Atomic Continuum Research Articles

Spectral Module for Photon Monte Carlo Calculations in Hypersonic Nonequilibrium Radiation

In this paper, efficient spectral modules and random number databases are developed for atomic and diatomic species for use in photon Monte Carlo (PMC) modeling of hypersonic nonequilibrium flow radiation. To model nonequilibrium flow conditions, the quasisteady state assumption was used to generate electronic state populations of atomic and diatomic gas species in the databases. For atomic species (N and O), both bound-bound transitions and continuum radiation were included and were separately databased as a function of electron temperature and number density as well as the ratio of atomic ion to neutral number density. For the radiating diatomic species of N2+, N2, O2, and NO databases were generated for each electronic molecular electronic system. In each molecular electronic system, the rovibrational transition lines were separately databased for each electronic upper state population forming the electronic system. The spectral module for the PMC method was optimized toward computational efficiency for emission calculations, wavelength selections of photon bundles and absorption coefficient calculations in the ray tracing scheme.

Low temperature Rosseland opacities with varied abundances of carbon and nitrogen

Context. With certain assumptions, radiative energy transport can be modelled by the diffusion approximation. In this case, the Rosseland mean opacity coefficient characterises the interaction between radiation and matter. The opacity data are usually available in pre-tabulated form, and in the generation of the data one assumes a distinct heavy element mixture, which is usually a scaled solar one. Therefore, presently available data is unable to cover the full parameter range of some astrophysical problems, in which the chemical composition of the medium being considered varies. Aims. We attempt to produce low temperature opacity data incorporating the effects of varied abundances of the elements carbon and nitrogen. For our temperature range of interest, molecules represent the dominant opacity source. Our dataset covers a wide metallicity range and is meant to provide important input data for stellar evolution models and other applications. Methods. We conduct chemical equilibrium calculations to evaluate the partial pressures of neutral atoms, ions, and molecules. Based on a large dataset containing atomic line and continuum data and, most importantly, a plethora of molecular lines, we calculate Rosseland mean opacity coefficients not only for a number of different metallicities, but also for varied abundances of the isotopes 12 C and 14 N at each metallicity. The molecular data comprise the main opacity sources for either an oxygen-rich or carbon-rich chemistry. We tabulate the opacity coefficients as a function of temperature and, basically, density. Results. Due to the special role of the CO molecule, within a certain chemistry regime an alteration to the carbon abundance causes considerable changes in the Rosseland opacity. The transition from a scaled solar (i.e. oxygen-rich) mixture to a carbon-rich regime results in opacities that can, at low temperatures, differ by orders of magnitude from to the initial situation. The reason is that the mean opacity in either case is due to different molecular absorbers. Variations in the abundance of nitrogen have less pronounced effects but, nevertheless, cannot be neglected. Conclusions. In typical astrophysical applications, it is indispensable to take into account opacity variations due to chemistry changes. In this respect, the new data is superior to previous compilations, but is, however, still subject to uncertainties.

Net emission of H2O–air–MgCl2/CaCl2/NaCl thermal plasmas

This paper is devoted to the calculation of the net radiation emitted by water–air–MgCl2/CaCl2/NaCl mixtures. Assuming a spherical, homogeneous and isothermal plasma, the net emission coefficient (NEC) is calculated for several pressures between 1 and 10 bar in the temperature range 300–30 000 K under the assumption of local thermodynamic equilibrium. The various contributions to the NEC (atomic continuum, molecular continuum, molecular band and spectral lines) are presented and described. The influence of the pressure and of the proportion of air, MgCl2, CaCl2 or NaCl is analysed. It is also demonstrated that the alkaline salt resonance lines (Ca+, Ca2+, Na+, Mg+, Mg2+, Cl+ and Cl2+) have a significant effect on the NEC value in spite of their strong self-absorption.

