X-ray Photoionization Research Articles

Atomic Data Needs for X-ray Astronomy

AbstractWith the launches of the Chandra X-ray Observatory and XMM-Newton, high resolution X-ray spectra of cosmic sources are broadening our understanding of the physical conditions, such as temperature, density, ionization state, and elemental abundances. X-ray emitting astrophysical plasmas can be generally classified by their dominant ionization mechanism, either collisional ionization or X-ray photoionization. The atomic data needs are significantly different for these two cases; however, for both cases it is important that we identify robust and accurate diagnostics and that we verify completeness of the broadband models. We discuss the status of the atomic data currently used in atomic databases for X-ray astronomy, in view of theoretical and experimental atomic physics considerations.

Bipolar Charging of Aerosol Nanoparticles by a Soft X-ray Photoionizer.

A new bipolar charger for nanometer aerosols using soft X-ray photoionization was developed. The concentrations and mobility distributions of the positive and negative ions generated by the X-ray charger were investigated, and then the charging probability of nanoparticles was evaluated experimentally and theoretically. All experimental data of the X-ray charger were compared with those of a 241Am α-ray bipolar charger. It was found that the X-ray bipolar charger could produce at least about 3.5 times as high concentration of gaseous ions as the α-ray charger and thus could have particles attain the equilibrium bipolar charging state in shorter residence time than the α-ray charger. The charging state of particles attained by the X-ray charger was explained by theoretical calculations as well as α-ray bipolar charging. It was concluded that the X-ray charger would be a useful instrument for charging high number concentration nanoparticles in the electrical aerosol measurements.

Heating and Ionization of the Intergalactic Medium by an Early X‐Ray Background

Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts just over 6. A number of models have been developed to describe the process of reionization and the effects of the ionizing photons from the first luminous objects. In this paper, we study the impact of an X-ray background, such as high-energy photons from early quasars, on the temperature and ionization of the IGM prior to reionization, before the fully ionized bubbles associated with individual sources have overlapped. X-rays, which have large mean free paths relative to EUV photons, and their photoelectrons can have significant effects on the thermal and ionization balance. We find that hydrogen ionization is dominated by the X-ray photoionization of neutral helium and the resulting secondary electrons. Thus, the IGM may have been warm and weakly ionized prior to full reionization. We examine several related consequences, including the filtering of the baryonic Jeans mass scale, signatures in the cosmic microwave background, and the H$^{-}$-catalyzed production of molecular hydrogen.

Separationof inner-shell vacancy transfer mechanisms in collisions of slowAr17+ ions with SiO2

We have studied the spectrum of x-rays emitted when 130 and 200 keV kineticenergy hydrogen-like argon ions impact silicon dioxide surfaces.Specifically, we were interested in the mechanism for creationof K-shell holes in the silicon target atoms, which can befilled with the release of a 1.75 keV x-ray. Two mechanisms havebeen hypothesized for the vacancy transfer between the K-shellsof silicon and argon atoms: `direct vacancy transfer' and`projectile-decay-product-mediated vacancy transfer'. Toseparate these mechanisms, we used a target with a metalliccoating (preventing close collisions between Si and Ar butallowing x-ray transmission) and a target without such acoating. We found that x-ray photoionization is the dominantmechanism in both cases and measured an upper limit for thecontribution from the `direct mechanism' on the uncoatedsample. Furthermore, we measured the relative strengths of theKα, Kβ and Kγ lines of the argonprojectile as a function of kinetic energy and foundsatisfactory agreement with charge exchange and cascade modelcalculations.

