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Brendan M. McLaughlin

Publications and source records attributed to Brendan M. McLaughlin.

12 recordsLinked to original sources
arXiv2019

Single photoionization of the Kr-like Rb II ion in the photon energy range 22 - 46.0 eV

Single photoionization cross sections for Kr-like Rb$^+$ ions are reported in the energy (wavelength) range 22 eV (564 Å) to 46 eV (270 Å). Theoretical cross section calculations for this {\it trans}-Fe element are compared with measurements from the ASTRID radiation facility in Aarhus, Denmark and the dual laser plasma (DLP) technique, at respectively 40 meV and 35 meV FWHM energy resolution. In the photon energy region 22 - 32 eV the spectrum is dominated by excitation autoionizing resonance states. Above 32 eV the cross section exhibit classic Fano window resonances features, which are analysed and discussed. Large-scale theoretical photoionization cross-section calculations, performed using a Dirac Coulomb $R$-matrix approximation are bench marked against these high resolution experimental results. Comparison of the theoretical work with the experimental studies allowed the identification of resonance features and their parameters in the spectra in addition to contributions from excited metastable states of the Rb$^+$ ions.

physics.atom-ph↗
arXiv2018

Photodissociation of CS from Excited Rovibrational Levels

Accurate photodissociation cross sections have been computed for transitions from the X $^1Σ^+$ ground electronic state of CS to six low-lying excited electronic states. New ab initio potential curves and transition dipole moment functions have been obtained for these computations using the multi-reference configuration interaction approach with the Davidson correction (MRCI+Q) and aug-cc-pV6Z basis sets. State-resolved cross sections have been computed for transitions from nearly the full range of rovibrational levels of the X $^1Σ^+$ state and for photon wavelengths ranging from 500 $\textÅ$ to threshold. Destruction of CS via predissociation in highly excited electronic states originating from the rovibrational ground state is found to be unimportant. Photodissociation cross sections are presented for temperatures in the range between 1000 and 10,000 K, where a Boltzmann distribution of initial rovibrational levels is assumed. Applications of the current computations to various astrophysical environments are briefly discussed focusing on photodissociation rates due to the standard interstellar and blackbody radiation fields.

physics.atom-ph↗
arXiv2017

Radiative association of C(${}^3P$) and H${}^+$: Triplet states

The radiative association of C(${}^3P$) and H${}^+$ is investigated by calculating cross sections for photon emission into bound ro-vibrational states of CH${}^+$ from the vibrational continua of initial triplet d$\,{}^3Π$ or b$\,{}^3Σ^-$ states. Potential energy curves and transition dipole moments are calculated using multi-reference configuration interaction (MRCI) methods with AV6Z basis sets. The cross sections are evaluated using quantum-mechanical methods and rate coefficients are calculated. The rate coefficients are about 100 times larger than those for radiative association of C${}^+({}^2{P^o})$ and H from the A$\,{}^1Π$ state. We also confirm that the formation of CH${}^+$ by radiative association of C${}^+({}^2{P^o})$ and H via the triplet c$\,{}^3Σ^+$ state is a minor process.

physics.atom-ph↗
arXiv2016

Formation of silicon monoxide by radiative association: the impact of resonances

Detailed quantum chemistry calculations within the multireference configuration interaction approximation with the Davidson correction (MRCI+Q) are presented using an aug-cc-pV6Z basis set, for the potential energy curves and transition dipole moments between low lying molecular states of singlet spin symmetry for the SiO molecule. The high quality molecular data are used to obtain radiative association cross sections and rate coefficients for collisions between ground state Si and O atoms. Quantal calculations are compared with semiclassical results. Using a quantum kinetic theory of radiative association in which quasibound levels are assumed to be in local thermodynamic equilibrium, we find that resonances play an important role in enhancing the rate coefficients at low temperatures by several orders of magnitude from that predicted by standard quantum scattering formulations. These new molecular formation rates may have important implications for applications in astrophysics.

physics.atom-ph↗
arXiv2016

Towards a spectroscopically accurate set of potentials for heavy hydride laser cooling candidates: effective core potential calculations of BaH

