Theoretical Photoionization Research Articles

L X-ray intensity ratios for high Z elements induced with X-ray tube

Abstract We have studied the intensity ratios I(Lα1,2)/I(Lβ1,2), I(Lα1,2)/I(Lγ) and I(Lβ1,2)/I(Lγ) for elements Ta, W, Au and Pb by 13.1 keV bremsstrahlung radiation. In this work, experimental values were compared with the theoretical results and other experimental results. Theoretical results of the intensity ratios were calculated with theoretical subshell photoionization cross sections, fractional X-ray emission rates, fluorescence yields, and Coster–Kronig transition probabilities. Good agreement can be observed between experimental values and theoretical results. Comparing with L1 and L2 subshells, the ionization cross section of L3 subshell shows a large increase for Ta and W with the variation of excitation energy from 59.5 keV to 13.1 keV.

Photoionization of the valence shells of the neutral tungsten atom

Results from large-scale theoretical cross section calculations for the total photoionization (PI) of the 4f, 5s, 5p and 6s orbitals of the neutral tungsten atom using the Dirac Coulomb R-matrix approximation (DARC: Dirac-atomic R-matrix codes) are presented. Comparisons are made with previous theoretical methods and prior experimental measurements. In previous experiments a time-resolved dual laser approach was employed for the photo-absorption of metal vapours and photo-absorption measurements on tungsten in a solid, using synchrotron radiation. The lowest ground state level of neutral tungsten is , with J = 0, and requires only a single dipole matrix for PI. To make a meaningful comparison with existing experimental measurements, we statistically average the large-scale theoretical PI cross sections from the levels associated with the ground state (J = 0, 1, 2, 3, 4) levels and the excited metastable level. As the experiments have a self-evident metastable component in their ground state measurement, averaging over the initial levels allows for a more consistent and realistic comparison to be made. In the wider context, the absence of many detailed electron-impact excitation (EIE) experiments for tungsten and its multi-charged ion stages allows current PI measurements and theory to provide a road-map for future EIE, ionization and di-electronic cross section calculations by identifying the dominant resonance structure and features across an energy range of hundreds of eV.

Theoretical photoionization spectra for the ground state of a Mg-like Fe14+ ion converging to the Fe15+ 3l thresholds

The photoionization cross sections for the ground-state of a Mg-like Fe14+ ion converging to the Fe15+ 3l thresholds are calculated in LS coupling by using the variational eigenchannel R-matrix method, where is a much simpler technique than other extensive calculations such as the relativistic Breit-Pauli R-matrix method. The major approximation for this calculational formalism concerns the use of a Hamiltonian H that does not refer to the full atomic system, but represents only the valence electrons plus a central effective potential to represent the effects of the inner shell. The present calculational results are compared with the previous results obtained by considering the relativistic effects. With detailed descriptions for the resonances of the cross section across the Fe15+ 3l thresholds, we show that the relativistic effects are not so crucial in the energy range below the Fe15+ 3d threshold. The resonant states are identified and analyzed by using the resonance positions E r, effective quantum numbers n *, and widths Gr based on the idea of eigenphase gradients.

Theoretical photoionization spectra for the Mg-like S4+ Ion in the energy range between the S5+ 4s and 4f thresholds

The partial and the total photoionization cross sections of the Mg-like S4+ ion, leaving the residual S5+ 3l and 4l states from the ground 3s21Se and the excited 3s3p3,1Po initial states, have been calculated for photon energies ranging from the S5+ 4s to the S5+ 4f threshold. The complex features appearing in the cross sections represent the tangled autoionizing Rydberg resonances converging to the different S5+ 4l thresholds. The present results are compared with the available previous National Institute of Standards and Technology(NIST) and Opacity Project(OP) results.

THEORETICAL EVOLUTION OF OPTICAL STRONG LINES ACROSS COSMIC TIME

We use the chemical evolution predictions of cosmological hydrodynamic simulations with our latest theoretical stellar population synthesis, photoionization and shock models to predict the strong line evolution of ensembles of galaxies from z=3 to the present day. In this paper, we focus on the brightest optical emission-line ratios, [NII]/H-alpha and [OIII]/H-beta. We use the optical diagnostic Baldwin-Phillips-Terlevich (BPT) diagram as a tool for investigating the spectral properties of ensembles of active galaxies. We use four redshift windows chosen to exploit new near-infrared multi-object spectrographs. We predict how the BPT diagram will appear in these four redshift windows given different sets of assumptions. We show that the position of star-forming galaxies on the BPT diagram traces the ISM conditions and radiation field in galaxies at a given redshift. Galaxies containing AGN form a mixing sequence with purely star-forming galaxies. This mixing sequence may change dramatically with cosmic time, due to the metallicity sensitivity of the optical emission-lines. Furthermore, the position of the mixing sequence may probe metallicity gradients in galaxies as a function of redshift, depending on the size of the AGN narrow line region. We apply our latest slow shock models for gas shocked by galactic-scale winds. We show that at high redshift, galactic wind shocks are clearly separated from AGN in line ratio space. Instead, shocks from galactic winds mimic high metallicity starburst galaxies. We discuss our models in the context of future large near-infrared spectroscopic surveys.

