Relativistic quantum mechanic calculation of photoionization cross-section of hydrogenic and non-hydrogenic states using analytical potentials

Photoionization process is a subject of special importance in many areas of physics. Numerical methods must be used in order to obtain photoionization cross-sections for non-hydrogenic levels. The atomic data required to calculate them is huge so self-consistent calculations increase computing time considerably. Analytical potentials are a useful alternative because they avoid the iterative procedures typical in self-consistent models. In this work, we present a relativistic quantum calculation of photoionization cross-sections for isolated ions based on an analytical potential to obtain the required atomic data, which is valid both for hydrogenic and non-hydrogenic ions. Comparisons between our results and others obtained using either widely used analytical expressions for the cross-sections or more sophisticated calculations are done.     

实验室光致电离等离子体中激发过程的研究

光致电离等离子体在宇宙中广泛存在于强辐射场附近. 近年来随着高能量密度实验装置的发展, 在实验室内也能构造出光致电离等离子体. RCF是一个基于The Flexible Atomic Code 数据的针对光致电离等离子体的辐射碰撞模型, 该模型模拟了两个光致电离实验, 其 理论结果中电离态分布和光谱与测量值符合得很好. 在理论模拟中发现, 光致电离等离子体中光致激发和碰撞激发过程对离子态布居和发射光谱都有非常重要的影响. 光致激发过程可以通过将离子激发到双激发态从而间接电离离子; 碰撞激发过程会因为电子将基态离子激发到电离截面小的单激发态而抑制光子对等离子体的电离. 光致激发过程可以加强类锂离子的类氦离子的卫线, 而碰撞激发过程会影响类氦离子谱线的线强之比.     

Calculation of the photoionization cross sections for the ions of the rubidium isoelectronic sequence by the Dirac-Fock method

The photoionization cross sections for the ground and some excited states of 11 ions of the rubidium isoelectronic sequence from Rb through Ag¹⁰⁺ are calculated by the relativistic Dirac-Fock self-consistent field method. The energy dependence of the photoionization cross sections and the changes in the threshold values along the isoelectronic sequence are considered. The influence of the exchange effect and the choice of gauge on the calculated values of photoionization cross sections is examined. For practical use, the calculated cross sections are approximated by an analytical expression. Along with the total photoionization cross sections, the asymmetry parameter in the angular distribution of photoelectrons and the degree of their polarization are also calculated.     

Molecular photoionization cross-sections and their dependence on excitation energy

A general formula for the photoionization cross-section of a linear molecule in terms of atomic subshell cross-sections and diffraction effects is given. Example calculations are carried out for line intensities of CO and N₂ as a function of incident photon energy from threshold up to the soft X-ray region. The importance of associating CNDO coefficients to orthogonal Slater AO's is pointed out. General rules for dependence of photoionization cross-sections on excitation energy are discussed.     

Calculation of the radiative opacity of laser-produced plasmas using a relativistic-screened hydrogenic model

In this work, we use a relativistic-screened hydrogenic model to compute the radiative opacity of laser-produced plasmas. The model is based on a set of screening charges which allow one to easily calculate atomic properties of isolated ions. These screened charges have been fitted to a fourth-order polynomial depending on the nuclear charge Z for ground and single excited states of ions belonging to the isoelectronic sequences comprised between He-like to U-like. In the opacity model used, ionic populations are obtained by solving the Saha equation including degeneracy corrections. Bound-bound transitions are determined using a Voigt profile for line shape, which includes natural, collisional, Doppler and UTA widths. Bound-free and free-free opacities are evaluated using the Kramer cross-sections with appropriate corrections. Scattering processes are computed through the use of the Thomson formula with corrections. The results are compared with other screened hydrogenic models and more sophisticated self-consistent codes.     

Photoionization cross sections of Fe XXI

Total and partial photoionization cross sections for (Fe XXI+hν→Fe XXII+e) are presented for the ground and excited bound states with n?10 and l?9. Fe XXI is prevalent in high-temperature astrophysical plasmas as well as in photoionized plasmas excited by hard X-rays. Results are reported for the first time for the high-energy photoionization with core excitations to n=2,3 states. Details of photoionization, especially the high-energy features that often dominate considerably over the low energy ones, are illustrated. These prominent features will affect the photoionization and the recombination rates in high-temperature plasmas. Calculations are carried out in the close coupling (CC) approximation using the R-matrix method. A large CC wavefunction expansion for Fe XXII which includes the ground and 28 excited core states from n=2 and 3 complexes and spans over a wide energy range is used. A total of 835 discrete bound states of Fe XXI in the singlet, triplet, and quintet symmetries are obtained. Total photoionization cross sections, σ_PI(nLS), for ionization into all 29 states are presented for all 835 final bound states and partial photoionization cross sections, σ_PI(g,nLS), for ionization into the ground ²P⁰ state of the core are presented for 685 states. While the n=2 core excitations are at relatively lower energy range (within 15 Ry from the ionization threshold), the n=3 excitations lie at considerably higher energy, 73 Ry and above, yet introduce resonant features and enhancements more prominent than those of n=2 states. Larger numbers of resonances are formed due to Rydberg series of autoionizing states converging on to the 29 core states. However, most noticeable structures are formed in the excited state cross sections by the photoexcitation-of-core (PEC) resonances in the photon energy range of 73-82 Ry. All these high-energy features are absent in the currently available results. The present results should enable more accurate modeling of the emission spectrum of highly excited plasma from the optical to far-ultraviolet region.     

