K. Kirby

Theoretical study of the rovibrationally resolved transitions of CaH

Comprehensive absorption line lists for 40CaH have been calculated for electronic transitions from the X 2Σ+ ground state to the A 2Π, B/B′ 2Σ+, and E 2Π low-lying excited states. The lists include transition energies and oscillator strengths for all possible allowed transitions and were computed using the most recent set of theoretical potential energy curves and dipole transition moment functions, with adjustments to account for experimental dissociation energies and asymptotic limits. Good agreement with previous calculations and available experimental data has been obtained. Oscillator strengths for the transition from the X 2Σ+ state to the D 2Σ+ state are also given, but due to the large uncertainty of the available dipole transition moment function the line list is highly uncertain. For the C 2Σ+←X 2Σ+ transition, a Franck–Condon approximation has been used.

Molecular Line Opacity of LiCl in the Mid-Infrared Spectra of Brown Dwarfs

We present a complete line list for the X1Σ+ electronic ground state of 7Li35Cl, computed using fully quantum-mechanical techniques. This list includes transition energies and oscillator strengths in the spectral region 0.3-39,640.7 cm-1 for all allowed rovibrational transitions in absorption within the electronic ground state. The calculations were performed using an accurate hybrid potential constructed from a spectral inversion fit of experimental data and from recent multireference single- and double-excitation configuration interaction calculations. The line list was incorporated into the stellar atmosphere code PHOENIX to compute spectra for a range of young to old T dwarf models. The possibility of observing a signature of LiCl in absorption near 15.8 μm is addressed and the proposal to use this feature to estimate the total lithium elemental abundance for these cool objects is discussed.

Ab initio configuration interaction study of the low-lying 1Σ+ electronic states of LiCl

Ab initio configuration interaction calculations have been performed for the X 1Sigma+ and B 1Sigma+ electronic states of LiCl. Potential energy curves, dipole moment functions, and dipole transition moments have been computed for internuclear distances between R = 2.5a0 and 50a0. Single- and double-excitation configuration interaction wave functions were constructed using molecular orbitals obtained from a two-state averaged multiconfiguration self-consistent-field calculation. This procedure yielded an accurate energy splitting between the covalent and ionic separated-atom limits. The calculated avoided crossing of the X and B state curves occurs at R = 16.2a0, in close agreement with previous calculations using a semiempirical covalent-ionic resonance model. X 1Sigma+ state spectroscopic constants are in excellent agreement with experimental values.

Radiative L-Shell Transitions in Fe XIX and Fe XX Ions

We compute the wavelengths and oscillator strengths for the 3s, 3d → 2p, and 3p → 2s emission lines in Fe XIX and Fe XX ions by using a configuration interaction Dirac-Fock and Dirac-Fock-Sturm method combined with second-order Brillouin-Wigner perturbation theory. We provide a complete list of computed wavelengths and oscillator strengths in both the velocity and length gauge for these transitions, many of which have never previously been reported. A comparison of our data with laboratory measurements and other theoretical predictions allows us to estimate an uncertainty of ~2 mA in the wavelengths and an uncertainty of 2%-3% in the oscillator strengths. We expect that our calculations will provide a means of identifying emission lines from astrophysical sources and improve the ability to detect blending in X-ray grating spectra from Chandra and XMM-Newton. As an example, we present a simulated emission spectrum of Capella and find improved agreement between the observations and our calculations, compared with previous data sets.

Photodissociation of lithium chloride: Interferometric predissociation

Most of the lithium observed throughout the Universe is believed to be primordial. Understanding the abundance of Li has crucial implications for theories of the early Universe, interstellar processes, and Galactic and stellar evolution. In low-temperature environments, a large amount of Li can be sequestered into molecular species such as LiCl. Photodissociation cross sections for the production of ground-state Li and Cl atoms were computed up to temperatures of 1500 K, at which LiCl is believed to be the primary Li-bearing gas in cool dwarf atmospheres. Narrow Rydberg resonances in the total absorption spectrum are found to dominate the thermally averaged cross sections.

Accurate wavelengths for X‐ray spectroscopy and the NIST hydrogen‐like ion database

We have developed an ab initio multi‐configuration Dirac‐Fock‐Sturm method for the precise calculation of X‐ray emission spectra, including energies, transition wavelengths and transition probabilities. The calculations are based on non‐orthogonal basis sets, generated by solving the Dirac‐Fock and Dirac‐Fock‐Sturm equations. Inclusion of Sturm functions into the basis set provides an efficient description of correlation effects in highly charged ions and fast convergence of the configuration interaction procedure.A second part of our study is devoted to developing a theoretical procedure and creating an interactive database to generate energies and transition frequencies for hydrogen‐like ions. This procedure is highly accurate and based on current knowledge of the relevant theory, which includes relativistic, quantum electrodynamic, recoil, and nuclear size effects.