Measurements of absolute cross sections for the {\it K}-shell photoionization of B-like atomic nitrogen ions were carried out utilizing the ion-photon merged-beam technique at the SOLEIL synchrotron radiation facility in Saint-Aubin, France. High-resolution spectroscopy with E/$Δ$E $\approx$ 13,500 was the maximum resolution achieved. We have investigated two photon energy regions: 404 eV -- 409 eV and 439 eV -- 442 eV. Resonance peaks found in the experimental measured cross sections are compared with theoretical estimates from the multi-configuration Dirac - Fock, R-matrix and empirical methods, allowing identification of the strong 1s $\rightarrow$ 2p and the weaker 1s $\rightarrow$ 3p resonances in the observed {\it K}-shell spectra of this B-like nitrogen ion.
We present $R$-matrix calculations of photoabsorption and photoionization cross sections across the K-edge of Mg, Si, S, Ar, and Ca ions with more than 10 electrons. The calculations include the effects of radiative and Auger damping by means of an optical potential. The wave functions are constructed from single-electron orbital bases obtained using a Thomas--Fermi--Dirac statistical model potential. Configuration interaction is considered among all states up to $n=3$. The damping processes affect the resonances converging to the K-thresholds causing them to display symmetric profiles of cons…
Absolute cross sections for the {\it K}-shell photoionization of Be-like and Li-like atomic nitrogen ions were measured by employing the ion-photon merged-beam technique at the SOLEIL synchrotron radiation facility in Saint-Aubin, France. High-resolution spectroscopy at nominal resolutions of 38 meV, 56 meV, 111 meV, 133 meV FWHM for Be-like and 125 meV FWHM for Li-like atomic nitrogen ions was achieved for the photon energies ranging from 410 eV up to 460 eV. The experimental measurements are compared with theoretical estimates from the multi-configuration Dirac-Fock (MCDF), R-matrix and an e…
We present R-matrix calculations of photoabsorption and photoionization cross sections across the K edge of the Li-like to Ca-like ions stages of Ni. Level-resolved, Breit-Pauli calculations were performed for the Li-like to Na-like stages. Term-resolved calculations, which include the mass-velocity and Darwin relativistic corrections, were performed for the Mg-like to Ca-like ion stages. This data set is extended up to Fe-like Ni using the distorted wave approximation as implemented by AUTOSTRUCTURE. The R-matrix calculations include the effects of radiative and Auger dampings by means of an …
The photon-ion merged-beams technique for the photoionization of mass/charge selected ionized atoms, molecules and clusters by x-rays from synchrotron radiation sources is introduced. Examples for photoionization of atomic ions are discussed by going from outer-shell ionization of simple few-electron systems to inner-shell ionization of complex many-electron ions. Fundamental ionization mechanisms are elucidated and the importance of the results for applications in astrophysics and plasma physics is pointed out. Finally, the unique capabilities of the photon-ion merged-beams technique for the …
We photoionize nitrogen molecules with a train of extreme ultraviolet attosecond pulses together with a weak infrared field. We measure the phase of the two-color two-photon ionization transition (molecular phase) for different states of the ion. We observe a 0.76 $π$ shift for the electrons produced in the ionization channels leading to the X$^2 Σ_g^+$, $v'=1$ and $v'=2$ states. We relate this phase shift to the presence of a complex resonance in the continuum. By providing both a high spectral and temporal resolution, this general approach gives access to the evolution of extremely short liv…
Absolute cross sections for the {\it K}-shell photoionization of boron-like (B-like) O$^{3+}$ ions were measured by employing the ion-photon merged-beam technique at the SOLEIL synchrotron-radiation facility in Saint-Aubin, France. High-resolution spectroscopy with E/$Δ$E $\approx$ 5000 ($\approx$ 110 meV, FWHM) was achieved with photon energy from 540 eV up to 600 eV. Several theoretical approaches, including R-Matrix, Multi-Configuration Dirac-Fock and Screening Constant by Unit Nuclear Charge were used to identify and characterize the strong 1s $\rightarrow$ 2p and the weaker 1s $\rightarro…
