Photoionization Cross Sections Research Articles

Quantitative Comparison of LDA + DMFT and ARPES Spectral Functions

The emergence of angle-resolved photoemission spectroscopy (ARPES) made it possible to observe electronic dispersion directly as a spectral function map. On the other hand, a spectral function map can be obtained theoretically, for example, in the LDA+DMFT method. The electronic band on such a map is characterized not only by its energy position at a given $k$-point, but also by its width and intensity. To illustrate a way of quantitative comparison of theoretical spectral functions and ARPES data, spectral functions obtained by the LDA+DMFT method are chosen. It is shown that the theoretical spectral functions should take into account a number of experimental features: the photoionization cross section, the experimental energy and angular resolution, as well as the effects of the photohole lifetime arising in the process of photoemission. In this article, we present a robust procedure for taking these experimental features into account by the example of iron-based high-temperature superconductors (HTSC) systems: NaFeAs and FeSe on a SrTiO$_3$ substrate.

Intense-terahertz-laser modulated photoionization cross section of shallow-donor impurity in semiconductors in a magnetic field

The effects of intense terahertz laser and magnetic fields on the photoionization cross section (PICS) of shallow-donor impurity in semiconductors are investigated within the Faraday configuration. The donor ground-state energy level and its wave function are calculated using a combination of nonperturbative and variational methods where radiation field is exactly included through a laser-dressed Coulomb potential (LdCP). We find that a competition between donor ground-state binding energy and optical integrals in tailoring the magnitudes and peak positions of PICS is achieved by external fields via the LdCP. In this way, the magnitudes and peak positions of PICS can be decreased or increased and effectively tuned with an appropriate choice of external fields. The overall shape of PICS is optically anisotropic, which is more pronounced in the z-polarization direction for large laser field intensity or low magnetic field. In the vicinity of ωc, the overall shape of PICS in the z-polarization direction changes into that of PICS in the x-polarization direction. Our theoretical findings hold promising applications in designing and developing new efficient impurity-based devices manipulated by external fields.

Accuracy limitations for composition analysis by XPS using relative peak intensities: LiF as an example

Although precision in XPS can be excellent, allowing small changes to be easily observed, obtaining an accurate absolute elemental composition of a solid material from relative peak intensities is generally much more problematical, involving many factors such as background removal, differing analysis depths at different photoelectron kinetic energies, possible angular distribution effects, calibration of the instrument transmission function, and variations in the distribution of photoelectron intensity between “main” peaks (those usually used for analysis) and associated substructure following the main peak, as a function of the chemical bonding of the elements concerned. The last item, coupled with the use of photoionization cross sections and/or relative sensitivity factors (RSFs), is the major subject of this paper, though it is necessary to consider the other items also, using LiF as a test case. The results show that the above issues, which are relevant to differing degrees in most XPS analyses, present significant challenges to highly accurate XPS quantification. LiF, using the Li1s and F1s XPS peaks, appears, at first sight, to be an ideal case for high accuracy. Only 1s core levels are involved, removing any possible angular effects, and it is a wide bandgap material, resulting in the main Li1s and F1s peaks being well separated from the following scattered electron backgrounds. There are, however, two serious complications: (1) the main F1s and F2s levels have a major loss of intensity diverted into satellite substructure spread over ∼100 eV KE from the main line, whereas the Li1s level has very much less diversion of intensity; (2) there is serious overlap of the substructure from F2s (∼30 eV BE) with the main line of Li1s at ∼56 eV. We report here a detailed analysis of the LiF XPS, plus a supporting theory analysis of losses of intensity from Li1s and F1s to satellite structure, based on the cluster models of LiF. We conclude that, if the overlap from the F2s substructure is correctly subtracted from Li1s, and the intensity from satellites for F1s and Li1s properly estimated, the atomic composition of the single crystal LIF may be recovered to within 5%, using the photoionization cross sections of Scofield, inelastic mean free path lengths based on Tanuma, Powell, and Penn, and the calibrated instrument transmission function. This refutes the claim by Wagner et al., based on their empirical determination of RSFs, (which applied only to the instruments and the analysis procedure they used, in 1981) that Scofield values are too low in general and, for Li1s in particular, are low by a factor of ∼2. This is important because Wagner-based RSFs (sometimes modified and sometimes not) are still embedded in quantification software on modern commercial instruments, and so analysts need to be aware of how those RSFs were obtained/modified. Incorrect use can lead to large quantification errors.

