We have considered the processes of excitation and ionization of light multicharged ions by impact of high-energy particles, which proceed with participation of the $ns$ electrons. The screening corrections to the energy levels and photoionization cross sections are evaluated analytically within the framework of the non-relativistic perturbation theory with respect to the electron-electron interaction. The universal scalings for the excitation and ionization cross sections are studied for arbitrary principal quantum numbers $n$.
Modelling of massive stars and supernovae (SNe) plays a crucial role in understanding galaxies. From this modelling we can derive fundamental constraints on stellar evolution, mass-loss processes, mixing, and the products of nucleosynthesis. Proper account must be taken of all important processes that populate and depopulate the levels (collisional excitation, de-excitation, ionization, recombination, photoionization, bound-bound processes). For the analysis of Type Ia SNe and core collapse SNe (Types Ib, Ic and II) Fe group elements are particularly important. Unfortunately little data is cur…
We explore the effects of correlation on the ground-state energies and on photoionization dynamics in atomic Be and Ne. We apply the time-dependent restricted-active-space self-consistent-field method for several excitation schemes and active orbital spaces with and without a dynamic core to address the effects systematically at different levels of approximation. For the ground-state many-electron wave functions, we compare the correlation energies with entropic measures of entanglement. A larger magnitude of the correlation energy does not always correspond to a larger value of the considered…
We have studied the role of virtual excitations of inner shells upon outer shell photoionization. The calculations were performed in the frames of the Random Phase Approximation and its generalized version GRPAE that take into account variation of the atomic field due to electron elimination and the inner vacancies decay. We employ both analytic approximation and numeric computations. The results are presented for 3p electrons in Ar and 4d-electrons in Pd near inner shells thresholds. The effect proved to be noticeable.
Partial photoionization cross sections from the ^1S ground and the ^3P metastable states of the Be isoelectronic series to different final states including a number of satellite (ionization plus excitation) channels are studied using the R-matrix method. As expected, the ratio of each satellite partial cross section to the dominant partial cross section is found to be a monotonic decreasing function of Z for both the ground and the metastable states. The complicated patterns of these ratios as functions of photon energy are analyzed.
A detailed theoretical analysis of the 1s photoionization of neon is presented. It is found that the most significant many-electron correlation in computing photoionization of inner shells is the rearrangement of the outer shells caused by the inner vacancy. Further noticeable effects are: (i) the polarization of the ion core by the outgoing photoelectron and (ii) the coherent effect of double excitation/ionization. The core polarization increases the photoionization cross section by about 10% at the 1s threshold, and the coherent excitation results in further increases by about 5%. Incoherent…
Photoionization of an atom $A$ by a strong laser field in the presence of a spatially well-separated neighboring atom $B$ is considered. The laser field frequency is assumed to lie below the ionization potential of atom $A$ and be resonant with a dipole-allowed transition in atom $B$. In this situation, the ionization may occur either directly by multiphoton absorption from the laser field at the first atomic center. Or via an indirect pathway involving two-center electron-electron correlations, where the neighbor atom $B$ is first photoexcited and, afterwards, transfers its energy upon deexci…
Considering the photoionization of Ar@C60, we predict resonant femtosecond decays of both Ar and C60 vacancies through the continua of atom-fullerene hybrid final states. The resulting resonances emerge from the interference between simultaneous autoionizing and intercoulombic decay (ICD) processes. For Ar 3s-->np excitations, these resonances are far stronger than the Ar-to-C60 resonant ICDs, while for C60 excitations they are strikingly larger than the corresponding Auger features. The results indicate the power of hybridization to enhance decay rates, and modify lifetimes and line profiles.