ChemInform Abstract: Photoelectron Spectroscopic Studies of the Electronic Structure of . alpha.-SiC

The electronic structure of single crystal {alpha}-SiC (silicon carbide) has been determined with variable photon energy valence band photoelectron spectroscopy. Synchrotron radiation in the range 30-250 eV was used in addition to a conventional Al K{alpha} x-ray source to follow the intensity changes of valence band features as a function of the incident photon energy. The dominant atomic orbital contributions have been assigned to valence band features by comparing them to theoretical atomic photoionization cross sections. The results indicate that C 2s based ionizations appear at the highest binding energy, near 15 eV, while Si 3s features are evident near 10 eV. The highest occupied energy levels (from approximately 1-8-eV binding energies) are predominantly C 2p based. Differences between the photon energy dependence of the data and theoretical atomic orbital cross sections show the valence levels to be highly mixed. Comparison of the valence band spectra with a molecular orbital calculation confirms the experimental assignments, but the calculation appears to underestimate the extent of atomic orbital mixing. The valence band spectrum of a hot-pressed polycrystalline SiC sample compares quite well with the single-crystal spectrum. The implications of these results on the chemisorption behavior and reactivity of SiC are discussed.

Parameterization and generation of photon-induced K cross-sections

Theoretical K-shell photoionization and K X-ray fluorescence (K XRF) cross sections have been fitted empirically in polynomials of photon energy E and atomic number Z. This has been used to develop a computer code KCSPIF to generate K-shell photo-ionization and K XRF cross-sections for any element in the range 5 ≤Z≤95 and for photon energies, above K-edge–1500 keV, when only the atomic number and photon energy are supplied as the input.

Experimental and theoretical photoionization cross sections for Se+ from 18 eV to 31 eV

Absolute Se photoionization cross-section measurements and Dirac-Coulomb R-matrix calculations are reported for the photon energy range 18.0 eV – 31.0 eV, which spans the ionization thresholds of the 4S03/2 ground state and the low-lying 2D03/2,5/2 and 2P01/2,3/2 metastable states. The determination of the photoionization and recombination properties of n-capture element ions is motivated by their astrophysical detection and the importance of their elemental abundances in testing theories of nucleosynthesis and stellar structure.

Photoionization of the excited He atom in Debye plasmas

We present theoretical photoionization cross sections for He 1s2s 1S and He 1s2p 1P states in a Debye plasma environment by the complex coordinate rotation method, using a finite L 2 basis set constructed from one electron Laguerre orbitals. The plasma environment is found to appreciably influence the photoionization cross sections near the ionization threshold. In this regard, the photoionization cross sections of isolated He are compared with other theoretical and experimental results. Our results are in good agreement with the previous results. A new minimum appears in the photoionization cross section curve for the metastable 1s2s 1S state. Results are given for the S- and D-wave partial photoionization for the excited 1s2p 1P state.

Use of partial-wave decomposition to identify resonant interference effects in the photoionization–excitation of argon

We have studied simultaneous photoionization and excitation of Ar in the range of incident photon energies between 36.00 and 36.36 eV, where the resonant production of doubly excited neutral Ar states imbedded in the ionization continuum is dominant. By measuring the relative Stokes parameters of the fluorescence from residual Ar+* (3p4 [3P] 4p) ions (2P1/2, 465.8 nm transition; 2P3/2, 476.5 nm; 2D3/2, 472.7 nm; 2D5/2, 488.0 nm; 4P5/2, 480.6 nm; 4D5/2, 514.5 nm) we demonstrate a technique for determining individual partial-wave cross sections in photoionizing collisions. This procedure is shown to be important in sorting out competing dynamical ionization mechanisms, particularly with regard to resonant production of intermediate doubly excited autoionizing states. Comparison with theoretical photoionization cross sections demonstrates that spin–orbit coupling between different states of Ar II needs to be accounted for in the calculations.