Double K-shell photoionization of low-Z atoms and He-like ions

We report on the investigation of the photon energy dependence of double 1s photoionization of light atoms and compare the cross sections for hollow atom and He-like ion production. Measurements of the Kα hypersatellite x-ray spectra of Mg, Al, and Si were carried out using the Fribourg high-resolution x-ray spectrometer installed at the ID21 and ID26 beam lines at the ESRF. The double-to-single photoionization cross section ratios were derived as a function of the incident photon beam energy and compared to convergent close-coupling (CCC) calculations for He-like ions. The dynamical electron-electron scattering contribution to the DPI cross-sections was found to be more important for neutral atoms than for the He isoelectronic series.     

自由电子涨落对光电截面阈值的影响

基于含温有界相对论Hartree-Fock-Slater自洽场平均原子模型,运用巨正则系综理论和最大熵原理,研究了热动平衡等离子体内自由电子涨落对离子能级的影响,比较细致地计算了热等离子体内光电截面阈值的变化情况. 关键词：     

Electron correlation and hybridization in late-transition-metal compounds

In this article we discuss the transition-metal d-d Coulomb interaction U and the anion-to-cation charge transfer energy jfor nickel halides and relate these to the observed bandgaps. In addition, we describe the degree of hybridization for NiCl 2 and NiBr 2 and transition-metal chlorides by considering the ionicity of the chemical bond of these compounds and the results obtained with the Phillips-Van Vechten dielectric theory. The degree of hybridization in transition-metal chlorides has been obtained by considering a simple model which utilizes the photon energy dependence of the photoionization cross-sections for the 3p band of the chlorine ions and the 3d band of the transition-metal ions. Good agreement between the two approaches has been found and a degree of hybridization between 0.15 and 0.25 for transition-metal chlorides is confirmed.     

Helium atom in the monochromatic electromagnetic field

We apply a non-perturbative procedure for the calculation of the total photoionization cross-section of two-electron atomic systems. The procedure is based on the Floquet-Fourier representation of the time-dependent Schr?dinger equation. With the use of the Hylleraas-type basis functions, the total photoionization cross-sections obtained are within the accuracy of a fraction of a percent, which, we believe, is the most accurate estimate for the cross-sections available. The total photoionization cross-sections for neutral helium deviate notably from the benchmark experimental data [J.A.R. Samson et al., J. Phys. B 27, 887 (1994)].     

Electron-ion recombination and photoionization of Fe XXI

Results for electron-ion recombination and photoionization of , with emphasis in high-temperature region, are presented from ab initio unified method. The unified method, based on close coupling (CC) approximation and R-matrix method, (i) subsumes both the radiative recombination (RR) and dielectronic recombination (DR), (ii) enables self-consistent sets of photoionization and recombination cross sections from using an identical wavefunction for both the processes, and (iii) provides state-specific recombination rates of a large number of bound states. A large CC wavefunction expansion, which includes the ground and 28 core excitations of n=2 and 3 complexes and span a wide energy range, has been used. Compared to Δn=2-2, Δn=2-3 core excitations are found to introduce strong resonant structures and enhance the background photoionization cross sections (σ_PI) in the high-energy region. These features along with prominent photoexcitation-of-core (PEC) resonances at n=3 core thresholds have increased the unified total recombination rate coefficients (α_R(T)) at temperatures , region of maximum abundance of the ion in collisional equilibrium, by a factor of 1.6 over previous calculations. State-specific recombination rate coefficients α_R(nLS), which include both the RR and DR, are presented for the first time for 685 bound states with n?10 and l?9. The unified total recombination rate with photoelectron energy α_R(E) is presented and the role of low-energy near-threshold fine structure resonances is illustrated. The present results should provide a reasonably complete self-consistent set of recombination rates and photoionization cross sections for astrophysical modelings of high-temperature plasmas from optical to far-ultraviolet wavelength regions.     