The effects of hydrostatic pressure and temperature on photoionization cross section of impurities in semiconductors under magnetic and intense terahertz laser fields

Abstract We have presented a theoretical study on shallow-donor-related photoionization cross section (PICS) in semiconductors subjected to hydrostatic pressure, magnetic field, and intense laser field within the Vogit configuration. Calculations are performed for the donor ground state using a combination of nonperturbative and variational methods in which intense laser field is exactly taken into account via a laser-dressed Coulomb potential (LdCP). We find that the overall shape of shallow-donor-related PICS depends upon the incident optical field polarization, which is more significant when the incident optical field polarization is parallel to the magnetic field. The donor ground-state binding energy and the optical integrals act in an opposite manner to tailor the magnitudes and peak positions of shallow-donor-related PICS by changing radiation field through the LdCP or by changing magnetic field, hydrostatic pressure, and temperature, leading to a blueshift or redshift of shallow-donor-related PICS spectrum and to a decrease or increase of shallow-donor-related PICS magnitude manipulated by these factors. Thus, we can control the properties of shallow-donor-related PICS with an appropriate choice of two external applied fields, hydrostatic pressure, and temperature to meet some special needs in designing and developing new efficient optoelectronic devices.

Spectroscopic factors of atomic subshells for HAXPES applications

Theoretical photoionization cross sections which are used in the XPS and HAXPES data processing correspond to the main line along with shake-up satellites and shake-off continuum. The main line intensity is proportional to the photoionization cross section and the spectroscopic factor. Nevertheless photoionization cross sections can be used in XPS for estimation of relative intensities since the spectroscopic factors of the currently used XPS lines are about the same for outer shells and close to the value 0.8. Recent progress in HAXPES technique and research interests in investigation of deeper atomic shells require reliable theoretical cross sections of main lines for inner atomic shells. In the present work spectroscopic factors for all atomic subshell of all atoms 2≤Z≤100 are calculated using the sudden approximation and the Dirac–Fock electron wave functions. The spectroscopic factor of deep atomic shells was found may be less by ∼20% than that of outer shells. The factors are essential for the main line intensity determination and should be used in the data processing in the HAXPES region.

Photoionization cross section calculations of Ne-like Mo XXXIII

Within the framework of Breit Pauli R-matrix approach, a close-coupling calculation has been carried out for the process of photoionization of ground and excited states of Ne-like Mo XXXIII ion. The calculation includes 60 fine-structure levels and 15 parities for the target states. Level specific photoionization cross sections are computed for all levels at a fine energy mesh of 10−5 scaled Ryd, with 19500 energies for each Jπ = 0± - 6± symmetry, up to 2s22p43s (2S1/2) threshold at 212.33 Ryd above the first ionization threshold, 1s22s22p5 2Po3/2. In this article are presented cross sections for the photoionization of ground 1s22s22p6 1S0 and 1s22s22p5(2Po)3p 3P0, 1s22s22p5(2Po)3p 1S0, 1s22s2p63s 1S0, 1s22s22p53s(3Po2,1,0), 1s22s22p53s (1Po1) excited states. To explore the role of particle-hole, channels coupling and electron-electron correlation effects on the photoionization cross sections we have compared two calculation models, namely 8-target basic configurations and 5-target basic configurations, with and without 2s2p53l target states included into the model. Effects are highlighted in the photoionization cross section behavior of the excited of 2s2p63s of Mo XXXIII.We believe that this is the first such calculation for this ion to obtain the photoionization cross section and results are important for the modeling and diagnostics of laboratory and astrophysical plasmas.

Radiative recombination data for low-charged tungsten ions: IV. W[formula omitted]W[formula omitted

The radiative recombination and photoionization cross sections as well as the radiative recombination rate coefficients and the radiated power loss rate coefficients are presented for the low-charged tungsten ions from W3+ to W13+. The data are essential to diagnostic and modeling the divertor plasmas impurities in the ITER tokamak because tungsten has been resolved to use as a plasma-facing material. Partial cross sections and rates are given for the ground and excited electron states with principal quantum numbers n≤10 and orbital momenta ℓ≤4 in the wide energy range. Relativistic calculations have been performed taking into account all significant multipoles of the radiative field. Electron wave functions have been found by the Dirac–Fock method. The relativistic Maxwell–Jüttner distribution of continuum electrons has been used in calculations of radiative recombination and radiated power loss rates. Photoionization cross sections have been fitted by an analytical expression with five fit parameters. Peculiarities of behavior of the cross sections and rates for low-charged ions as well as ensuing problems of the cross section fitting are considered. The radiative recombination rates are compared with other recent calculations for the low-charged tungsten ions.