Photoionization of an atom in the presence of a neighboring atom of different species is studied. The latter first undergoes resonant photoexcitation, leading to an autoionizing state of the diatomic system. Afterwards, the excitation energy is transferred radiationlessly via a two-center Auger process to the other atom, causing its ionization. Assuming a fixed internuclear distance, it has been predicted theoretically that this indirect ionization pathway can strongly dominate over the direct photoionization. Here, we extend the theory of resonant two-center photoionization by including the n…
A two-step photoionization strategy of an ultracold rubidium beam for application in a focused ion beam instrument is analyzed and implemented. In this strategy the atomic beam is partly selected with an aperture after which the transmitted atoms are ionized in the overlap of a tightly cylindrically focused excitation laser beam and an ionization laser beam whose power is enhanced in a build-up cavity. The advantage of this strategy, as compared to without the use of a build-up cavity, is that higher ionization degrees can be reached at higher currents. Optical Bloch equations including the ph…
By considering photoionization of the C$_{60}$ fullerene, we elucidate the contributions of various classical and quantum physics phenomena appearing in this process. By comparing the results, based on the {\it ab initio} and model approaches, we map the well-resolved features of the photoabsoprtion spectrum to single-particle and collective excitations which have the different physical nature. It is demonstrated that the peculiarities arising in the photoionization spectrum of C$_{60}$ atop the dominating plasmon excitations have the quantum origin. In particular, we demonstrate that a series…
Results from large-scale theoretical cross section calculations for the total photoionization of the 4f, 5s, 5p and 6s orbitals of the neutral tungsten atom using the Dirac Coulomb R-matrix approximation (DARC: Dirac-Atomic R-matrix codes) are presented. Comparisons are made with previous theoretical methods and prior experimental measurements. In previous experiments a time-resolved dual laser approach was employed for the photo-absorption of metal vapours and photo-absorption measurements on tungsten in a solid, using synchrotron radiation. The lowest ground state level of neutral tungsten i…
Measurements of the single photoionization cross section of Cu-like Zn$^+$ ions are reported in the energy (wavelength) range 17.5 eV (709 Å) to 90 eV (138 Å). The measurements on this {\it trans}-Fe element were performed at the Advanced Light Source synchrotron radiation facility in Berkeley, California at a photon energy resolution of 17 meV using the photon-ion merged-beams end-station. Below 30 eV the spectrum is dominated by excitation autoionizing resonance states. The experimental results are compared with large-scale photoionization cross-section calculations performed using a Dirac-C…
Photoionization cross sections are obtained using the relativistic Dirac Atomic R-matrix Codes (DARC) for all valence and L-shell energy ranges between 27-270eV. A total of 557 levels arising from the dominant configurations 3s$^2$3p$^4$, 3s3p$^5$, 3p$^6$, 3s$^2$3p$^3$[3d, 4s, 4p], 3p$^5$3d, 3s$^2$3p$^2$3d$^2$, 3s3p$^4$3d, 3s3p$^3$3d$^2$ and 2s$^2$2p$^5$3s$^2$3p$^5$ have been included in the target wavefunction representation of the Ar III ion, including up to 4p in the orbital basis. We also performed a smaller Breit-Pauli (BP) calculation containing the lowest 124 levels. Direct comparisons …
Single photoionization cross sections for Kr-like Rb$^+$ ions are reported in the energy (wavelength) range 22 eV (564 Å) to 46 eV (270 Å). Theoretical cross section calculations for this {\it trans}-Fe element are compared with measurements from the ASTRID radiation facility in Aarhus, Denmark and the dual laser plasma (DLP) technique, at respectively 40 meV and 35 meV FWHM energy resolution. In the photon energy region 22 - 32 eV the spectrum is dominated by excitation autoionizing resonance states. Above 32 eV the cross section exhibit classic Fano window resonances features, which are anal…
In a joint experimental and theoretical endeavour, photoionization of metastable C4+(1s 2s 3S1) ions via intermediate levels with hollow, double-K-vacancy configurations 2s2p, 2s3p, 2p3s, 2p3d, 2s4p, 2p4s and 2p4d has been investigated. High-resolution photon-ion merged-beams measurements were carried out with the resolving power reaching up to 25,000 which is sufficient to separate the leading fine-structure components of the 2s2p 3P term. Many-body perturbation theory was employed to determine level-to-level cross sections for K-shell excitation with subsequent autoionization. The resonance …
Single, double, and triple ionization of the C+ ion by a single photon have been investigated in the energy range 286 to 326 eV around the K-shell single-ionization threshold at an unprecedented level of detail. At energy resolutions as low as 12 meV, corresponding to a resolving power of 24000, natural linewidths of the most prominent resonances could be determined. From the measurement of absolute cross sections, oscillator strengths, Einstein coefficients, multi-electron Auger decay rates and other transition parameters of the main K-shell excitation and decay processes are derived. The cro…