Above-threshold ionization of atoms in linearly polarized laser fields of relativistic intensities

Within the framework of the relativistic quantum theory the multiphoton above-threshold ionization (ATI) of hydrogenlike atoms in the field of super-strong electromagnetic radiation of linear polarization, taking into account the photoelectron stimulated bremsstrahlung (SB) on the residual ion, is investigated. The latter is considered in the Born approximation by the Coulomb field which allows us to simplify the previous results in the generalized eikonal approximation upon the scattering potential and obtain an analytic formula for the relativistic ATI rate summed over the intermediate transitions of a photoelectron through the virtual states of the atomic continuum. The energetic spectra and angular distributions of photoelectrons are obtained with the secondary SB of photoelectrons. Due to the latter the partial ATI rates are 4 times larger than the ATI rates of the direct process. In contrast to the nonrelativistic case, the main peaks in the angular distributions of photoelectrons are shifted toward the wave propagation direction and have a tendency to be aligned about this direction with the increase of the field intensity.

A New Method for Stress and Deformation Analysis from Nano to Macro Dimensional Scales

This paper presents a concurrent multiscale method for the stress analysis of solids using a coupled meshless and molecular dynamic analysis. A new transition algorithm using transition particles was employed to ensure the compatibility of both displacements and their gradients. The equivalent continuum strain energy density was obtained locally based on the atomic potential and Cauchy-Born rule, and hence plasticity can be easily handled in not only the atomic domain but also the continuum domain. Numerical examples demonstrated that the present multiscale technique has a promising potential of application to multiscale systems subjected to deformation.

An atomic interaction-based continuum model for adhesive contact mechanics

A micro/nano-scale computational contact mechanics model is proposed to study the adhesive contact between deformable bodies. To model adhesive contact, an interatomic interaction potential is incorporated into the framework of nonlinear continuum mechanics. The ensuing contact model is cast into an efficient finite element formulation which is implemented using an updated Lagrangian approach. The scaling of the model with respect to its geometrical size and the strength of adhesion is investigated. The proposed computational contact model is validated by a comparison with the analytical JKR and Maugis–Dugdale models.

Suzaku Spectroscopic Study of Hard X-Ray Emission in the Arches Cluster

We present the results of a Suzaku study of the Arches cluster in the Galactic center region. A high signal-to-noise ratio spectrum in the 3–12 keV band was obtained with the X-ray Imaging Spectrometer (XIS) onboard the Suzaku observatory. We found that the spectrum consists of a thermal plasma, a hard power-law tail, and two Gaussian line components. The plasma component with a temperature of $\sim 2.2 \,\mathrm{keV}$ is established from the presence of Ca XIX and Fe XXV $\mathrm{K}\alpha$ lines as well as absence of the Fe XXVI $\mathrm{K}\alpha$ line. The two Gaussian lines represent the $\mathrm{K}\alpha$ and $\mathrm{K}\beta$ lines from iron at lower ionization stages at $\sim 6.4$ and $\sim 7.1 \,\mathrm{keV}$. Both the line centers and the intensity ratio of these two lines are consistent with neutral iron. The hard power-law tail with a photon index of $\sim 0.7$ was found to have no pronounced iron K edge feature. Compared with the published Chandra spectra constructed separately for point-like and diffuse emission, we conclude that the thermal component is from an ensemble of point-like sources plus thermal diffuse emission concentrated at the cluster center, while the Gaussian and the hard tail components are from non-thermal diffuse emission extended in a larger scale. In band-limited images of the XIS field, the distribution of the 7.5–10.0 keV emission resembles that of the 6.4 keV emission, including local excess at the Arches cluster. This strongly suggests that the power-law emission is related to the 6.4 and 7.1 keV lines in the underlying physics. We discuss two ideas to explain both the hard continuum and the lines: (1) X-ray photoionization that produces fluorescence lines and the Thomson scattering continuum and (2) non-thermal electron impact ionization of iron atoms and bremsstrahlung continuum. However, whichever scenario is adopted, the photon or particle flux from the Arches cluster is too low to account for the observed lines and continuum intensity.

