Total Photoionization Cross Sections Research Articles

Photoionization of hydrogen halides using the R-matrix method

Abstract In this study, we use the UK Molecular R-matrix (UKRMol) codes in the close-coupling approximation to examine the photoionization of hydrogen halides (hydrogen fluoride, hydrogen chloride, and hydrogen bromide). This article reports the total and partial photoionization cross sections for the X2Π, A2Σ+, and B2Σ+ ionic states of these halides. The calculated cross sections are compared with the available literature, which does not accurately represent the effective cross sections near the threshold region, which is dominated by the Rydberg series autoionization resonances converging to the A2Σ+ ionic state. There seems to have been minimal effort to investigate the Rydberg-bound states of these halides. Meanwhile, the R-matrix approaches have traditionally excelled at characterizing such studies. This indicates the effectiveness of this method for molecular photoionization as well as for understanding the resonant contribution to the photoionization cross sections. The detailed cross sections calculated comprise the complex autoionizing resonance structures capable of significantly contributing to the computations of total photoionization rates, which are necessary to maintain a steady state of ionization in astrophysical plasmas. Comparisons with the experimental measurements and the theoretical data generally show reasonable agreement across the reported energy range.

Influence of relativistic effects on photoionization process of hydrogen-like ions in plasma environment

In this study, the photoionization cross sections of C<sup>5+</sup>, Al<sup>12+</sup>, and W<sup>73+</sup> ions in a Debye plasma environment are calculated in the dipole approximation. The main emphasis is placed on investigating the influence of relativistic effects on shape resonances, Cooper minima, and virtual state effects. The relativistic effects lead to fine-structure splittings, allowing the appearance of double-shape resonance peaks in the total cross-section. Because the width and energy position of resonance peak are affected by the near critical screening length, the increase of nuclear charge Z leads to the significant differences in the size, width, and position of the double-shape resonance peak. The energy position of Cooper minimum in the photoelectrons is related to the critical screening length corresponding to the final continuum state. Unlike the deeper minima observed in the total photoionization cross-sections for C<sup>5+</sup> and Al<sup>12+</sup> ions, for the higher nuclear charge of W<sup>73+</sup> ions, the significant fine-structure splitting arising from relativistic effects results in substantial differences in the positions of the Cooper minima in the partial cross-sections. Therefore, when superimposed on the total cross section, these minima appear shallower. The W<sup>73+</sup> ion has a higher nuclear charge, and the screening length related to the virtual state effect is completely different from that of C<sup>5+</sup> ion and Al<sup>12+</sup> ion. Moreover, for the same screening length, there is a significant difference in the virtual state enhancement amplitude between C<sup>5+</sup> ion and Al<sup>12+</sup> ion in the low energy region .

Resonant Photoionization of CO2 up to the Fourth Ionization Threshold.

We present a comprehensive theoretical study of valence-shell photoionization of the CO2 molecule by using the XCHEM methodology. This method makes use of a fully correlated molecular electronic continuum at a level comparable to that provided by state-of-the-art quantum chemistry packages in bound-state calculations. The calculated total and angularly resolved photoionization cross sections are presented and discussed, with particular emphasis on the series of autoionizing resonances that appear between the first and the fourth ionization thresholds. Ten series of Rydberg autoionizing states are identified, including some not previously reported in the literature, and their energy positions and widths are provided. This is relevant in the context of ongoing experimental and theoretical efforts aimed at observing in real-time (attosecond time scale) the autoionization dynamics in molecules.

Photoionization of the water molecule with XCHEM.

We have evaluated total and partial photoionization cross sections, β asymmetry parameters, and molecular frame photoelectron angular distributions (MFPADs) of the water molecule by using the XCHEM methodology. This method accounts for electron correlation in the electronic continuum, which is crucial to describe Feshbach resonances and their autoionization decay. We have identified a large number of Feshbach resonances, some of them previously unknown, in the region between 12.2 and 18.7eV, for which we provide energy positions and widths. Many of these resonances lead to pronounced peaks in the photoionization spectra, some of them remarkably wide (up to 0.2eV, for resonances converging to the third ionization threshold), which should be observable in high-energy resolution experiments. We show that, in the vicinity of these peaks, both asymmetry parameters and MFPADs vary very rapidly with photoelectron energy, which, as in atoms and simpler molecules, reflects the interference between direct ionization and autoionization, which is mostly driven by electron correlation.