We introduce the time-dependent restricted active space Configuration Interaction method to solve the time-dependent Schrödinger equation for many-electron atoms, and particularly apply it to the treatment of photoionization processes in atoms. The method is presented in a very general formulation and incorporates a wide range of commonly used approximation schemes, like the single-active electron approximation, time-dependent Configuration Interaction with single-excitations, or the time-dependent R-matrix method. We proof the applicability of the method by calculating the photoionization cro…
The photon-ion merged-beams technique has been employed at the new Photon-Ion spectrometer at PETRA III (PIPE) for measuring multiple photoionization of Xe$^{q+}$ (q=1-5) ions. Total ionization cross sections have been obtained on an absolute scale for the dominant ionization reactions of the type hν+ Xe$^{q+}$ $\to$ Xe$^{r+}$ + (q-r) e$^-$ with product charge states q+2 $\le$ r $\le$ q+5. Prominent ionization features are observed in the photon-energy range 650-750 eV, which are associated with excitation or ionization of an inner-shell 3d electron. Single-configuration Dirac-Fock calculation…
Opacity determines radiation transport through material media. In a plasma source the primary contributors to atomic opacity are bound-bound line transitions and bound-free photoionization into the continuum. We review the theoretical methodology for state-of-the-art photoionization calculations based on the R-matrix method as employed in the Opacity Project, the Iron Project, and solution of the heretofore unsolved problem of plasma broadening of autoionizing resonances due to electron impact, Stark (electric microfields), Doppler (thermal), and core-excitations. R-matrix opacity calculations…
Experimental and theoretical results are presented for double, triple, and quadruple photoionization of Si$^+$ and Si$^{2+}$ ions and for double photoionization of Si$^{3+}$ ions by a single photon. The experiments employed the photon-ion merged-beams technique at a synchrotron light source. The experimental photon-energy range 1835--1900 eV comprises resonances associated with the excitation of a $1s$ electron to higher subshells and subsequent autoionization. Energies, widths, and strengths of these resonances are extracted from high-resolution photoionization measurements, and the core-hole…
Photoionization of an atom $A$, in the presence of a neighboring atom $B$, can proceed via resonant excitation of $B$ with subsequent energy transfer to $A$ through two-center electron-electron correlation. We demonstrate that this two-center mechanism can strongly outperform direct photoionization at nanometer internuclear distances and possesses characteristic features in its time development and the spectrum of emitted electrons.
Heavy-metal hot subdwarfs (sdB and sdO) represent a small group of stars with unusually high concentrations of trans-iron elements in their atmospheres, having abundances ~ 10000 times solar. One example is LS IV-14$^{\circ}$ 116, where a number of heavy-metal absorption lines of Sr II, Y III and Zr IV have been observed in the optical band 4000 - 5000 A. We use a fully relativistic Dirac atomic R-Matrix (DARC) to calculate photoionization cross sections of Sr$^{0}$, Y$^{+}$ and Zr$^{2+}$ from their ground state to the twentieth excited level. We use the cross sections and the oscillator stren…
It is shown that the M$^{2+}$ ion yield in the double photoionization of the aromatic hydrocarbons, pyrene and coronene, can be expressed as a superposition of a contribution from a resonance involving carbon atoms on the perimeter and coherent contributions from carbon atoms inside the perimeter. In the case of pyrene, the two interior atoms are associated with a resonance peak at 10 eV and linear behavior above 75 eV. The resonance peak is an optically excited state of the interior carbon pair. The linear behavior arises from the coherent emission of two electrons with equal energy and oppos…
We report on experimental and theoretical results for the photoionization of Ag-like xenon ions, Xe$^{7+}$, in the photon energy range 95 to 145~eV. The measurements were carried out at the Advanced Light Source at an energy resolution of $Δ$E = 65 meV with additional measurements made at $Δ$E = 28 meV and 39 meV. Small resonance features below the ground-state ionization threshold, at about 106 eV, are due to the presence of metastable Xe$^{7+} (4d^{10} 4f~^2{\rm F}^{\circ}_{5/2,7/2})$ ions in the ion beam. On the basis of the accompanying theoretical calculations using the Dirac Atomic R-mat…
Resonant two-photon ionization in a system consisting of two spatially well-separated atoms is studied. Due to two-center electron-electron correlations, the ionization may also proceed through photo-excitation of both atoms with subsequent interatomic Coulombic decay. We show that this channel may dominate the photoionization process and qualitatively change its dependence on the field intensity and the spectra of emitted electrons.