Electron‐Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. V. Relativistic Calculations for Fe xxiv and Fe xxv for X‐Ray Modeling

Photoionization and recombination cross sections and rate coefficients are calculated for Li-like Fe XXIV and He-like Fe XXV using the Breit-Pauli R-matrix (BPRM) method. A complete set of total and level-specific parameters is obtained to enable X-ray photoionization and spectral modeling. The ab initio calculations for the unified (e + ion) recombination rate coefficients include both the non-resonant and the resonant recombination (radiative and di-electronic recombination, RR and DR, respectively) for (e + Fe XXV) -> Fe XXIV and (e + Fe XXVI) -> Fe XXV. The level specific rates are computed for all fine structure levels up to n = 10, enabling accurate computation of recombination-cascade matrices and effective rates for the X-ray lines. The total recombination rate coefficients for both Fe XXIV and Fe XXV differ considerably, by several factors, from the sum of RR and DR rates currently used to compute ionization fractions in astrophysical models. As the photoionization/recombination calculations are carried out using an identical eigenfunction expansion, the cross sections for both processes are theoretically self-consistent; the overall uncertainty is estimated to be about 10-20%. All data for Fe XXIV and Fe XXV (and also for H-like Fe XXVI, included for completeness) are available electronically.

Plasma diagnostics for x-ray driven foils at Z

We report the development of techniques to diagnose plasmas produced by x-ray photoionization of thin foils placed near the Z-pinch on the Sandia Z Machine. The development of 100+ TW x-ray sources enables access to novel plasma regimes, such as the photoionization equilibrium. To diagnose these plasmas one must simultaneously characterize both the foil and the driving pinch. The desired photoionized plasma equilibrium is only reached transiently for a 2-ns window, placing stringent requirements on diagnostic synchronization. We have adapted existing Sandia diagnostics and fielded an additional gated three-crystal Johann spectrometer with dual lines of sight to meet these requirements. We present sample data from experiments using 1-cm, 180-eV tungsten pinches to photoionize foils made of 200 Å Fe and 300 Å NaF co-mixed and sandwiched between 1000 Å layers of Lexan (C16H14O3), and discuss the application of this work to benchmarking astrophysical models.

Argon coated alkali halide clusters: the effect of the coating on the ionization and fragmentation dynamics

We present an investigation of the formation, ionization and fragmentation processes of a new class of clusters consisting of an ionically bound cluster core, coated by a shell of Van der Waals bound atoms. Element specific soft X-ray photoionization spectroscopy was used to investigate the alkali halide cluster core and the Ar coating separately. The thickness of the coating controls the mechanism of ionization and fragmentation of these inhomogeneous clusters. Unusual metastable structures of alkali halide clusters were observed when the thickness of the coating exceeded two monolayers.

Soft x-ray photoionization and fragmentation of SO2 studied by threshold photoelectron-photoion-photoion coincidence (TPEPIPICO) spectroscopy

The photoionization of SO2 has been studied in the photon energy range 160-560 eV using the SRS at Daresbury Laboratory, UK. Threshold photoelectron and total ion-yield spectra are presented in the region of the S 2p and O 1s edges. Ion branching ratios are obtained from threshold photoelectron-photoion coincidence (TPEPICO) data and the detailed dynamics of the fragmentation process determined using the threshold photoelectron-photoion-photoion coincidence (TPEPIPICO) technique. The data are compared with other results at the S 2p edges, and with previous studies of SO2 in the vacuum ultraviolet spectral region.

ASCAObservation of the Dipping X‐Ray Source XB 1916−053

We present the results of timing and spectral studies of the dipping X-ray source XB 1916-053, observed by ASCA during its performance verification phase. The detected dipping activity is consistent with previous observations, with a period of 3008 s and an intermittent secondary dip observed roughly 0.4 out of phase with the primary dip. The energy spectra of different intensity states are fitted with a power law with partial covering fraction absorption and interstellar absorption. The increase in the hardness ratio during the primary and secondary dips, and the increase in the covering fraction and column density with decreasing X-ray intensity, all imply that the dipping is caused by the photoabsorbing materials that have been suggested to be where the accreted flow hits the outer edge of the disk materials. The spectra at all intensity levels show no apparent evidence for Fe or Ne emission lines. This may be due to the low metal abundance in the accretion flow. Alternatively, the X-ray luminosity of the central source may be too weak to excite emission lines, which are assumed to be produced by X-ray photoionization of the disk materials.