A theoretical study on dissociative photoionization and photoionization cross-sections of a typical ketohydroperoxide in n-butane low-temperature oxidation

Ketohydroperoxides (KHP) are closely relevant to the low-temperature reactivity of hydrocarbons and their derivatives. Photoionization mass spectrometry (PIMS) technology has played a crucial role in detecting and quantifying KHP in recent years. However, the dissociative photoionization behavior at the near-threshold may bias the collection of KHP signals, resulting in a significant underestimation of the concentration. In the present study, the dissociative photoionization mechanism of 3-hydroperoxybutanal (3-KHP), the most abundant KHP in n-butane oxidation, was investigated by quantum chemical calculations. The results suggest that a two-step dehydration reaction channel with a low barrier accounts for the observation of fragments at the near-threshold. The appearance energy of possible fragments was obtained at a high theoretical level. The photoionization cross-sections (PICS) of 3-KHP at photon energies ranging from the threshold to 11 eV were obtained by theoretical calculations in this study. By comparing the calculated PICS with the photoionization efficiency curve measured in a previous study, we found that approximately 70% of the 3-KHP signals (m/z = 104) were converted into cation fragments (m/z = 29, 43, 71, etc.) at 10 eV. The theoretically predicted dissociative photoionization fragment information and total PICS were finally used to refine the quantification of 3-KHP in the previous experiment. Our study indicates that for highly reactive species such as KHP, theoretical investigations on the dissociative photoionization behavior and photoionization cross-section will substantially benefit the species quantification by PIMS.

Angular Distribution of Atomic Photoelectrons as a Function of Photon Field Polarization: Case of Ns-type Light Atoms

The angular distribution of the atomic photoelectrons gives us information on the evolution of the field of the photoelectrons in different directions concerning the incident direction of the electromagnetic radiation which is absorbed by the atom and its direction of polarization. Here, the angular distribution of atomic photoelectrons as a function of photon field polarization was studied theoretically using monocentric wave functions to investigate the influence of polarization on the theoretical results. A GEANT4 modeling calculation based on the Monte Carlo code was made on the helium atom. The calculations were performed at low energy above the atomic ionization threshold. The results obtained by analytical calculation for the total photoionization cross-section were compared with those obtained by a simulation calculation using GEANT4 modeling. A reasonable agreement was observed following this comparison for a range of energy considered in this study.

Theoretical study of the enhancement of saturable absorption of Kr under x-ray free-electron laser

The generation of hollow atoms will reduce the probability of light absorption and provide a high-quality diffraction image in the experiment. In this paper, we calculated the ionization rate of the Kr atom under x-ray free-electron laser (XFEL) using Hartree–Fock–Slater model and simulated the ionization model of Kr atom using Monte–Carlo method to determine the response of the hollow atom of Kr atom to the XFEL photon energy. Calculating the correlation between the total photoionization cross-section of the ground state of Kr atom and the photon energy, we determined three particular photon energies of 1.75 keV, 1.90 keV, and 14.30 keV. The dynamics simulation under the experimental condition’s 17.50 keV photon energy was achieved by implementing the Monte–Carlo method and calibrating the photon flux modeling parameters. Consequently, our calculated data are more consistent with experimental phenomena than previous theoretical studies. The saturable absorption of Kr at 1.75 keV, 1.90 keV, 14.30 keV, and 17.50 keV energies was further investigated by using the optimized photon flux model theory. We compared the statistics on main ionization paths under those four specific photon energies and calculated the population changes of various Kr hollow atoms with different configurations. The results demonstrate that the population of hollow atoms produced at the critical ionization photon energy is high. Furthermore, the change of population with respect to position is smooth, which shows a significant difference between the generation mode of ions with low and high photon energies. The result is important for the study of medium- and high-Z element hollow atoms, which has substantial implications for the study of hollow atoms with medium and high charge states, as well as for the scaling of photon energy of free electron lasers.

Contribution of near-edge processes to the L X-ray emission lines for various lanthanum (III) compounds

The present measurement illustrates the study of near-edge processes and the intensity ratios [ILl/ILα, ILβ1/ILα and ILl/ILβ1] for Mn Kα1,2 X-rays across the L2 edge of lanthanum in its various chemical forms viz., lanthanum oxide (La2O3), lanthanum oxalate (C6La2O12) and lanthanum acetate (C6H9LaO6) at four different scattering angles i.e. 90°, 105°, 115° and 125° with a lab-scale X-ray fluorescence (XRF) spectrometer. The current observation reveals that the Lβ1 X-ray emission lines exhibit a variance of ∼ (21–40) % between measured and calculated fluorescence cross-sections. Apart from this, the measured values of intensity ratios have been compared with the three data sets of calculated values using L shell emission rates based on the Dirac-Fock (DF) model, total X-ray emission rates and photoionization cross-section values based on the relativistic Hartree-Slater (RHS) model and three sets of Coster-Kronig (CK) and fluorescence yields. The correlation between crystal structure, bond distance and intensity ratio are also discussed in this work.