Excitation of Highly Charged Ions in Plasma by Ultrashort Electromagnetic Pulses

Excitation of highly charged ions in plasmas under the action of ultrashort electromagnetic pulses is investigated theoretically in the frame of perturbation theory. The study is based on analytical expression for probability of bound‐bound transition which is derived for Gaussian pulse and Doppler broadening of spectral line shape of the transition. Electronic transitions from the ground state of hydrogen‐like ions are considered with account for fine splitting of upper energy levels. The main attention is paid to the dependence of the excitation probability on pulse duration for various ion charges and carrier frequencies of the pulse. The results obtained are of interest for plasma diagnostics based on ion excitation from ground state by ultrashort pulses. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Screening of a charged particle field in rarefied ionized gas

A self-consistent field of a charged micron-size particle placed in a rarefied ionized gas is created by both free ions moving along infinite trajectories and trapped ions moving in closed orbits. The character of screening of the particle field is analyzed under dynamic conditions in a nonequilibrium plasma where the temperature (or the mean energy) of electrons greatly exceeds the ion temperature. Under these conditions, trapped ions are generated in a restricted region of the particle field where the transitions between closed ion orbits resulting from resonant charge exchange dominate. This leads to a higher number density of trapped ions compared to that of free ions. The parameters of the self-consistent field of the particle and ions are found when free or trapped ions determine the screening of the particle field, and a similarity law is established for a simultaneous variation of the number density of plasma particles and the particle size. In dusty plasmas of the Solar System, which result from the interaction of the solar wind with dust, formation of trapped ions increases the plasma number density compared to that in the solar wind.     

Relative intensities in x-ray photoelectron spectra. Part II

Experimental data on the relative intensities of X-ray photoelectron lines of some elements with 22 ? Z ? 56, and the calculations for the photoionization cross-sections for inner levels of some elements with 21 ? Z ? 63 are reported. The relationship between photoionization relative cross-sections and line relative intensities is examined. Theoretical values of the photoionization cross-sections were used in the calculations of relative intensities of molecular valence levels for AO₄^x? (A = Cl, S, P, Se, As), AF₆^x? (A = S, Si, Al), COS, CS₂ and H₂S.     

Photoionization cross-section measurements from the 2p, 3d and 3s excited states of lithium

The photoionization cross-sections from the 2p²P_{1/2, 3/2}, 3d²D_{3/2, 5/2} and 3s²S_1/2 excited states of lithium have been measured at different ionizing laser wavelengths, above the first ionization threshold. The experiments are performed by using a thermionic diode working in the space charge limited mode and the cross-sections are measured by employing the saturation technique. By changing the ionization photon energy, a smooth frequency dependence of the cross-sections has been observed for the 2p and 3d states. The cross-section from the 3s excited state has been measured at a single photon energy. The measured values of the photoionization cross are compared with the available data.     

Partial photoionization cross-sections for CO2 and N2O from 20 to 6OeV

Measurements of the partial photoionization cross-sections of the molecules CO₂ and N₂O are reported for the energy range 20–6OeV. Large contributions to the photoionization cross-section from multiple electron transitions are observed, particularly above 40eV, in accord with the results of recent many-body Green's function-calculations. The cross-section for total photabsorption as well as the ionization efficiency have also been measured.     

含温有界原子模型下电子与离子碰撞激发和电离截面的理论研究

采用扭曲波玻恩交换近似方法，在自由原子模型下计算了电子与离子碰撞激发、电离截面，计算值与实验一致；在含温有界平均原子模型下，系统研究了不同温度、密度等离子体中离子的电子碰撞直接电离截面，发现由于温度、密度效应导致离子的能级漂移，引起等离子体中离子的碰撞电离截面比自由原子情形发生较大变化. 关键词： 平均原子模型 扭曲波波恩交换近似 电离截面     

温稠密等离子体中Al12+离子的光电离过程

本文采用多组态Dirac-Hartree-Fock方法研究了温稠密等离子体环境中Al12+离子的光电离过程. 等离子体效应采用SM屏蔽势来模拟(Stanton和Murillo在2015年提出). 计算中固定标度温度1.3eV,在强耦合等离子体环境下,研究了等离子体密度对能级,连续态波函数以及光电离截面的影响. 通过与相同等离子体条件下的Debye-Hückel模型势计算的光电离截面对比,发现两种模型对应的结果中未同时出现势形共振峰,这是由于本文采用的屏蔽模型包含等离子体简并、有限温度对动能的梯度修正和量子交换关联效应,对光电离截面的大小和共振结构具有显著影响.     

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.