Atomic data for opacity calculations. II. Computational methods

Experimental and Theoretical Study of Photoionization of Cl III

Photoionization of Cl III ions into Cl IV was studied theoretically using the ab initio relativistic Breit–Pauli R-matrix (BPRM) method and experimentally at the Advanced Light Source (ALS) synchrotron at the Lawrence Berkeley National Laboratory. A relative-ion-yield spectrum of Cl IV was measured with a photon energy resolution of 10 meV. The theoretical study was carried out using a large wave-function expansion of 45 levels of configurations 3s23p2, 3s3p3, 3s23p3d, 3s23p4s, 3s3p23d, and 3p4. The resulting spectra are complex. We have compared the observed spectrum with photoionization cross sections (σPI) of the ground state 3s23p3(4S3/2o) and the seven lowest excited levels 3s23p3(2D5/2o), 3s23p3(2D3/2o), 3s23p3(2P3/2o), 3s23p3(2P1/2o), 3s3p4(4P5/2), 3s3p4(4P3/2) and 3s3p4(4P1/2) of Cl III, as these can generate resonances within the energy range of the experiment. We were able to identify most of the resonances as belonging to various specific initial levels within the primary Cl III ion beam. Compared to the first five levels, resonant structures in the σPI of excited levels of 3s3p4 appear to have a weaker presence. We have also produced combined theoretical spectra of the levels by convolving the cross sections with a Gaussian profile of experimental width and summing them using statistical weight factors. The theoretical and experimental features show good agreement with the first five levels of Cl III. These features are also expected to elucidate the recent observed spectra of Cl III by Sloan Digital Scan Survey project.

Photoionization and core resonances from range-separated density-functional theory: General formalism and example of the beryllium atom

We explore the merits of linear-response range-separated time-dependent density-functional theory (TDDFT) for the calculation of photoionization spectra. We consider two variants of range-separated TDDFT, namely, the time-dependent range-separated hybrid (TDRSH) scheme, which uses a global range-separation parameter, and the time-dependent locally range-separated hybrid (TDLRSH), which uses a local range-separation parameter, and compare with standard time-dependent local-density approximation (TDLDA) and time-dependent Hartree-Fock (TDHF). We show how to calculate photoionization spectra with these methods using the Sternheimer approach formulated in a non-orthogonal B-spline basis set with appropriate frequency-dependent boundary conditions. We illustrate these methods on the photoionization spectrum of the Be atom, focusing, in particular, on the core resonances. Both the TDRSH and TDLRSH photoionization spectra are found to constitute a large improvement over the TDLDA photoionization spectrum and a more modest improvement over the TDHF photoionization spectrum.

Database NORAD-Atomic-Data for Atomic Processes in Plasma

The online atomic database of NORAD-Atomic-Data, where NORAD stands for Nahar OSU Radiative, is part of the data sources of the two international collaborations of the Opacity Project (OP) and the Iron Project (IP). It contains large sets of parameters for the dominant atomic processes in astrophysical plasmas, such as, (i) photo-excitation, (ii) photoionization, (iii) electron–ion recombination, (iv) electron–impact excitations. The atomic parameters correspond to tables of energy levels, level-specific total photoionization cross-sections, partial photoionization cross-sections of all bound states for leaving the residual ion in the ground state, partial cross-sections of the ground state for leaving the ion in various excited states, total level-specific electron–ion recombination rate coefficients that include both the radiative and dielectronic recombination, total recombination rate coefficients summed from contributions of an infinite number of recombined states, total photo-recombination cross-sections and rates with respect to photoelectron energy, transition probabilities, lifetimes, collision strengths. The database was created after the first two atomic databases, TOPbase under the OP and TIPbase under the IP. Hence the contents of NORAD-Atomic-Data are either new or from repeated calculations using a much larger wave function expansion making the data more complete. The results have been obtained from the R-matrix method using the close-coupling approximation developed under the OP and IP, and from atomic structure calculations using the program SUPERSTRUCTURE. They have been compared with available published results which have been obtained theoretically and experimentally, and are expected to be of high accuracy in general. All computations were carried out using the computational facilities at the Ohio Supercomputer Center (OSC) starting in 1990. At present it contains atomic data for 154 atomic species, 98 of which are lighter atomic species with nuclear charge Z ≤ 28 and 56 are heavier ones with Z > 28. New data are added with publications.

Atomic Data for Plasma Spectroscopy: The CHIANTI Database, Improvements and Challenges

CHIANTI is an atomic database and software package for modeling emission lines and continua from hot astrophysical plasmas. It is freely available to all researchers and has been widely used in the Heliophysics and Astrophysics communities for almost 25 years. In this review, we summarize the properties of the current version of the database and give an overview of the relevant atomic processes. We also discuss progress towards a complete implementation of collisional-radiative modeling, simultaneously solving for atomic level and ion populations for individual elements.

Solar UV and X-ray spectral diagnostics

X-ray and ultraviolet (UV) observations of the outer solar atmosphere have been used for many decades to measure the fundamental parameters of the solar plasma. This review focuses on the optically thin emission from the solar atmosphere, mostly found at UV and X-ray (XUV) wavelengths, and discusses some of the diagnostic methods that have been used to measure electron densities, electron temperatures, differential emission measure (DEM), and relative chemical abundances. We mainly focus on methods and results obtained from high-resolution spectroscopy, rather than broad-band imaging. However, we note that the best results are often obtained by combining imaging and spectroscopic observations. We also mainly focus the review on measurements of electron densities and temperatures obtained from single ion diagnostics, to avoid issues related to the ionisation state of the plasma. We start the review with a short historical introduction on the main XUV high-resolution spectrometers, then review the basics of optically thin emission and the main processes that affect the formation of a spectral line. We mainly discuss plasma in equilibrium, but briefly mention non-equilibrium ionisation and non-thermal electron distributions. We also summarise the status of atomic data, which are an essential part of the diagnostic process. We then review the methods used to measure electron densities, electron temperatures, the DEM, and relative chemical abundances, and the results obtained for the lower solar atmosphere (within a fraction of the solar radii), for coronal holes, the quiet Sun, active regions and flares.