Absolute cross sections for the K-shell photoionization of C-like nitrogen ions were measured by employing the ion-photon merged-beam technique at the SOLEIL synchrotron radiation facility in Saint-Aubin, France. High-resolution spectroscopy with E/$Δ$E $\approx$ 7,000 was achieved with the photon energy from 388 to 430 eV scanned with a band pass of 300 meV, and the 399.4 to 402 eV range with 60 meV. Experimental results are compared with theoretical predictions made from the multi-configuration Dirac-Fock (MCDF) and R-matrix methods. The interplay between experiment and theory enabled the id…
Absolute cross sections for the K-shell photoionization of Be-like boron ions were measured with the ion-photon merged-beams technique at the Advanced Light Source synchrotron radiation facility. High-resolution spectroscopy with E/$Δ$E up to 8800 ($Δ$E $\sim$ 22 meV) covered the energy ranges 193.7 -- 194.7 eV and 209 -- 215 eV. Lifetimes of the strongest resonances are determined with relative uncertainties down to approximately 4% for the broadest resonance. The measured resonance strengths are consistent with 60 % $\rm 1s^22s^2$ $^1S$ ground-state and 40 % $\rm 1s^22s2p$ $^3P^o$ metastable…
We review recent work on the photoionization of atomic ions of astrophysical interest that has been carried out at the photon-ion merged-beams setup PIPE, a permanently installed end station at the XUV beamline P04 of the PETRAIII synchrotron radiation source operated by DESY in Hamburg, Germany. Our results on single and multiple L-shell photoionization of Fe+, Fe2+, and Fe3+ ions and on single and multiple K-shell photoionization of C-, C+, C4+, Ne+, and Si2+ ions are discussed in astrophysical contexts. Moreover, these experimental results bear witness of the fact, that the implementation o…
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.
The cross section for single K-shell ionization by a high-energy photon is evaluated in the next-to-leading order of the nonrelativistic perturbation theory with respect to the electron-electron interaction. The screening corrections are of particular importance for light helium-like ions. Even in the case of neutral He atom, our analytical predictions turn out to be in good agreement with the numerical calculations performed with the use of the sophisticated wave functions. The universal high-energy behavior is studied for the ratio of double-to-single photoionization cross sections. We also …
Experimental and theoretical cross sections are reported for single-photon single ionization of W$^{5+}$ ions. Absolute measurements were conducted employing the photon-ion merged-beams technique. Detailed photon-energy scans were performed at (67$\pm$10)~meV resolution in the 20 -- 160 eV range. In contrast to photoionization of tungsten ions in lower charge states, the cross section is dominated by narrow, densely-spaced resonances. Theoretical results were obtained from a Dirac-Coulomb R-matrix approach employing a basis set of 457 levels providing cross sections for photoionization of W$^{…
Triple photoionization of Xe3+, Xe4+ and Xe5+ ions has been studied in the energy range 670-750 eV, including the 3d ionization threshold. The photon-ion merged-beam technique was used at a synchrotron light source to measure the absolute photoionization cross sections. These cross sections exhibit a progressively larger number of sharp resonances as the ion charge state is increased. This clearly visualizes the re-ordering of the $ε$f continuum into a regular series of (bound) Rydberg orbitals as the ionic core becomes more attractive. The energies and strengths of the resonances are extracte…
Absolute cross-section measurements for valence-shell photoionization of Ar$^{+}$ ions are reported for photon energies ranging from 27.4 eV to 60.0 eV. The data, taken by merging beams of ions and synchrotron radiation at a photon energy resolution of 10 meV, indicate that the primary ion beam was a statistically weighted mixture of the $^2P^o_{3/2}$ ground state and the $^2P^o_{1/2}$ metastable state of Ar$^{+}$. Photoionization of this C$\ell$-like ion is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. Observed resonanc…
We discuss the results of the calculation of the photoionization with de-excitation of excited He and helium-like ions Li$^{+}$ and B$^{3+}$ at high but non-relativistic photon energies $ω$. Several lower $^{1}S$ and $^{3}S$ states are considered. We present and analyze the ratios $R_{d}^{+\ast}$ of the cross sections of photoionization with de-excitation, $σ_{(d)}^{+\ast}(ω)$, and of the photo-ionization with excitation, $σ^{+\ast}(ω)$. The dependence of $R_{d}^{+\ast}$ on the excitation of the target object and the charge of its nucleus is presented. Apart to theoretical interest, results ob…