DoubleK-Vacancy Production in Molybdenum by X-Ray Photoionization

We report the observation of double $K$-vacancy production in x-ray photoionization of molybdenum. From these data we deduce the relative probabilities for production of double and single $K$ vacancies by photon impact at 50 keV and report the most precise measure of the ratio of double to single $K$-vacancy production by photon impact in a heavy atom. That ratio provides an important probe of electron-electron correlations in high- $Z$ atoms where theoretical descriptions must consider those correlations simultaneously with relativity. Our results suggest the need for theoretical treatments to properly deal with such systems.

X-ray photoionization in the presence of a bichromatic laser field

X-ray photoionization of hydrogen in the presence of a bichromatic laser field is considered. The expressions for the $T$ matrix and the cross sections for the laser-assisted x-ray photoionization are presented. The initial state is the laser-field-dressed hydrogen-atom ground state, while the final state is the improved Coulomb-Volkov wave. The gauge consistency is ensured by working in the $\mathbf{r}\ensuremath{\cdot}\mathbf{E}$ gauge. It turns out that the matrix elements (the explicit form of which is given in the Appendix) are much simpler in this case. This enables one to consider the symmetry properties of the $T$ matrix and the cross sections analytically. It was shown that the symmetry $(\ensuremath{\varphi}+\ensuremath{\pi},\ensuremath{\pi}\ensuremath{-}\ensuremath{\theta})\ensuremath{\leftrightarrow}(\ensuremath{\varphi},\ensuremath{\theta})$, where $\ensuremath{\varphi}$ is the relative phase between the laser-field components and $\ensuremath{\theta}$ is the polar angle of the outgoing electron, is exact in our case. In addition to this, there are approximate symmetries $\ensuremath{\varphi}\ensuremath{\leftrightarrow}\ensuremath{-}\ensuremath{\varphi}$ and $\ensuremath{\varphi}\ensuremath{\leftrightarrow}\ensuremath{\varphi}+\ensuremath{\pi}$. All these symmetries, as well as the behavior of the differential and total cross sections as functions of $\ensuremath{\varphi}$, $\ensuremath{\theta}$, and the number of exchanged photons, are analyzed in numerous examples. The results presented show the possibility of the coherent phase control of the laser-assisted x-ray photoionization process. It was also shown that in the monochromatic case our model gives results that are in good agreement with the results of previous work.

SimultaneousHubble Space TelescopeandASCAObservations of LMC X‐4: X‐Ray Ionization Effects on a Stellar Wind

We present first results from simultaneous ultraviolet (UV; HST/GHRS) and X-ray (ASCA) observations of the 13.5s pulsar LMC X-4 taken in 1996 May. The GHRS data are the highest resolution (both temporal and spectral) UV spectra ever taken of LMC X-4. Fits to the UV continuum using a model that incorporates X-ray heating of the companion star and the accretion disk yields a mass accretion rate Mdot=4.0x10^(-8) Msun/yr; the X-ray luminosity implied by this value is consistent with the X-ray flux measured during simultaneous observations. The model accurately predicts observed B magnitude and ultraviolet variations over both orbital and long-term periods. The UV P-Cygni lines show dramatic changes with orbital phase: strong broad absorption near X-ray eclipse and narrow absorption when the X-ray source is in the line-of-sight. We interpret this as a result of X-ray photoionization of the stellar wind; when the neutron star is in front of the normal star, the wind absorption disappears and mainly the photospheric absorption lines are visible. The X-ray pulse period measured during our observations, 13.5090+/-0.0002s, is consistent with steady spin-down over the past 10 years. No pulsations were detected in the UV observations with upper limits to the pulsed fraction around N V and C IV of 1.8% and 2.7% in the continuum and 12.4% and 7% in the absorption troughs.

Correlation effects in the valence x‐ray photoionization spectra of ethylene, butadiene, and hexatriene

The x-ray photoemission spectra of the ethylene, butadiene, and hexatriene compounds were investigated using one-particle Green's function calculations which are exact up to third order in correlation. This study points to the importance of initial-state correlation and cross-section effects for a consistent reproduction of satellite bands recorded under x-ray photoionization. By extrapolating the results obtained to larger unsaturated hydrocarbons, it is clear that the signal recorded in the inner-valence region of polyacetylene must be exclusively assigned to many-body structures. This study also provides evidence for a partial contamination of the outer-valence band by shake-up lines. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 465–481, 1997

