoretical results remove the earlier discrepancies between theory and experiment.

Photoabsorption of atomic oxygen in the vicinity of the K-edge

Ab initio cross section calculations using the R-matrix approach have been performed for the inner-shell photoabsorption of atomic oxygen in the photon energy region 520-555 eV. The prominent (1s2(P)np ) resonance series, converging to the series limit of atomic oxygen, dominate the cross section in the vicinity of the K-edge. A comparison of the calculations with two recent sets of experimental measurements taken at the Advanced Light Source and at the Wisconsin synchrotron facility is made.

Accurate transition probabilities for some spectral lines of singly ionized oxygen

Transition probabilities based on sophisticated configuration interaction wave functions have been obtained for transitions between the fine structure levels of the 26 lowest lying 2s22p3, 2s2p4, 2s22p23s, 2s22p23p, 2s22p23d states of singly ionized oxygen. The calculations were performed using the CIV3 code of Hibbert and small empirical adjustments were introduced to the diagonal Hamiltonian matrix elements in order to achieve accurate energy splittings between the levels. The present results are believed to be the most accurate available and, for several allowed transitions, a significant departure from LS coupling, due to relativistic effects, is found.

Photoionization cross sections for the trans-iron element Se + from 18 to 31 eV

Absolute photoionization cross-section calculations are presented for Se+ using large-scale close-coupling calculations within the Breit–Pauli and Dirac–Coulomb R-matrix approximations. The results from our theoretical work are compared with recent measurements (Esteves 2010 PhD Thesis publication number AAI3404727, University of Reno, NV, USA; Sterling et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 025701; Esteves et al 2011 Phys. Rev. A 84 013406) made at the advanced light source (ALS) radiation facility in Berkeley, CA, USA. We report on results for the photon energy range 18.0–31.0 eV, which spans the ionization thresholds of the 4So3/2 ground state and the low-lying 2Do5/2,3/2 and 2Po3/2,1/2 metastable states. Metastable fractions are inferred from our present work. Resonance energies and quantum defects of the prominent Rydberg resonances series identified in the spectra are compared for the 4p → nd transitions with the recent ALS experimental measurements made on this complex trans-iron element.

Photoionization cross section calculations for the halogen-like ions Kr + and Xe +

Photoionization cross section calculations on the halogen-like ions; Kr+ and Xe+ have been performed for a photon energy range from each ion threshold to 15 eV, using large-scale close-coupling calculations within the Dirac–Coulomb R-matrix approximation. The results from our theoretical work are compared with recent measurements made at the ASTRID merged-beam set-up at the University of Aarhus in Denmark and from the Fourier transform ion cyclotron resonance trap method at the SOLEIL synchrotron radiation facility in Saint-Aubin, France Bizau et al (2011 J. Phys. B: At. Mol. Opt. Phys. 44 055205) and the advanced light source Muller (2012 private communication), Aguliar et al (2012 J. Phys.: Conf. Ser. at press). For each of these complex ions our theoretical cross section results over the photon energy range investigated are seen to be in excellent agreement with experiment. Resonance energy positions and quantum defects of the prominent Rydberg resonances series identified in the spectra are compared with experiment for these complex halogen-like ions.