Changes in the crystal and electronic structure of LiCoO2 and LiNiO2 upon Li intercalation and de-intercalation

Li(x)CoO(2) and Li(x)NiO(2) (0.5 < x < 1) are used as prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue when used as cathode material during electrochemical cycling. In order to analyze the change of the structure and the electronic structure of Li(x)CoO(2) and Li(x)NiO(2) as a function of Li content x in detail, we have performed X-ray diffraction studies, photoelectron spectroscopy (PES) investigations and band structure calculations for a series of compounds Li(x)(Co,Ni)O(2) (0 < x < or = 1). The calculated density of states (DOS) are weighted by theoretical photoionization cross sections and compared with the DOS gained from the PES experiments. Consistently, the experimental and calculated DOS show a broadening of the Co/Ni 3d states upon lithium de-intercalation. The change of the shape of the experimental PES curves with decreasing lithium concentration can be interpreted from the calculated partial DOS as an increasing energetic overlap of the Co/Ni 3d and O 2p states and a change in the orbital overlap of Co/Ni and O wave functions.

Irregularity of Autoionizing Rydberg Series in Photoionization Leading to the Si3+ 3l States from the Mg-Like Si2+ Ions

We report the theoretical photoionization cross sections in the low energy region for Mg-like Si 2+ ion, with arousing desire for experiments. With the advent of third-generation synchrotron radiation sources, it is expected to study for this ionic system using merged-beam technique. Since the Mg-like Si 2+ ions are expected to be admixed with the ground-state and several low-lying excited-states, we have calculated the photoionization cross sections from the initial ground-state 3 s 2 1 S , and the excited-states 3 s 3 p 1,3 P , 3 s 3 d 1,3 D , and 3 s 4 s 1,3 S of Mg-like Si 2+ ion. For these calculations, we have used a most elaborate noniterative eigenchannel R -matrix method. We briefly discuss the results for autoionizing resonances and interlopers in the structure of photoionization cross sections, and compare with the previous results of Opacity Project.

Theoretical photoionization spectra in the UV photon energy range for a Mg-like Al+ ion

In the present work, we report the photoionization cross sections of the Al+ ion calculated for the photon energy range 20–26 eV and 30–50 eV. We have expanded our previous calculation (2007 J. Phys. Soc. Japan 76 014302) with an optimized admixture of the initial ground state 3s21S and exited states 3s3p1,3P, 3s3d1,3D and 3s4s1,3S, and obtained significantly improved predictions for the main background and autoionizing resonance structures of the reported experimental spectra. The absolute measurements of the photoionization cross sections of the Al+ ion in these energy ranges have been performed by West et al (2001 Phys. Rev. A 63 052719), and they reported that the prominent peaks around 21 eV were attributed to the effects of the significant influence of the small fraction of the fourth-order radiation with energies around 84 eV from the synchrotron source. In our previous work, the main shape for these cross sections was calculated assuming an admixture of initial 3s21S and 3s3p3P states, only with a rough overall estimate for the experimental spectra in the photon energy range 20–26 eV, and without these peaks around 21 eV. The report of the experimental assignment attributes these peaks to the excitation of a 2p electron from the core. However, our present results with the new admixture reveal similar peaks without considering the possibility of the core excitation.

Differential and total M-shell X-ray production cross-sections of some selected elements between Au and U at 5.96 keV

Differential M-shell X-ray production (MXRP) cross-sections for selected heavy elements between Au and U have been measured at 5.59keV incident photon energy, respectively at seven angles varying from 120° to 150° a Si(Li) detector. The differential M-shell X-ray production cross-sections have been derived, using M-shell fluorescence yields, experimental total M X-ray production cross-sections and theoretical M-shell photoionization cross-sections. The differential M-shell X-ray production cross-sections have been compared with the semi-empirical fits. The measured differential M X-ray production cross-sections have been found within experimental error. Differential M X-ray production cross-section can be fitted to the ∑nanZn (n=2) as a function of cosθ. Total M X-ray production cross-sections have been derived using the fitted values.

Many-electron character of Rydberg series converging to the ionization thresholds of the 4p 45s states of KrII

The photoionization cross sections of the 4p shell and the 4s main level and 4p 4(3 P) 5s 4 P 5/2, 3/2 satellite subvalence levels of KrII have been calculated in the 4s-near-threshold range of excitation energies from 28.48 to 28.70 eV. The calculation takes into account the core relaxation by the methods of the theory of non-orthogonal orbitals, the interaction between resonant states through autoionization channels by solving the complex secular equation, and the interaction between the channels of the continuous spectrum in all orders of the perturbation theory by the K-matrix method. Good quantitative agreement between the energy-integrated theoretical and experimental photoionization cross sections for the satellite levels has been obtained for the first time. It is shown that only simultaneous consideration of the above-mentioned effects leads to such agreement. The resonant structure of the photoionization cross sections in this excitation energy range is related to the autoionization decay of the 4p 45s(4 P 1/2)np and 4p 45s(2 P 3/2)np Rydberg series. The specificity of this process is that both series manifest themselves not independently but owing to their strong electrostatic interaction with the prominent 4p 4(1 D)5s 2 D 5/2 6p 3/2 resonance, which lies in this excitation energy range.