Correlation effects in the valence x-ray photoionization spectra of ethylene, butadiene, and hexatriene

The x-ray photoemission spectra of the ethylene, butadiene, and hexatriene compounds were investigated using one-particle Green's function calculations which are exact up to third order in correlation. This study points to the importance of initial-state correlation and cross-section effects for a consistent reproduction of satellite bands recorded under x-ray photoionization. By extrapolating the results obtained to larger unsaturated hydrocarbons, it is clear that the signal recorded in the inner-valence region of polyacetylene must be exclusively assigned to many-body structures. This study also provides evidence for a partial contamination of the outer-valence band by shake-up lines. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 465–481, 1997

HIGH ENERGY EMISSION FROM B STARS AND ITS RELATIONSHIP TO STELLAR WINDS

ABSTRACT In this work I present results of detailed studies of the high energy emission of two B stars using the EUVE, ROSAT, and ASCA telescopes. I also present the results of a ROSAT survey of 27 near-main-sequence B stars. These observations and related efforts at modeling the data are used to address several outstanding questions about B stars, their high energy emission, and their stellar winds. These questions involve the applicability of standard radiation-driven wind theory to B stars, the ability of the line-force instability mechanism and other wind-shock mechanisms to account for the EUV and X-ray emission from B stars, and the need to invoke non-wind emission mechanisms for anomalous B stars and those B stars with spectral type later than about B2. The first part of this study involves the simultaneous analysis of an EUVE emission line spectrum and the ROSAT PSPC data of the normal B2 II star Epsilon CMa. I find that a continuous temperature distribution in the emitting plasma as well as attenuation by the overlying stellar wind is needed to explain both data sets. The attenuation is consistent with the predictions of the wind instability model, while the temperature distribution and emission measure may require some slight modifications in the applications of the theories of the line-force instability and radiation-driven winds, respectively, to Epsilon CMa. In order to analyze these data sets I performed calculations of the ionization structure of the stellar wind which, for the first time, took into account the influence of X-rays on the photoionization rates. I also performed numerical simulations of the wind of Epsilon CMa to explore the applicability of the line-force instability mechanism. The next part of the study is the ROSAT survey in which I detected 20 of 27 near-main-sequence B stars. This result is in direct contradiction to earlier surveys in which only five to ten percent of stars later than B1 were detected. I find that the X-ray luminosities fall rapidly beyond B1, but they do not disappear. It is nearly impossible for these observations of mid- and late-B stars to be understood within the context of nearly any reasonable wind-shock model. On the other hand, the early-B stars in this survey have temperature distributions and wind filling factors similar to those found for Epsilon CMa. The degree of wind attenuation in these stars is also significant. Because the spectral models I employ account for wind attenuation I am able to derive both the emergent and the generated X-ray luminosities. One implication of these results, and also of the model fits to Epsilon CMa is that the well known LX/LBol ~10-7 law is the result of wind attenuation more than it is the result of anything inherent to the emission process. Finally, I used moderate resolution ASCA data to study the high energy processes on the unusual B0 V star Tau Sco. This star is anomalous in several ways, including extreme youth, which may affect its X-ray properties. It has a high X-ray luminosity (LX/LBol ~10-6) and a very hard spectrum. The ASCA data reveal emission line complexes from very highly ionized iron, magnesium, silicon, and sulfur which help constrain the plasma temperature distribution. I show that there must be a significant quantity of gas with temperatures in excess of 28 million degrees, which is far too hot to be explained by line-force instability wind shocks. It seems that coronal mechanisms may need to be invoked to explain the high energy properties of some mid- and late-B stars as well as a small number of anomalous earlier-type OB stars. The inclusion of Tau Sco in this category indicates that youth may play an important role in this non-wind high energy mechanism, whatever it may turn out to be. Normal early-B stars, however, do not strain the current wind-shock theories too badly, as long as the mass loss rates have been underestimated by a factor of two or three. New calculations of the line force in these winds, taking the effects of X-ray photoionization into account, seems warranted at this time.