Photoelectron spectra and angular distribution in sequential two-photon double ionization in the region of autoionizing resonances of ArII and KrII

Autoionizing hole states with electron configuration nsnp5 mp are studied in Ar+ and Kr+. Total and partial photoionization cross sections, photoelectron spectra and photoelectron angular distributions in the region of the resonances are obtained theoretically in extensive R-matrix calculations. The states of Ar+ are observed by means of excitation by a free-electron laser operating in the vacuum- and extreme-ultraviolet wavelength regime combined with angle-resolved photoelectron spectroscopy. Fine tuning of the photon energy allows scanning of the resonances and the observation of the shape of the partial cross section ratio, as well as the asymmetry parameter of the angular distribution of the photoelectrons. The calculations are in good agreement with the experimental data.

Multiple Sequential Ionization of Valence n = 4 Shell of Krypton by Intense Femtosecond XUV Pulses

Sequential photoionization of krypton by intense extreme ultraviolet femtosecond pulses is studied theoretically for the photon energies below the 3d excitation threshold. This regime with energetically forbidden Auger decay is characterized by special features, such as time scaling of the level population. The model is based on the solution of rate equations with photoionization cross sections of krypton in different charge and multiplet states determined using R-matrix calculations. Predictions of the ion yields and photoelectron spectra for various photon fluence are presented and discussed.

Statistical and quantum photoionization cross sections in plasmas: Analytical approaches for any configurations including inner shells

Statistical models combined with the local plasma frequency approach applied to the atomic electron density are employed to study the photoionization cross-section for complex atoms. It is demonstrated that the Thomas–Fermi atom provides surprisingly good overall agreement even for complex outer-shell configurations, where quantum mechanical approaches that include electron correlations are exceedingly difficult. Quantum mechanical photoionization calculations are studied with respect to energy and nl quantum number for hydrogen-like and non-hydrogen-like atoms and ions. A generalized scaled photoionization model (GSPM) based on the simultaneous introduction of effective charges for non-H-like energies and scaling charges for the reduced energy scale allows the development of analytical formulas for all states nl. Explicit expressions for nl = 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, and 5s are obtained. Application to H-like and non-H-like atoms and ions and to neutral atoms demonstrates the universality of the scaled analytical approach including inner-shell photoionization. Likewise, GSPM describes the near-threshold behavior and high-energy asymptotes well. Finally, we discuss the various models and the correspondence principle along with experimental data and with respect to a good compromise between generality and precision. The results are also relevant to large-scale integrated light–matter interaction simulations, e.g., X-ray free-electron laser interactions with matter or photoionization driven by a broadband radiation field such as Planckian radiation.

Multiple photoionization cross sections for Fe2+ K shell

Multiple photoionization cross sections from the K shell are studied for all levels of the Fe2+ 3d6 configuration. The study shows that the quadruple photoionization leads to the largest cross sections. A large variation in the multiple photoionization cross sections is determined among the levels of the Fe2+ 3d6 configuration. Main decay branches of radiative and Auger cascades, following the photoionization of the K shell for the ground configuration of the Fe2+ ion, are identified. The radiative and Auger cascade is studied by considering transitions among energy levels and subconfigurations. The obtained data for ion yields are compared with previous calculations produced for configuration averages.

Photoionization of Xe 5s: angular distribution and Wigner time delay in the vicinity of the second Cooper minimum

The angular distribution and photoionization Wigner time delay of Xe 5s photoelectrons are studied in the region of the second Cooper minimum (SCM) using (i) the relativistic multiconfiguration Tamm–Dancoff approximation, (ii) the relativistic-random-phase approximation (RRPA) and (iii) the RRPA-with-relaxation to demonstrate how differing treatments of correlation, and the relativistic interactions, affect the results. The results of the three methods are compared with each other and with available experimental data. The comparison reveals the importance of electron correlations for which a multiconfiguration description of the initial state is essential. The spin-resolved and spin-averaged photoionization time delay results show important signatures in the region of the SCM in the Xe 5s photoionization cross-section.

Carbon monoxide interacting with free-electron-laser pulses

We study the interaction of a heteronuclear diatomic molecule, carbon monoxide, with a free-electron laser (FEL) pulse. We compute the ion yields and the intermediate states by which the ion yields are populated. We do so using rate equations, computing all relevant molecular and atomic photo-ionisation cross-sections and Auger rates. We find that the charge distribution of the carbon and oxygen ion yields differ. By varying the photon energy, we demonstrate how to control higher-charged states being populated mostly by carbon or oxygen. Moreover, we identify the differences in the resulting ion yields and pathways populating these yields between a homonuclear molecule, molecular nitrogen, and a heteronuclear molecule, carbon monoxide, interacting with an FEL pulse. These two molecules have similar electronic structure. We also identify the proportion of each ion yield which accesses a two-site double-core-hole state and tailor pulse parameters to maximise this proportion.