The host galaxies and classification of active galactic nuclei

We present an analysis of the host properties of 85224 emission-line galaxies selected from the Sloan Digital Sky Survey. We show that Seyferts and LINERs form clearly separated branches on the standard optical diagnostic diagrams. We derive a new empirical classification scheme which cleanly separates star-forming galaxies, composite AGN-H ii galaxies, Seyferts and LINERs and we study the host galaxy properties of these different classes of objects. LINERs are older, more massive, less dusty and more concentrated, and they and have higher velocity dispersions and lower [OIII] luminosities than Seyfert galaxies. Seyferts and LINERs are most strongly distinguished by their [OIII] luminosities. We then consider the quantity L[OIII]/σ 4 , which is an indicator of the black hole accretion rate relative to the Eddington rate. Remarkably, we find that at fixed L[OIII]/σ 4 , all differences between Seyfert and LINER host properties disappear. LINERs and Seyferts form a continuous sequence, with LINERs dominant at low L/LEDD and Seyferts dominant at high L/LEDD . These results suggest that the majority of LINERs are AGN and that the Seyfert/LINER dichotomy is analogous to the high/low-state transition for X-ray binary systems. We apply theoretical photo-ionization models and show that pure LINERs require a harder ionizing radiation field with lower ionization parameter than Seyfert galaxies, consistent with the low and high X-ray binary states.

Theoretical studies on the photoionization cross-sections of solid silver

An alternative expression for photoionization cross-section of atoms or molecules and a dielectric influence function (DIF) in a high-density system proposed recently are used to study the photoionization cross-sections of solid silver. It is suggested that a density turning point (DTP) of a photoionized system may be viewed as the critical point where the photoionization properties of atoms in a real system may have a notable change. The results show that the present theoretical photoionization cross-sections are in good agreement with the experimental results of a silver crystal both in structure and in magnitude.

HIGH DISPERSION OPTICAL SPECTROSCOPY OF PLANETARY NEBULAE

Chemical compositions of planetary nebulae are of interest for a study of the late stage of stellar evolution and for elemental contributions to the interstellar medium of reprocessed elements since possibly a large fraction of stars in 0.8 - 8 <TEX>$M_{\bigodot}$</TEX> range go through this stage. One of the methods for getting chemical composition is a construction of theoretical photoionization models, which involves geometrical complexities and a variety of physical processes. With modelling effort, one can analyze the high dispersion and find the elemental abundances for a number of planetary nebulae. The model also gives the physical parameter of planetary nebula and its central star physical parameter along with the knowledge of its evolutionary status. Two planetary nebulae, NGC 7026 and Hu 1-2, which could have evolved from about one solar mass progenitor stars, showed radically different chemical abundances: the former has high chemical abundances in most elements, while the latter has extremely low abundances. We discuss their significance in the light of the evolution of our Galaxy.

L X-ray production cross sections, average L shell fluorescence yield and intensity ratios in heavy elements

L X-ray fluorescence cross sections, and intensity ratios were measured for elements in the 70£Z£92 atomic range at the excitation energy 59.5 keV using a Si(Li) detector. Furthermore, L X-ray fluorescence cross sections and intensity ratios were calculated for elements in the same range. The average L shell fluorescence yields were derived using experimental L X-ray fluorescence cross sections and theoretical photoionization cross sections. The obtained results were compared with other experimental and theoretical values.

Absolute Photoionization Cross Section Measurements of O ii Ions from 29.7 to 46.2 eV

Absolute photoionization cross sections have been measured for a mixture of ground-state and metastable O ii (O + ) ions at photon energies ranging from 29.9 to 46.0 eV (414.7 to 269.5 A ˚ ). All measurements were performed by merging an O + beam with synchrotron radiation from an undulator beam line at the Advanced Light Source (ALS). At a resolution of 17 meV, more than 70 spectral features have been resolved, most of them identified and characterized. These measurements are compared with two independent R-matrix calculations and the data in TOPbase. All three calculations agree within 25% on the direct photoionization cross section, and with the absolute measurements within 40%. Some differences are noted in the predicted resonance positions among the three close-coupling R-matrix calculations, the TOPbase data being the least accurate. The estimated total experimental uncertainty varies from 15% to 20%. Such measurements benchmark theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project. Subject headings: atomic data — atomic processes — methods: laboratory