Photoionisation of the CH radical using the B-spline R-matrix method

Aims. The primary motivation for this paper is to provide accurate data for the photoionisation of the CH radical, including the absolute total photoionisation cross-section, partial cross-sections, and photoelectron angular distribution. In addition, the near threshold features in the photoionisation curve (which are absent in previous studies) are produced with high precision. Methods. A multichannel wavefunction based on the R-matrix approach, which uses the configuration interaction (CI) method to describe electronic correlation, is carried out in the present calculations. A set of B-spline orbitals is employed to represent the accurate continuum. The distinctive feature of the present calculations allows us to generate a more accurate description of the bound and continuum states than those employed before. Results. Total photoionisation cross-sections from the ground state of CH radicals and partial cross-sections corresponding to 1π, 3σ, and 2σ states of CH+ ions are presented for photon energies ranging from threshold to 80 eV. Extensive resonance structures, which are absent in previous studies, are observed for the first time near the ionisation threshold. The cross-section dataset obtained from the present calculations is expected to be sufficiently accurate and comprehensive for most current modelling applications involving the photon and CH radical scattering system.

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.

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.

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.

Partial Photoionization Cross Sections of Chromium from the Ground and Excited States

Partial and total photoionization cross sections of iron-peak elements are important for the determination of abundances in late-type stars and nebular objects. We have investigated photoionization of neutral chromium from the ground and excited states in the low energy region from the first ionization threshold at 6.77 eV to 30 eV. Accurate descriptions of the initial bound states of Cr I and the final residual Cr II ionic states have been obtained in the multiconfiguration Hartree-Fock method together with adjustable configuration expansions and term-dependent non-orthogonal orbitals. The B-spline R-matrix method has been used for the calculation of photoionization cross sections. The 194 LS final ionic states of Cr II 3d44s, 3d34s2, 3d5, 3d44p, and 3d34s4p principal configurations have been included in the close-coupling expansion. The inclusion of all terms of these configurations has significant impact on the near-threshold resonance structures as well as on the nonresonant background cross sections. Total photoionization cross sections from the ground 3d54sa7S and excited 3d54sa5S, 3d44s2a5D, 3d54pz5P, and 3d44s4py5P states of Cr I have been compared with other available R-matrix calculation to estimate the likely uncertainties in photoionization cross sections. We analyzed the partial photoionization cross sections for leaving the residual ion in various states to identify the important scattering channels, and noted that 3d electron ionization channel becomes dominant at higher energies.

Measurements of mass attenuation coefficients for 51Sb over energy region 4 keV–14 keV using synchrotron radiation

In the present work, the X-ray mass attenuation coefficients have been measured for 51Sb (a medium-Z element) at forty energies across its Li (i = 1–3) sub-shell absorption edges covering an extended energy region within 4.0 keV–14.0 keV. The aim of this study is to experimentally determine the key X-ray fundamental parameters for 51Sb with improved accuracy by using tunable energy synchrotron radiation. From the present measured mass attenuation coefficients, the Li (i = 1–3) sub-shell photoionization cross sections (σLiP) have been deduced at twenty four energies across the Li (i = 1–3) edge-energies of 51Sb covering the region 4.150 keV–5.0 keV by using experimentally deduced Li (i = 1–3) edge jump ratios. Two theoretical datasets of X-ray mass attenuation coefficients and total photoionization cross sections were served to evaluate their consistency with the present experimental values and reveal possible discrepancies, in particular, across the Li (i = 1–3) edge-energies of 51Sb.

X-ray Photoionization Cross Section Spectra of Water and Ammonia Bonded on Polycyclic Aromatic Hydrocarbons: A Quantum Mechanical Interpretation to the Absorption Spectra on Graphene.

A theoretical study of the K-shell total photoabsorption and photoionization cross section spectra of water and ammonia bonded to benzene (C6H6) and the polycyclic aromatic hydrocarbons (PAHs) naphthalene (C10H8), coronene (C24H12) and circumcoronene (C54H18) by van der Waals (vdW) forces is presented. The discretized electronic pseudospectra at the oxygen and nitrogen K-edges, covering the discrete and the continuum spectral regions, were obtained at the time-dependent density functional theory (TDDFT) level with dispersion correction. An analytic continuation procedure based on the Padé approximants was used in order to obtain the K-shell cross sections of the structures at the discrete and the continuum regions of the spectra. By examining the electronic spectra of water and ammonia bonded to coronene and circumcoronene, we observed that our results agree well with the experiments performed with graphene. This work provides a quantum mechanical interpretation to the NEXAFS experiments of water and ammonia adsorbed on graphene in terms of a physisorption model of these molecules by van der Waals forces.