Double ionization of atoms by ion impact: two-step models

Total cross sections for the double ionization of He and Li atoms by the impact of H+, He2+ and Li3+ are calculated at intermediate and high energies within two-step models. The double ionization of He by the impact of other bare projectiles at a fixed energy is obtained as well. Single ionization probabilities are calculated within the continuum distorted wave –eikonal-initial-state (CDW–EIS) approximation. The required atomic bound and continuum wave functions are evaluated by numerically solving the atomic wave equation with an optimized potential model (OPM). Correlation between events is introduced by considering ion relaxation. The final state electronic correlation is considered by means of the so-called Gamow factor. We compare the transition probabilities resulting from our approach with those resulting from the use of a Rootham–Hartree–Fock initial state and a Coulomb continuum state with an effective charge. We find that the use of OPM waves gives a better agreement with the experimental results than with Coulomb waves.

BSR: B-spline atomic R-matrix codes

BSR is a general program to calculate atomic continuum processes using the B-spline R-matrix method, including electron–atom and electron–ion scattering, and radiative processes such as bound–bound transitions, photoionization and polarizabilities. The calculations can be performed in LS-coupling or in an intermediate-coupling scheme by including terms of the Breit–Pauli Hamiltonian. New version program summary Title of program: BSR Catalogue identifier: ADWY Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADWY Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Computers on which the program has been tested: Microway Beowulf cluster; Compaq Beowulf cluster; DEC Alpha workstation; DELL PC Operating systems under which the new version has been tested: UNIX, Windows XP Programming language used: FORTRAN 95 Memory required to execute with typical data: Typically 256–512 Mwords. Since all the principal dimensions are allocatable, the available memory defines the maximum complexity of the problem No. of bits in a word: 8 No. of processors used: 1 Has the code been vectorized or parallelized?: no No. of lines in distributed program, including test data, etc.: 69 943 No. of bytes in distributed program, including test data, etc.: 746 450 Peripherals used: scratch disk store; permanent disk store Distribution format: tar.gz Nature of physical problem: This program uses the R-matrix method to calculate electron–atom and electron–ion collision processes, with options to calculate radiative data, photoionization, etc. The calculations can be performed in LS-coupling or in an intermediate-coupling scheme, with options to include Breit–Pauli terms in the Hamiltonian. Method of solution: The R-matrix method is used [P.G. Burke, K.A. Berrington, Atomic and Molecular Processes: An R-Matrix Approach, IOP Publishing, Bristol, 1993; P.G. Burke, W.D. Robb, Adv. At. Mol. Phys. 11 (1975) 143; K.A. Berrington, W.B. Eissner, P.H. Norrington, Comput. Phys. Comm. 92 (1995) 290].

The Computational Materials Design Facility (CMDF): A powerful framework for multi-paradigm multi-scale simulations

AbstractPredicting the properties and behavior of materials by computer simulation from a fundamental, ab initio perspective has long been a vision of computational material scientists. The key to achieving this goal is utilizing hierarchies of paradigms and scales that connect macrosystems to first principles quantum mechanics (QM). Here we describe a new software environment, the “Computational Materials Design Facility” (CMDF), capable of simulations of complex materials studies using a variety of simulation paradigms. The CMDF utilizes a Python scripting layer to integrate different computational tools to develop multi-scale simulation applications. We have integrated DFT QM methods, the first principles ReaxFF reactive force field, empirical all atom force fields (FFs), mesoscale and continuum methods. The central data structure Extended OpenBabel (XOB) plays a critical role as glue between applications. We demonstrate the usefulness of CMDF in examples that couple complex chemistry and mechanical properties during dynamical failure processes, as for example in a study of cracking of Ni under presence of O2.

Mechanical properties of graphite oxides:Ab initiosimulations and continuum theory

The mechanical properties of oxidized graphitic layers are calculated by atomic ab initio simulations and continuum theory of elasticity. The adsorption of atomic oxygen atoms induces a nanocorrugation as well as a local bonding reinforcement that modify the bending coefficient of a graphite layer. As a matter of fact, a surface oxygen composition of 12.5% leads to an increase of the rigidity coefficient by more than a factor of 40. We thus demonstrate that oxidation also has a considerable impact on the mechanical properties of graphite structures.