Detailed-term-accounting approximation calculations of the radiative opacity of aluminum plasmas: a systematic study

The spectrally resolved radiative opacity and the Rosseland and Planck mean opacities are calculated by using the detailed-term-accounting approximation for aluminum plasmas with varieties of density and temperature. The results are presented along a 40 eV isothermal sequence, a 0.01 g/cm(3) isodense sequence, and a sequence with average ionization degree Z* approximately 7.13. Particular attention is given to the influence of the detailed treatment of spectral lines on the Rosseland mean opacity under different thermodynamic conditions. The results show that at densities of 0.004 g/cm(3) and higher, the opacities are not very sensitive to the spectral linewidth within a reasonable range. As examples, the Rosseland mean opacity, which is most sensitive to the detailed linewidth, at 40 eV and 0.004 g/cm(3) changes no more than 15%, when we change the electron impact spectral linewidth artificially by reducing it by 50% or increasing it twice, and at 40 eV and 0.1 g/cm(3) it changes less than 5%. For comparison, we also carried out calculations by using an average atom model. For the Rosseland mean opacities, the two models show quite large differences, in particular at low densities, while for the Planck mean opacities the results of the two models are much closer.

Emission lines of [K V] in the optical spectra of gaseous nebulae

Recent R-matrix calculations of electron impact excitation rates in K v are used to derive the nebular emission line ratio R = I(4122.6 A)/I(4163.3 A) as a function of electron density (N(e)). This ratio is found to be very sensitive to changes in N(e) over the density range 10(3) to 10(6) cm(-3), but does not vary significantly with electron temperature, and hence in principle should provide an excellent optical N(e) diagnostic for the high-excitation zones of nebulae. The observed value of R for the planetary nebula NGC 7027, measured from a spectrum obtained with the Hamilton Echelle spectrograph on the 3-m Shane Telescope, implies a density in excellent agreement with that derived from [Ne iv], formed in the same region of the nebula as [K v]. This observation provides observational support for the accuracy of the theoretical [K v] line ratios, and hence the atomic data on which they are based. However, the analysis of a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope, reveals that the [K v] 4122.6 A line in this object is badly blended with Fe ii 4122.6 A. Hence, the [K v] diagnostic may not be used for astrophysical sources that show a strong Fe ii emission line spectrum.

Photoionization of metastable O+ ions: experiment and theory

High-resolution absolute experimental measurements and two independent theoretical calculations were performed for photoionization of O+ ions from the 2P(o) and 2D(o) metastable levels and from the 4S(o) ground state in the photon energy range 30-35.5 eV. This is believed to be the first comparison of experiment and theory to be reported for photoionization from metastable states of ions. While there is correspondence between the predicted and measured positions and relative strengths of the resonances, the cross-section magnitudes and fine structure are sensitive to the choice of basis states.

Benchmark nonperturbative calculations for the electron-impact ionization of Li(2s) and Li(2p)

Three independent nonperturbative calculations are reported for the electron-impact ionization of both the ground and first excited states of the neutral lithium atom. The time-dependent close-coupling, the R matrix with pseudostates, and the converged close-coupling methods yield total integral cross sections that are in very good agreement with each other, while perturbative distorted-wave calculations yield cross sections that are substantially higher. These nonperturbative calculations provide a benchmark for the continued development of electron-atom experimental methods designed to measure both ground and excited state ionization.

Nebular and auroral emission lines of [Cl III] in the optical spectra of planetary nebulae

Electron impact excitation rates in Cl III, recently determined with the R-matrix code, are used to calculate electron temperature (T(e)) and density (N(e)) emission line ratios involving both the nebular (5517.7, 5537.9 A) and auroral (8433.9, 8480.9, 8500.0 A) transitions. A comparison of these results with observational data for a sample of planetary nebulae, obtained with the Hamilton Echelle Spectrograph on the 3-m Shane Telescope, reveals that the R(1) = I(5518 A)/I(5538 A) intensity ratio provides estimates of N(e) in excellent agreement with the values derived from other line ratios in the echelle spectra. This agreement indicates that R(1) is a reliable density diagnostic for planetary nebulae, and it also provides observational support for the accuracy of the atomic data adopted in the line ratio calculations. However the [Cl iii] 8433.9 A line is found to be frequently blended with a weak telluric emission feature, although in those instances when the [Cl iii] intensity may be reliably measured, it provides accurate determinations of T(e) when ratioed against the sum of the 5518 and 5538 A line fluxes. Similarly, the 8500.0 A line, previously believed to be free of contamination by the Earth's atmosphere, is also shown to be generally blended with a weak telluric emission feature. The [Cl iii] transition at 8480.9 A is found to be blended with the He i 8480.7 A line, except in planetary nebulae that show a relatively weak He i spectrum, where it also provides reliable estimates of T(e) when ratioed against the nebular lines. Finally, the diagnostic potential of the near-UV [Cl iii] lines at 3344 and 3354 A is briefly discussed.