Chemical diversity among A–B stars with low rotational velocities: non-LTE abundance analysis

ABSTRACT We present accurate element abundance patterns based on the non-local thermodynamic equilibrium (non-LTE, NLTE) line formation for 14 chemical elements from He to Nd for a sample of nine A9 to B3-type stars with well-determined atmospheric parameters and low rotational velocities. We constructed new model atom of Zr ii–iii and updated model atoms for Sr ii and Ba ii by implementing the photoionization cross-sections from calculations with the Dirac B-spline R-matrix method. The NLTE abundances of He to Fe in the stars HD 17081, HD 32115, HD 160762, and HD 209459 are found to be consistent with the solar abundances, and HD 73666 being a Blue Straggler does not reveal deviations from chemical composition of the Praesepe cluster. Three of these stars with an effective temperature of lower than 10 500 K have supersolar abundances of Sr, Zr, Ba, and Nd, and our results suggest the presence of a positive correlation between stellar effective temperature and abundance. For each star, enhancement of Ba is higher than that for any other heavy element. We propose that the solar Ba abundance is not representative of the galactic Ba abundance at modern epoch. The status of HD 145788 was not clarified: This star has solar abundances of C to Si and enhancements of Sr to Ba similar to that for superficially normal stars of similar temperature, while Ca, Ti, and Fe are overabundant. The NLTE abundances of Vega support its status of a mild λ Bootis star.

Optically stimulated luminescence of cowrie shells

Characteristics of continuous wave optically stimulated luminescence (CW-OSL) and linear modulation OSL (LM-OSL) from three different types of cowrie shells are presented. Irradiated samples were optically stimulated at 72 mW/cm2 for CW-OSL and linearly ramped from zero to 72 mW/cm2 to obtain LM-OSL. The measured CW-OSL and LM-OSL curves of the shells are each determined to consist of three components namely fast, medium and slow components. The fast, medium and slow LM-OSL components peak at 17.0 ± 0.8, 45.0 ± 1.8 and 140.8 ± 6.7 s respectively for all samples. In addition, peak positions are independent of dose. The photoionization cross-sections of the OSL traps for the three components estimated from CW-OSL are similar to those determined using LM-OSL. The OSL of the three components increases linearly with dose. The coefficient of variation of responses from repeated measurements of same aliquot lies below 2.3%. The shells are thus potential materials for retrospective dosimetry.

Database NORAD-Atomic-Data for Atomic Processes in Plasma

The online atomic database of NORAD-Atomic-Data, where NORAD stands for Nahar OSU Radiative, is part of the data sources of the two international collaborations of the Opacity Project (OP) and the Iron Project (IP). It contains large sets of parameters for the dominant atomic processes in astrophysical plasmas, such as, (i) photo-excitation, (ii) photoionization, (iii) electron–ion recombination, (iv) electron–impact excitations. The atomic parameters correspond to tables of energy levels, level-specific total photoionization cross-sections, partial photoionization cross-sections of all bound states for leaving the residual ion in the ground state, partial cross-sections of the ground state for leaving the ion in various excited states, total level-specific electron–ion recombination rate coefficients that include both the radiative and dielectronic recombination, total recombination rate coefficients summed from contributions of an infinite number of recombined states, total photo-recombination cross-sections and rates with respect to photoelectron energy, transition probabilities, lifetimes, collision strengths. The database was created after the first two atomic databases, TOPbase under the OP and TIPbase under the IP. Hence the contents of NORAD-Atomic-Data are either new or from repeated calculations using a much larger wave function expansion making the data more complete. The results have been obtained from the R-matrix method using the close-coupling approximation developed under the OP and IP, and from atomic structure calculations using the program SUPERSTRUCTURE. They have been compared with available published results which have been obtained theoretically and experimentally, and are expected to be of high accuracy in general. All computations were carried out using the computational facilities at the Ohio Supercomputer Center (OSC) starting in 1990. At present it contains atomic data for 154 atomic species, 98 of which are lighter atomic species with nuclear charge Z ≤ 28 and 56 are heavier ones with Z > 28. New data are added with publications.