Implementation of exterior complex scaling in B-splines to solve atomic and molecular collision problems

B-spline methods are now well established as widely applicable tools for the evaluation of atomic and molecular continuum states. The mathematical technique of exterior complex scaling has been shown, in a variety of other implementations, to be a powerful method with which to solve atomic and molecular scattering problems, because it allows the correct imposition of continuum boundary conditions without their explicit analytic application. In this paper, an implementation of exterior complex scaling in B-splines is described that can bring the well-developed technology of B-splines to bear on new problems, including multiple ionization and breakup problems, in a straightforward way. The approach is demonstrated for examples involving the continuum motion of nuclei in diatomic molecules as well as electronic continua. For problems involving electrons, a method based on Poisson's equation is presented for computing two-electron integrals over B-splines under exterior complex scaling.

Elastic interaction of oxygen atoms on a graphite surface

The elastic interaction energy between two oxygen atoms deposited on a graphite basal plane is calculated by atomic ab initio simulations and continuum theory of elasticity. The comparison between the results obtained with the two different methods, despite their usual length scale domain of application, is very good; attractive and repulsive behaviors are found. We speculate that this type of interaction could play an important role in the rearrangement of oxygen atoms experimentally observed after a weak oxidation of a graphite surface.

Net emission coefficient of air thermal plasmas

We have calculated the net emission coefficient of air plasmas at atmospheric pressure in the temperature range between 300 and 40 000 K, in the assumption of local thermodynamic equilibrium and isothermal plasmas. This calculation takes into account the radiation due to the atomic continuum, the molecular continuum, the molecular bands (several systems for O2, N2, NO and N2+) and the atomic lines. Special attention has been devoted in this paper to the description of the molecular bands radiation. The results show that in the high temperature range where molecules are dissociated, the radiation properties of air plasmas are mainly due to those of nitrogen plasmas, the resonance atomic lines playing an important role in spite of their strong self-absorption. At low temperature (T<6000 K) the role of the molecular bands of oxygen (O2) and NO is predominant.

Sampling statistics and considerations for single-shot analysis using laser-induced breakdown spectroscopy

A statistical analysis of single-shot spectral data is reported for laser-induced breakdown spectroscopy (LIBS). Fluctuations in both atomic emission and plasma continuum emission are investigated in concert for a homogenous gaseous flow, and fluctuations in plasma temperature are reported based on iron atomic emission in an aerosol-seeded flow. Threshold irradiance for plasma initiation and plasma absorption were investigated for pure gaseous and aerosol streams, with detailed statistical measurements performed as a function of pulse energy in the breakdown regime. The ratio of the analyte atomic emission intensity to the continuum emission intensity (peak/base) provided a robust signal for single-shot LIBS analysis. Moreover, at optimal temporal delay, the precision of the LIBS signal was maximized for pulse energies within the saturation regime with respect to plasma absorption of incident energy. Finally, single-shot temperature measurements were analyzed, leading to the conclusion that spatial variations in the plasma volume formation and subsequent plasma emission collection, play important roles in the overall shot-to-shot precision of the LIBS technique for gaseous and aerosol analysis.

Low Frequency Science with the Square Kilometre Array

Over the past several years an international community of scientists and engineers has emerged with a common goal to solve the technical challenge required to construct a giant radio telescope with a collecting area of one square kilometre. The Square Kilometre Array (SKA) will have a hundred times more collecting area than our most powerful existing radio telescopes, providing sensitivity of a few tens of nanoJy in the centimetre/decimetre wavelength continuum. With a spatial resolution better than the Hubble Space Telescope, a field of view larger than the full moon, and the ability to simultaneously image a wide range of red shift, the SKA will be the worlds premier spectroscopic imaging telescope at any waveband.At long wavelengths the SKA will be able to detect emission from atomic hydrogen gas at extreme redshifts, allowing study of the “Dark Ages” of the Universe, before, and during, the transition phase when the initial stars formed and reionization occurred. The combination of sensitivity, wide field of view and high angular resolution, will allow high resolution imaging of the interstellar media and magnetic field of a vast number of galaxies to high redshift. Measurements of atomic hydrogen emission and continuum emission will trace the star formation history of the Universe from primordial galaxies to the present.