A comprehensive study of high-accuracy photoionization cross sections is carried out using the relativistic Breit-Pauli R-matrix (BPRM) method for (hnu + Fe XVII --> Fe XVIII + e). Owing to its importance in high-temperature plasmas the calculations cover a large energy range, particularly the myriad photoexciation-of-core (PEC) resonances including the n = 3 levels not heretofore considered. The calculations employ a close coupling wave function expansion of 60 levels of the core ion Fe XVIII ranging over a wide energy range of nearly 900 eV between the n = 2 and n = 3 levels. Strong coupling…
Relative cross sections for $m$-fold photoionization ($m=1,\ldots,5$) of Fe$^{3+}$ by single photon absorption were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source operated at DESY in Hamburg, Germany. The photon energies used spanned the range of $680-950\,\mathrm{eV}$, covering both the photoexcitation resonances from the $2p$ and $2s$ shells as well as the direct ionization from both shells. Multiconfiguration Dirac-Hartree-Fock (MCDHF) calculations were performed to simulate the total photoexcitation spectra. Good agreement was found with…
Using the photon-ion merged-beams technique at a synchrotron light source, we have measured relative cross sections for single and up to five-fold photoionization of Fe$^{2+}$ ions in the energy range 690--920 eV. This range contains thresholds and resonances associated with ionization and excitation of $2p$ and $2s$ electrons. Calculations were performed to simulate the total absorption spectra. The theoretical results show very good agreement with the experimental data, if overall energy shifts of up to 2.5 eV are applied to the calculated resonance positions and assumptions are made about t…
Absolute cross sections for m-fold photoionization (m=1,...,6) of Fe+ by a single photon were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany. Photon energies were in the range 680-920 eV which covers the photoionization resonances associated with 2p and 2s excitation to higher atomic shells as well as the thresholds for 2p and 2s ionization. The corresponding resonance positions were measured with an uncertainty of +- 0.2 eV. The cross section for Fe+ photoabsorption is derived as the sum of the individu…
Recent R-matrix calculations claim to produce a significant enhancement in the opacity of Fe XVII due to atomic core excitations [S. N. Nahar & A.K. Pradhan, Phys. Rev. Letters 116, 235003 (2016), arXiv:1606.02731] and assert that this enhancement is consistent with recent measurements of higher-than-predicted iron opacities [J. E. Bailey et al., Nature 517, 56 (2015)]. This comment shows that the standard opacity models which have already been directly compared with experimental data produce photon absorption cross-sections for Fe XVII that are effectively equivalent to (and in fact larger th…
Double to tenfold M-shell photoionization of singly charged lanthanum ions
Helmholtz Forschungsakademie Hessen für FAIR (HFHF)（黑森FAIR亥姆霍兹研究学院）；GSI Helmholtzzentrum für Schwerionenforschung（GSI重离子研究中心）；Institut für Experimentalphysik, Universität Hamburg（汉堡大学实验物理研究所）
The success of the R-matrix with pseudo-state (RMPS) method to model single photoionization processes, benchmarked against dedicated synchrotron light source measurements, is exploited and extended to investigate, single photon double ionization cross-sections for the He-like, Li$^+$ ion. We investigate these processes from both the ground state and the excited $\rm 1s2s^{1,3}S$ metastable levels of this He-like system. Comparisons of the results from the R-matrix plus pseudo-state (RMPS) method are made with other state-of-the-art theoretical approaches such as time-dependent close-coupling (…
Double ionization of an atom by single-photon absorption in the presence of a neighbouring atom is studied. The latter is, first, resonantly photoexcited and, afterwards, transfers the excitation energy radiationlessly to the other atom, leading to its double ionization. The process relies on the combined effect of interatomic and intraatomic electron correlations. It can dominate over the direct double photoionization by several orders of magnitude at interatomic distances up to few nanometers. The relative position of the neighbouring atom is shown to exert a characteristic influence on the …
We present multi-configuration Breit-Pauli AUTOSTRUCTURE calculations of distorted-wave photoionization (PI) cross sections, and total and partial final-state resolved radiative recombination (RR) and dielectronic recombination (DR) rate coefficients for the first six ions of the trans-iron element Se. These calculations were motivated by the recent detection of Se emission lines in a large number of planetary nebulae. Se is a potentially useful tracer of neutron-capture nucleosynthesis, but accurate determinations of its abundance in photoionized nebulae have been hindered by the lack of atom…
Considering the photoionization of Ar@C60 and Kr@C60 endofullerenes, the decay of C60 innershell excitations through the outershell continuum of the confined atom via the inter-Coulombic decay (ICD) pathway is detailed. Excitations to atom-C60 hybrid states demonstrate coherence between ICD and electron-transfer mediated decay (ETMD). This should be the dominant above-threshold decay process for a variety of confined systems, and the strength of these resonances is such that they should be amenable for study by photoelectron spectroscopy.