Photoionization of Kr$^+$ ions was studied in the energy range from 23.3 eV to 39.0 eV at a photon energy resolution of 7.5 meV. Absolute measurements were performed by merging beams of Kr$^+$ ions and of monochromatized synchrotron undulator radiation. Photoionization (PI) of this Br-like ion is characterized by multiple Rydberg series of autoionizing resonances superimposed on a direct photoionization continuum. Resonance features observed in the experimental spectra are spectroscopically assigned and their energies and quantum defects tabulated. The high-resolution cross-section measurement…
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 …
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…
Experimental and theoretical results are reported for single-photon single ionization of the tungsten ion W$^{4+}$. Absolute cross sections have been measured employing the photon-ion merged-beams setup at the Advanced Light Source in Berkeley. Detailed photon-energy scans were performed at 200~meV bandwidth in the 40 -- 105~eV range. Theoretical results have been obtained from a Dirac-Coulomb R-matrix approach employing basis sets of 730 levels for the photoionization of W$^{4+}$. Calculations were carried out for the $4f^{14}5s^2 5p^6 5d^2 \; {^3}{\rm F}_{J}$, $J$=2, ground level and the ass…
Photoionization cross sections calculations on the halogen-like ions; Kr$^+$ and Xe$^+$ have been performed for a photon energy range from each ion threshold to 15 eV, using large-scale close-coupling calculations within the Dirac-Coulomb R-matrix approximation. The results from our theoretical work are compared with recent measurements made at the ASTRID merged-beam set-up at the University of Aarhus in Denmark and from the Fourier transform ion cyclotron resonance (FT-ICR) trap method at the SOLEIL synchrotron radiation facility in Saint-Aubin, France and the Advanced Light Soure (ALS). For …
The absolute single photoionization cross-section was measured for Cl-like K$^{2+}$ over the photon energy range from 44.2 - 69.7 eV at a constant energy resolution of 0.045 eV. The experiments were performed by merging an ion beam with a beam of synchrotron radiation from an undulator. The ground-state ionization threshold was measured at 0.004 eV energy resolution to be 45.717 $\pm$ 0.030 eV. The measurements are rich in resonance structure due to multiple Rydberg series of transitions to autoionizing states. These series are assigned spectroscopically using the quantum defect method, guided…
High-resolution photoionization experiments were carried out using beams of Be-like C$^{2+}$, N$^{3+}$, and O$^{4+}$ ions with roughly equal populations of the $^1$S ground-state and the $^3$P$^o$ manifold of metastable components. The energy scales of the experiments are calibrated with uncertainties of 1 to 10 meV depending on photon energy. Resolving powers beyond 20,000 were reached allowing for the separation of contributions from the individual metastable $^3$P$^o_0$, $^3$P$^o_1$, and $^3$P$^o_2$ states. The measured data compare favourably with semi-relativistic Breit-Pauli R-matrix
The IRON Project, initiated in 1991, aims at two main objectives, i) study the characteristics of and calculate large-scale high accuracy data for atomic radiative and collisional processes, and ii) application in solving astrophysical problems. It focuses on the complex iron and iron-peak elements commonly observed in the spectra of astrophysical plasmas. The present report will illustrate the characteristics of the dominant atomic process of photoionization that have been established under the project and the preceding the Opacity Project and their importance in applications.
Energy levels, hyperfine interaction constants, and Landé $g_J$-factors are reported for n=2 states in beryllium-, boron-, carbon-, and nitrogen-like ions from relativistic configuration interaction calculations. Valence, core-valence, and core-core correlation effects are taken into account through single and double-excitations from multireference expansions to increasing sets of active orbitals. A systematic comparison of the calculated hyperfine interaction constants is made with values from the available literature.