Requirements for detection of ionospheric bubbles in the oxygen recombination EUV airglow

We present simulations that show the requirements for the space‐based detection of ionospheric bubbles in the atomic oxygen recombination continuum at 911 Å. In particular, we provide limits to the instrument sensitivity and integration time required for the remote sensing of these diverse and dynamic density depletions that percolate up through the nightside equatorial ionosphere. To first order, these are also the requirements to combine a series of observations and tomographically invert the two‐dimensional structure of the depleted flux tubes. We find that with a sensitivity of 1 count per second per rayleigh, a minimum of 1‐s integration is required to obtain a 3σ detection of the depletion over a 5° zenith angle bin. We compare the results of these simulations with the characteristics of three instruments from the Space Test Program 78–1, Tomographic Experiment using Radiative Recombinative Ionospheric EUV and Radio Sources, and Advanced Research and Global Observation Satellite missions, designed to measure this emission. This comparison shows that the state of the art of ultraviolet remote sensing has advanced to the point where spectrographs can be developed to obtain multidimensional images of bubbles and other similar ionospheric density structures.

Effect of beryllium nitrate vaporization on surface temperature in the pyrocoated graphite furnace

The vaporization of 20–50 μg beryllium from nitrate solution was observed in graphite furnace atomizers using pyrocoated and Ta-lined tubes. A charge coupled device (CCD) spectrometer was employed to follow the evolution of absorption spectra (200–475 nm), the light scattering and emission. Molecular bands of NO and NO 2 were observed below 1000°C. Beryllium absorption at 234.9 nm was prominent in spectra above 2200°C and 1900°C, respectively, in Ta-lined and pyrocoated tubes. The evolution profile of Be atomic absorption and of some bands indicated a faster vapor release in the pyrocoated tube. Light scattering occurred only in the pyrocoated tube, increasing with the tube age. When purge gas mini flow was applied, the scattering was observed at 1900–2200°C simultaneously with Be atomic absorption and emission continuum at long wavelength. The emission continuum showed the wavelength distribution characteristic of black body radiation. The temperature increase, due to the vaporization of the sample, was estimated using Planck’s equation. The maximum temperature increase reached 400°C, when the most intense Be atomic absorption, light scattering and emission was observed. According to the hypothesis proposed, the black body radiation was induced by the formation of Be carbide in the pyrographite layer. Low heat capacity across the pyrographite prevented the heat dissipation, and led to increase of surface temperature. This induced an increase of sample evaporation rate and the formation of a thermal gradient in the cross section of the tube. Both factors originated vapor supersaturation in the tube center, spatial condensation and, accordingly, light scattering. The results, together with those already obtained with Mg nitrate place limitations to the atomization theories based on the concept of isothermal equilibrium or on Arrhenius kinetic approach.

Multiple phase control in Mg through the continuum

We present results from perturbative calculations on a scheme of coherent control in Mg. The scheme involves the excitation of Mg in the vicinity of an autoionizing resonance lying above the first two ionization thresholds by a two-color field composed of a fundamental frequency and its third harmonic, whose relative phase can be continuously controlled. Four as well as two-photon coherent excitation pathways, involving appropriate combinations of the two frequencies, proceed through the atomic continuum simultaneously contributing to the excitation, while three-photon ionization by the fundamental is also energetically possible. As a result, some of the excitation processes involve above-threshold ionization (ATI). The calculated ionization yields for the various groups of photoelectrons reveal significant modulation as the relative phase of the field component frequencies is varied. The modulation patterns for the different photoelectron groups are mutually shifted due to the multiphoton matrix element phases associated both with the ATI process and the multichannel nature of the final state. The effect is particularly pronounced when the autoionizing state is in near four-photon resonance with the fundamental frequency. The role of an autoionizing state as a final state as well as that of the continuum as an intermediate state in coherent control processes is discussed.

Coherent population trapping in multilevel laser-induced continuum structure system including cascade two-photon processes

We have investigated the atomic coherent population trapping in a multilevel laser-induced continuum structure system including cascade two-photon processes, in which two sets of quasicontinua of excited levels to be modeled as the Bixon-Jortner quasicontinuum are laser-embedded into a previously structureless atomic continuum. The condition leading to the atomic coherent population trapping and the coherent population trapping state are given explicitly. And the effects of the Fano factors, the laser intensities and the detunings on the amount of population trapped in the atomic bound states and the populations distributed in the two sets of quasicontinua are also examined.