Evidence for a partial breakdown of the molecular orbital picture in the ionization spectra of large saturated hydrocarbons

The x-ray photoionization spectra of large saturated hydrocarbons have been investigated by means of one-particle Green’s function calculations. These spectra saturate overall rather quickly to their asymptotic form with increasing system size. The results obtained indicate a severe breakdown of the molecular orbital picture of ionization above a binding energy threshold of about 23 eV, for n-alkane chains ranging from n-propane to n-nonane, or cycloalkane compounds such as cyclobutane, cyclopentane, and cyclohexane. In spite of the fast multiplication of satellite solutions, shake-up lines remain confined above that threshold, as a result of the delocalization properties of one-electron canonical states. The ring closure is shown to emphasize the dispersion of photoionization intensity into satellites. Conformational changes, on the other hand, have only marginal effects on the convoluted correlation bands.

Photoionization-Driven X-Ray Line Emission in Cygnus X-3

The ASCA SIS spectrum of the X-ray binary Cygnus X-3 is shown to contain a narrow recombination continuum of hydrogenic sulfur near 3.5 keV, which indicates that the temperature of the X-ray emission-line region is far below the coronal equilibrium temperature required to support S15+. The presence of this feature immediately confirms that X-ray photoionization produces extreme overionization in the circumsource medium, and that the X-ray emission features are energized by the hard X-ray continuum. Therefore, we conclude that spectral analyses of Cygnus X-3 and, most likely, other accretion-powered X-ray sources must be conducted in the context of plasma emission models that account for population kinetics dominated by recombination. Furthermore, the temperature dependence of the shapes of recombination continua provides a simple diagnostic of the electron temperature. We reanalyze the SIS data using a spectral model developed to account for recombination cascades, and, using these diagnostics, derive constraints on the temperatures in the S15+, Si13+, and Mg11+ ionization zones.

Theoretical study of the X‐ray photoionization spectra of polycycloalkanes

The dependence of X-ray photoionization spectra on structural characteristics is assessed by one-particle Green's function calculations on the nearly isomeric adamantane, 1,4-dimethyl bicyclo [2,2,2] octane and 1,1,4,4-tetramethyl cyclohexane compounds. These calculations are carried out using the quasi-particle approximation and the diagonal 2ph-TDA renormalization scheme for the self-energy. The simulated spectra fingerprint remarkably the atomic connectivity from their inner valence part, through the interplay of energy degeneracies and the influence of torsional strains on electron binding energies. The boundary region between the inner and outer valence bands provides also specific signatures for molecular characteristics pertaining to conformational questions, namely the enforced conversion from the chair to the boat form of the cyclohexane units generating these compounds. © 1996 John Wiley & Sons, Inc.

Theoretical study of the X-ray photoionization spectra of polycycloalkanes

Theoretical study of the X-ray photoionization spectra of polycycloalkanes

Ab initio molecular orbital calculations on the electronic structure of phosphate glasses. Sodium phosphate glasses

Ab inition molecular orbital theory was applied to investigate the local structure of vitreous P 2O 5 and sodium phosphate glasses. Small clusters such as H 4P 2O 7, H 3P 2O 7Na and H 2P 2O 7Na 2 were employed to model the local structure of P 2O 5, Na 2O · 2P 2O 5 and Na 2O · P 2O 5 glasses, respectively, and their geometries were optimized at the 3-21G + d(O, P) level. Single-point calculations were then performed at the 3-21G + d(O, P, Na) level by using the 3-21G + d(O, P) geometries. The calculations have led to the following. First, each Na 2O unit introduced into the phosphate network causes two POO 3 2 tetrahedra to convert to two PO 2 O 2 2 − tetrahedra with only two different types of oxygen, namely, bridging and terminal oxygens. Second, The positive charge of Na + is balanced by the negative charge which is almost equally delocalized between the two terminal oxygens in the PO 2 O 2 2 − tetrahedron. Finally, the average π-bonding character per PO bond decreases with increasing Na 2O content. It has also been shown that the previous spectroscopic data for sodium phosphate glasses, such as X-ray photoionization, X-ray emission and UV excitation energies, and the composition dependence of the ionic conductivity for sodium phosphate and silicate glasses can be interpreted in terms of the present molecular orbital calculations.