The relations between photoionization cross sections and photon radius are obtained on basis of quantum mechanics and the particle-like properties of a photon. The photoionization cross sections of H atom and H-like ions, He atom and He like ions, alkali metal atoms, and Rydberg atoms are calculated using the relations. The calculation results are found to be good agreement with the known experimental data. The results show that the photoionization cross section is always smaller than the cross section of the photon to ionize the atom or ion and can be expressed as the product of the cross sec…
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…
Absolute cross sections for the K-shell photoionization of ground-state Li-like carbon [C$^{3+}$(1s$^2$2s $^2$S)] ions were measured by employing the ion-photon merged-beams technique at the Advanced Light Source. The energy ranges 299.8--300.15 eV, 303.29--303.58 eV and 335.61--337.57 eV of the [1s(2s2p)$^3$P]$^2$P, [1s(2s2p)$^1$P]$^2$P and [(1s2s)$^3$S 3p]$^2$P resonances, respectively, were investigated using resolving powers of up to 6000. The autoionization linewidth of the [1s(2s2p)$^1$P]$^2$P resonance was measured to be $27 \pm 5$ meV and compares favourably with a theoretical result o…
We produce large numbers of low-energy ions by photoionization of laser-cooled atoms inside a surface-electrode-based Paul trap. The isotope-selective trap loading rate of $4\times10^{5}$ Yb$^{+}$ ions/s exceeds that attained by photoionization (electron impact ionization) of an atomic beam by four (six) orders of magnitude. Traps as shallow as 0.13 eV are easily loaded with this technique. The ions are confined in the same spatial region as the laser-cooled atoms, which will allow the experimental investigation of interactions between cold ions and cold atoms or Bose-Einstein condensates.
The objective of our proposal is to study the photoionization of Chlorine, Sulfur, and Phosphorus ions using the unique high resolution capability of 10.0.1 beamline in the ALS. These species are relevant in astrophysics and as a benchmark for state of the art theoretical models of the quantum properties of ions. We also propose to test a technique to quench the metastable states from the ion-beam based on recent experimental evidence and on a well known technique in chemistry. The work of this proposal may lay the foundations for a definite solution to control the metastable component of the …
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 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…
We report on the design and performance of a double-sided coincidence velocity map imaging spectrometer optimized for electron-ion and ion-ion coincidence experiments studying inner-shell photoionization of gas-phase molecules with soft X-ray synchrotron radiation. The apparatus employs two microchannel plate detectors equipped with delay-line anodes for coincident, time- and position-resolved detection of photo- and Auger electrons with kinetic energies up to 300\,eV on one side of the spectrometer and photoions up to 25\,eV per unit charge on the opposite side. We demonstrate its capabilitie…
Energy levels, normal and specific mass shift parameters as well as electronic densities at the nucleus are reported for numerous states along the beryllium, boron, carbon, and nitrogen isoelectronic sequences. Combined with nuclear data, these electronic parameters can be used to determine values of level and transition isotope shifts. The calculation of the electronic parameters is done using first-order perturbation theory with relativistic configuration interaction wave functions that account for valence, core-valence and core-core correlation effects as zero-order functions. Results are c…
Experimental and theoretical results are reported for single-photon single ionization of W$^{2+}$ and W$^{3+}$ tungsten ions. Experiments were performed at the photon-ion merged-beam setup of the Advanced Light Source in Berkeley. Absolute cross sections and detailed energy scans were measured over an energy range from about 20~eV to 90~eV at a bandwidth of 100~meV. Broad peak features with widths typically around 5~eV have been observed with almost no narrow resonances present in the investigated energy range. Theoretical results were obtained from a Dirac-Coulomb $R$-matrix approach. The cal…
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…
The Photon-Ion Spectrometer at PETRA III - in short, PIPE - is a permanently installed user facility at the "Variable Polarization XUV Beamline" P04 of the synchrotron light source PETRA III operated by DESY in Hamburg, Germany. The careful design of the PIPE ion-optics in combination with the record-high photon flux at P04 has lead to a breakthrough in experimental studies of photon interactions with ionized small quantum systems. This short review provides an overview over the published scientific results from photon-ion merged-beams experiments at PIPE that were obtained since the start of …