From inherent correlation to constrained measurement: Model-assisted calibration in MBMS experiments

Synchrotron-based molecular-beam mass spectrometry (MBMS) can provide detailed species-resolved information to help develop, validate and optimize combustion kinetic models. While quantification of stable species can be achieved within 30% uncertainty, the measured mole fractions of reactive intermediates often have large systematic errors, mainly due to the large uncertainties associated with estimated photoionization cross sections. These measurements are therefore less effective in improving the model accuracy, and it remains a challenge to make full use of those data for important reactive intermediates with relatively large uncertainties. In the present work, we propose a model-assisted calibration method to reduce the uncertainty of the measurements for those reactive species in the MBMS experiments. The method takes advantage of the inherent correlation of the systematic uncertainty in the MBMS measurements and uses the accurate model predictions to calibrate the correlated experimental data. By global uncertainty analysis, the kinetic model for the methanol/O2 flame was analyzed to select the optimal experimental conditions for which the model prediction of the hydroxymethyl radical (CH2OH) has the smallest uncertainty. Then the correlation factor for the systematic uncertainty is determined by analyzing the new measurement and the model prediction under the designed condition. The correlation factor determined has been successfully used to calibrate the peak mole fraction of the CH2OH radical in a laminar premixed methanol flame, reported earlier.

Photoionization of superheavy atoms: correlation and relativistic effects

Calculations of the photoionization cross sections of the ground states of the superheavy atoms No (Z = 102), Cn (Z = 112) and Og (Z = 118) have been performed using fully relativistic methodologies. The photoionization cross sections are dominated by interchannel coupling, in all cases, over a broad energy range including both outer and inner shells.

Measurement of collisions between laser-cooled cesium atoms and trapped cesium ions

We report the measurement of collision rate coefficient for collisions between ultracold Cs atoms and low-energy $\mathrm{C}{\mathrm{s}}^{+}$ ions. The experiments are performed in a hybrid trap consisting of a magneto-optical trap (MOT) for Cs atoms and a Paul trap for $\mathrm{C}{\mathrm{s}}^{+}$ ions. The ion-atom collisions impart kinetic energy to the ultracold Cs atoms, resulting in their escape from the shallow MOT and, therefore, in a reduction in the number of Cs atoms in the MOT. By monitoring, using fluorescence measurements, the Cs atom number, and the MOT loading dynamics and then fitting the data to a rate equation model, the ion-atom collision rate is derived. The $\mathrm{Cs}\ensuremath{-}\mathrm{C}{\mathrm{s}}^{+}$ collision rate coefficient $9.3(\ifmmode\pm\else\textpm\fi{}0.4)(\ifmmode\pm\else\textpm\fi{}1.2)(\ifmmode\pm\else\textpm\fi{}3.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{3}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, measured for an ion distribution with most probable collision energy of 95 meV $(\ensuremath{\approx}{\mathrm{k}}_{\mathrm{B}}\phantom{\rule{4pt}{0ex}}1100\phantom{\rule{4pt}{0ex}}\mathrm{K})$, is in fair agreement with theoretical calculations. As an intermediate step, we also determine the photoionization cross section of Cs $6{P}_{3/2}$ atoms at 473 nm wavelength to be $2.28(\ifmmode\pm\else\textpm\fi{}0.33)\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{2}$.

Glory oscillations in photoionization microscopy: Connection with electron dynamics and Stark spectral structures in the continuum

In classical scattering theory, the term “glory scattering” implies the divergence of the classical differential cross section that occurs as soon as the deflection function goes through zero for a nonzero value of the impact parameter. This critical effect also occurs in slow photoelectron imaging where near-threshold atomic photoionization is performed in the presence of an external static electric field. In this case, glory scattering manifests itself by the appearance of an intense peak at the center of the photoelectron momentum distribution. In the present work we examine the magnitude variation of this central peak as a function of electron energy. We experimentally study near-threshold two-photon ionization of ground state magnesium atoms, below as well as above the field-free ionization limit. It is found that, apart from its behavior of classical origin, the glory signal additionally exhibits strong oscillations and beating effects over the full spectral range of the recordings. Of particular interest are its oscillations above the zero-field limit, many aspects of which are expected to be independent of the atomic target. Our results are analyzed with the help of classical, semiclassical, and quantum mechanical calculations devoted to the hydrogenic Stark effect. It is theoretically found that these continuum glory oscillations are related to the resonantlike Stark structures appearing under certain conditions in the total photoionization cross section and implying energy quantization in the continuum. The striking outcome of the present study, however, is that both theory and experiment clearly support the connection between the energy- and static field-dependent periodicity of glory oscillations with the classical dynamics of electron motion. In particular, it is shown that the Fourier transform of the glory signal provides information on the differences between the origin-to-detector times of flight corresponding to specific pairs of classical electron trajectories.