Probing photon interaction with H2O and D2O

Investigation of the valence state photoionization of water using vacuum ultraviolet and soft x-rays gives important information for spectroscopic analysis and studies of radiation loss and cell biology. In this article, the total and valence-shell photoionization cross sections of H2O and D2O, generating the states 2B1, 2A1 and 2B2, and their isotopic consequences are studied. The effect of bonding on valence state photoionization is also reported. The calculations were made using R-matrix theory for photon energies up to 40 eV. Electronic transitions below 20 eV were considered to account for the autoionizing Rydberg transitions. This is the first report to demonstrate the effect of bonding on valence state photoionization. For the isotopic partner (D2O), we observed a quantitative change in cross section. Comparisons are made with the previously available results and a reasonable agreement is found. The present study gives a comprehensive understanding of the photoionization of water, thus providing deep insight into different non-dissociative processes of the molecule in various environments.

Lanczos-based equation-of-motion coupled-cluster singles-and-doubles approach to the total photoionization cross section of valence excited states.

Excitation energies and oscillator strengths of the first two electronically excited states of helium, water, sulfur dioxide, molecular nitrogen, and carbon monoxide were obtained from an asymmetric-Lanczos-based formulation of the equation-of-motion coupled cluster singles and doubles approach. The total photoionization cross sections were generated by two different methodologies: an analytic continuation procedure based on the Padé approximants and the Stieltjes imaging technique. The results are compared with theoretical photoionization cross sections from algebraic diagrammatic construction [ADC(2)] and ADC(2)-x calculations [M. Ruberti et al., J. Chem. Phys. 140, 184107 (2014)] and with available experimental data.

Restricted Correlation Space B-Spline ADC Approach to Molecular Ionization: Theory and Applications to Total Photoionization Cross-Sections.

Herein is presented a new approach to the ab initio algebraic diagrammatic construction (ADC) schemes for the polarization propagator, which is explicitly designed to accurately and efficiently describe molecular ionization. The restricted correlation space (RCS) version of the ADC methods up to second order of perturbation theory is derived via the intermediate state representation (ISR) and implemented in the multicenter B-spline basis set for the electronic continuum. Remarkably a general close-coupling structure of the RCS-ADC many-electron wave function, connecting the N-particle to the ( N - 1)-particle ADC intermediate states, emerges naturally as a nontrivial result of the RCS ansatz. Moreover, the introduced RCS-ADC schemes prove to be particularly manageable from a computational point of view, overcoming the practical limitations of the conventional ADC approaches. The quality of the new RCS-ADC( n) approaches is verified by performing a series of total photoionization cross-section calculations on a test set of molecules. The excellent agreement of the results with existing accurate benchmarks demonstrates that the RCS versions of the ADC schemes are optimal and quantitatively accurate methods for studying multichannel molecular photoionization.

Relativistic R-matrix calculations of photoionization cross sections of Cu XVIII

The total photoionization cross sections for the ground state 2p63s2(1S0) and the lowest three excited states 2p63s3p(3Po0,1,2) of the Mg-like Cu XVIII ion have been calculated by making use of the close-coupling Breit Pauli R-matrix (BPRM) approximation. The lowest 21 target level states of Cu XIX have been included in the photoionization calculations of Cu XVIII. Our calculated target state energies of the core ion Cu XIX show a good agreement with the observed energies tabulated in the NIST database. To further assess the accuracy of our BPRM results, photoionization cross section calculations for the ground state configuration have also been performed by employing the fully relativistic Dirac atomic R-matrix code (DARC). Our two independent calculations show a reasonable agreement between the calculated cross sections in the overall background shape and magnitude. We believe that our cross section data may be beneficial for the modelling and diagnostics of astrophysical and laboratory plasmas.

Relativistic R-matrix photoionization cross section calculations of Ne-like Co XVIII with resonance parameters

We report total photoionization cross sections of the ground state and the first three excited states () of Ne-like Co XVIII ion using the close-coupling Breit–Pauli R-matrix (BPRM) approximation. The target wavefunctions are constructed by configuration interaction technique (CIV3). Our calculated ionization threshold energies for the ground and excited states show a good agreement with the NIST ones. To further affirm the accuracy and reliability of our BPRM results, photoionization cross section calculations for the ground state have also been performed by employing the fully relativistic Dirac Atomic R-matrix Code. Our two independent calculations show a reasonable agreement both in magnitude and resonant photoionization cross sections. The resonance positions (Er), widths (Γ) and quantum defects (μ) due to ejection of a 2p or 2s electron from the ground state are given by adopting Quigley and Berrington method. We believe that our cross section data may be beneficial for the modelling and diagnostics of astrophysical plasmas.