Absolute photoionization cross sections for the Rb$^{2+}$ ion were recently measured at high resolution over the energy range 37.31 eV -- 44.08~eV, with autoionizing Rydberg resonance series identified, using the photon-ion merged-beam setup at the Advanced Light Source (\textsc{als}) in Berkeley {(Macaluso D A et. al. J. Phys. B: At. Mol. Opt. Phys. {\bf 49} (2016) 235002; {\bf 50} (2017) 119501)}. Detailed photon-energy scans taken at 13.5 $\pm$ 2.5 meV bandwidth illustrated multiple Rydberg resonance series associated with the ground and metastable states. Here we present theoretical cross …
The decay of inner-shell vacancy in an atom through radiative and non-radiative transitions leads to final charged ions. The de-excitation decay of 3s, 3p and 3d vacancies in Kr atoms are calculated using Monte-Carlo simulation method. The vacancy cascade pathway resulted from the de-excitation decay of deep core hole in 3s subshell in Kr atoms is discussed. The generation of spectator vacancies during the vacancy cascade development gives rise to Auger satellite spectra. The last transitions of the de-excitation decay of 3s, 3p and 3d holes lead to specific charged ions. Dirac-Fock-Slater wav…
Singly ionized nitrogen molecules in ambient air pumped by near-infrared femtosecond laser give rise to superradiant emission. Here we demonstrate coherent control of this superradiance by injecting a pair of resonant seeding pulses inside the nitrogen gas plasma. Strong modulation of the 391.4 nm superradiance with a period of 1.3 fs is observed when the delay between the two seeding pulses are finely tuned, pinpointing the essential role of macroscopic coherence in this lasing process. Based on this time-resolved method, the complex temporal evolution of the macroscopic coherence between two…
Reliable atomic data have been computed for the spectral modeling of the nitrogen K lines, which may lead to useful astrophysical diagnostics. Data sets comprise valence and K-vacancy level energies, wavelengths, Einstein $A$-coefficients, radiative and Auger widths and K-edge photoionization cross sections. An important issue is the lack of measurements which are usually employed to fine-tune calculations so as to attain spectroscopic accuracy. In order to estimate data quality, several atomic structure codes are used and extensive comparisons with previous theoretical data have been carried …
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…
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 $n$.
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）
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…
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…
Simple and effective techniques for loading barium ions into linear Paul traps are demonstrated. Two-step photoionization of neutral barium is achieved using a weak intercombination line (6s2 1S0 <-> 6s6p 3P1, 791 nm) followed by excitation above the ionization threshold using a nitrogen gas laser (337 nm). Isotopic selectivity is achieved by using a near Doppler-free geometry for excitation of the triplet 6s6p 3P1 state. Additionally, we report a particularly simple and efficient trap loading technique that employs an in-expensive UV epoxy curing lamp to generate photoelectrons.
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…
A fully relativistic calculation of the double photoionization of helium-like atoms is presented. The approach is based on the partial-wave representation of the Dirac continuum states and accounts for the retardation in the electron-electron interaction as well as the higher-order multipoles of the absorbed photon. The electron-electron interaction is taken into account to the leading order of perturbation theory. The relativistic effects are shown to become prominent already for the medium-Z ions, changing the shape and the asymptotic behaviour of the photon energy dependence of the ratio of…
Two-photon ionization of Rubidium atoms in a magneto-optical trap and a Bose-Einstein condensate (BEC) is experimentally investigated. Using 100 ns laser pulses, we detect single ions photoionized from the condenstate with a 35(10)% efficiency. The measurements are performed using a quartz cell with external electrodes, allowing large optical access for BECs and optical lattices.
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…
Absolute {\it K}-shell photoionization cross sections for atomic nitrogen have been obtained from both experiment and state-of-the-art theoretical techniques. Due to the difficulty of creating a target of neutral atomic nitrogen, no high-resolution {\it K}-edge spectroscopy measurements have been reported for this important atom. Interplay between theory and experiment enabled identification and characterization of the strong $1s$ $\rightarrow$ $np$ resonance features throughout the threshold region. An experimental value of 409.64 $\pm$ 0.02 eV was determined for the {\it K}-shell binding ene…
Photoionization cross sections and partial ion yields of Xe and XeF$_2$ from Xe 3d$_{5/2}$, Xe 3d$_{3/2}$, and F 1s subshells in the 660--740 eV range are compared to explore effects of the F ligands. The Xe 3d - $ε$f continuum shape resonances dominate the photoionization cross sections of both the atom and molecule, but prominent resonances appear in the XeF$_2$ cross section due to nominal excitation of Xe 3d and F 1s electrons to the lowest unoccupied molecular orbital (LUMO), a delocalized anti-bonding MO. The subshell ionization thresholds, the LUMO resonance energies and their oscillato…