BaH (and its isotopomers) is an attractive molecular candidate for laser cooling to ultracold temperatures and a potential precursor for the production of ultracold gases of hydrogen and deuterium. The theoretical challenge is to simulate the laser cooling cycle as reliably as possible and this paper addresses the generation of a highly accurate ab initio $^{2}Σ^+$ potential for such studies. The performance of various basis sets within the multi-reference configuration-interaction (MRCI) approximation with the Davidson correction (MRCI+Q) is tested and taken to the complete basis set limit. It is shown that the calculated molecular constants using a 46 electron Effective Core-Potential (ECP), the augmented polarized core-valence quintuplet basis set (aug-pCV5Z-PP) but only including three active electrons in the MRCI calculation are in close agreement with the available experimental values. The predicted dissociation energy D$_e$ for the X$^2Σ^+$ state (extrapolated to the complete basis set (CBS) limit) is 16895.12 cm$^{-1}$ (2.094 eV), which agrees within 0.1$\%$ of a revised experimental value of $<$16910.6 cm$^{-1}$, while the calculated r$_e$ is within 0.03 pm of the experimental result.

physics.atom-ph↗
arXiv2015

PAMOP: Petascale Atomic, Molecular and Optical Collision Calculations

Petaflop architectures are currently being utilized efficiently to perform large scale computations in Atomic, Molecular and Optical Collisions. We solve the Schrödinger or Dirac equation for the appropriate collision problem using the R-matrix or R-matrix with pseudo-states approach. We briefly outline the parallel methodology used and implemented for the current suite of Breit-Pauli and DARC codes. In this report, various examples are shown from our theoretical results compared with experimental results obtained from Synchrotron Radiation facilities where the Cray architecture at HLRS is playing an integral part in our computational projects.

physics.atom-ph↗
arXiv2015

Photoionization of the $3s^23p^4~ ^3P$ and the $3s^23p^4~^1D,~^1S$ states of sulfur: experiment and theory

Photoionization of neutral atomic sulfur in the ground and metastable states was studied experimentally at a photon energy resolution of 44 meV FWHM. Relative cross section measurements were recorded by using tunable vacuum ultraviolet (VUV) radiation in the energy range 9 -- 30 eV obtained from a laser-produced plasma and the atomic species were generated by photolysis of molecular precursors. Photoionization of this atom is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. A wealth of resonance features observed in the experimental spectra are spectroscopically assigned and their energies and quantum defects tabulated. The cross-section measurements are compared with state-of-the-art theoretical cross-section calculations obtained from the Dirac Coulomb R-matrix method. Resonances series in the spectra are identified and compared indicating similar features in both the theoretical and experimental spectra.

astro-ph.EP↗
arXiv2014

Petascale computations for Large-scale Atomic and Molecular collisions

Petaflop architectures are currently being utilized efficiently to perform large scale computations in Atomic, Molecular and Optical Collisions. We solve the Schroedinger or Dirac equation for the appropriate collision problem using the R-matrix or R-matrix with pseudo-states approach. We briefly outline the parallel methodology used and implemented for the current suite of Breit-Pauli and DARC codes. Various examples are shown of our theoretical results compared with those obtained from Synchrotron Radiation facilities and from Satellite observations. We also indicate future directions and implementation of the R-matrix codes on emerging GPU architectures.

cs.DC↗
arXiv2012

Photoionization, fluorescence, and inner-shell processes

Major advances in state-of-the-art theoretical methods coupled with advances in computational architectures have opened the doorway to large-scale computations on elements across the periodic table, allowing the inclusion of fully relativistic effects. Whenever possible results have been benchmarked against high resolution measurements obtained from either synchrotron radiation facilities or satellite observations. Various stages of ionization, necessary for the many applications in astrophysics, can be studied in the absence of experimental values to obtain the necessary data. A simple review is presented of photoionization, fluorescence and inner-shell processes recently investigated.

astro-ph.HE↗
arXiv2012

Photoionization Cross-Sections for the trans-iron element Se$^+$ from 18 eV 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 made at the Advanced Light Source (ALS) radiation facility in Berkeley, California, USA. We report on results for the photon energy range 18.0 eV -- 31.0 eV, which spans the ionization thresholds of the $\rm ^4S^o_{3/2}$ ground state and the low-lying $\rm ^2D^o_{5/2,3/2}$ and $\rm ^2P^o_{3/2,1/2}$ 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 $\rm 4p \rightarrow nd$ transitions with the recent ALS experimental measurements made on this complex trans-iron element.

astro-ph.EP↗
arXiv2000

Interstellar X-ray Absorption Spectroscopy of Oxygen, Neon, and Iron with the Chandra LETGS Spectrum of X0614+091

We find resolved interstellar O K, Ne K, and Fe L absorption spectra in the Chandra Low Energy Transmission Grating Spectrometer spectrum of the low mass X-ray binary X0614+091. We measure the column densities in O and Ne, and find 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.

astro-ph↗