An analytical formula is developed to represent accurately the photoabsorption cross section of O I for all energies of interest in X-ray spectral modeling. In the vicinity of the Kedge, a Rydberg series expression is used to fit R-matrix results, including important orbital relaxation effects, that accurately predict the absorption oscillator strengths below threshold and merge consistently and continuously to the above-threshold cross section. Further minor adjustments are made to the threshold energies in order to reliably align the atomic Rydberg resonances after consideration of both expe…
In an ensemble of laser-driven atoms involving strongly interacting Rydberg states, the excitation probability is usually strongly suppressed. In contrast, here we identify a regime in which the steady-state Rydberg excited fraction is enhanced by the interaction. This effect is associated with the build-up of many-body coherences, induced by coherent multi-photon excitations between collective states. The excitation enhancement should be observable under currently-existing experimental conditions, and may serve as a direct probe for the presence of coherent multi-photon dynamics involving col…
We present multi-configuration Breit-Pauli distorted-wave photoionization (PI) cross sections and radiative recombination (RR) and dielectronic recombination (DR) rate coefficients for the first six krypton ions. These were calculated with the AUTOSTRUCTURE code, using semi-relativistic radial wavefunctions in intermediate coupling. Kr has been detected in several planetary nebulae (PNe) and H II regions, and is a useful tracer of neutron-capture nucleosynthesis. PI, RR, and DR data are required to accurately correct for unobserved Kr ions in ionized nebulae, and hence to determine elemental K…
We present measurements of the pulse length of ultracold electron bunches generated by near-threshold two-photon photoionization of a laser-cooled gas. The pulse length has been measured using a resonant $3$ GHz deflecting cavity in TM$_{110}$ mode. We have measured the pulse length in three ionization regimes. The first is direct two-photon photoionization using only a $480$ nm femtosecond laser pulse, which results in short ($\sim 15$ ps) but hot ($\sim 10^{4}$ K) electron bunches. The second regime is just-above-threshold femtosecond photoionization employing the combination of a continuous…
The probability that photoionization of the caged atom in an endohedral system is accompanied by excitation of the fullerene shell is shown to be close to unity in broad intervals of the photoelectron energies. The result is obtained by summation of the perturbative series for the interaction between the photoelectron and the fullerene shell. This result means that the interaction between the photoelectron ejected from the cage atom and the fullerene shell cannot be described by a static potential, since inelastic processes become decisively important.
介绍了国际不透明度计划（OP）以来理论计算的最新进展，以及新的铁计划（IP）和扩展，用于恒星大气、星云和活动星系核等各种物体的计算。主要的原子过程是：电子碰撞激发（EIE），光电离，电子-离子复合，和束缚束缚跃迁，所有考虑使用准确和强大的R-矩阵方法，包括相对论效应。作为OP和IP的扩展，发展了一个自洽的、统一的光电离和复合理论。辐射和双电子复合（RR和DR）过程被认为是在一个统一的方式。光电离和复合截面计算具有相同的波函数展开，从而确保在从头算方式的自洽性。新的统一结果不同于以前的结果总和RR和DR的重要，但复杂的原子系统，如Fe I-V的一个因素，4.根本的区别是由于量子力学的干扰和耦合效应忽略在简单的近似，非物理治疗RR和DR分开，这不能独立测量或观察。我还将描述电子、网络交互式数据库TIPTOPBASE，以便于访问的方式存档OP/IP数据。TIPTOPBASE还将包括电子-离子复合数据和新的精细结构跃迁概率。还将提供M. J. Seaton开发的计算恒星中“定制”混合物不透明度和辐射加速度（“悬浮”）的有效代码。
R-Matrix calculations for opacities: IV. Convergence, completeness, and comparison of relativistic R-matrix and distorted wave calculations for FeXVII and FeXVIII
We study photoionization of argon atoms excited by attosecond pulses using an interferometric measurement technique. We measure the difference in time delays between electrons emitted from the $3s^2$ and from the $3p^6$ shell, at different excitation energies ranging from 32 to 42 eV. The determination of single photoemission time delays requires to take into account the measurement process, involving the interaction with a probing infrared field. This contribution can be estimated using an universal formula and is found to account for a substantial fraction of the measured delay.
We present an experimental and theoretical energy- and angle-resolved study on the photoionization dynamics of non-resonant one-color two-photon single valence ionization of neutral N$_2$ molecules. Using 9.3 eV photons produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and ions produced from one-color two-photon ionization in coincidence. Photoionization of N$_2$ populates the X $^2Σ^+_g$, A $^2Π_u$, and B $^2Σ^+_u$ ionic states of N$_2^+$, where the photoelectron angular distributions associated with the X $^2Σ^+_g$ and A $^2Π_u$ state…
Excitation and charge transfer in low-energy O+H collisions is studied; it is a problem of importance for modelling stellar spectra and obtaining accurate oxygen abundances in late-type stars including the Sun. The collisions have been studied theoretically using a previously presented method based on an asymptotic two-electron linear combination of atomic orbitals (LCAO) model of ionic-covalent interactions in the neutral atom-hydrogen-atom system, together with the multichannel Landau-Zener model. The method has been extended to include configurations involving excited states of hydrogen usi…
