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Register a new userK-shell photoionization of Nickel ions using R-matrix
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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 optical potential. The damping processes affect the absorption resonances converging to the K thresholds causing them to display symmetric profiles of constant width that smear the otherwise sharp edge at the K-shell photoionization threshold. These data are important for the modeling of features found in photoionized plasmas.
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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/$Delta$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 identification and characterization of the strong 1s $rightarrow$ 2p resonances observed in the spectra.
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 individually measured cross-sections for m-fold ionization. Calculations of the Fe+ absorption cross sections have been carried out using two different theoretical approaches, Hartree-Fock including relativistic extensions and fully relativistic Multi-Configuration Dirac Fock. Apart from overall energy shifts of up to about 3 eV, the theoretical cross sections are in good agreement with each other and with the experimental results. In addition, the complex deexcitation cascades after the creation of inner-shell holes in the Fe+ ion have been tracked on the atomic fine-structure level. The corresponding theoretical results for the product charge-state distributions are in much better agreement with the experimental data than previously published configuration-average results. The present experimental and theoretical results are valuable for opacity calculations and are expected to pave the way to a more accurate determination of the iron abundance in the interstellar medium.
X-ray photoabsorption cross sections have been computed for all magnesium ions using the $R$-matrix method. A comparison with the other available data for Mg II-Mg X shows good qualitative agreement in the resultant resonance shapes. However, for the lower ionization stages, and for singly-ionized Mg II in particular, the previous $R$-matrix results (Witthoeft et al.2009; Witthoeft et al. 2011) overestimate the K-edge position due to the neglect of important orbital relaxation effects, and a global shift downward in photon energy of those cross sections is therefore warranted. We have found that the cross sections for Mg I and Mg II are further complicated by the M-shell ($n=3$) occupancy. As a result, the treatment of spectator Auger decay of $1srightarrow np$ resonances using a method based on multichannel quantum defect theory and an optical potential becomes problematic, making it necessary to implement an alternative, approximate treatment of Auger decay for neutral Mg. The new cross sections are used to fit the Mg K edge in XMM-Newton spectra of the low-mass X-ray binary GS 1826-238, where most of the interstellar Mg is found to be in ionized form.
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 capabilities by measuring valence photoelectron and ion spectra of neon and nitrogen, and by studying channel-resolved photoelectron and Auger spectra along with fragment-ion momentum correlations for chlorine $2p$ inner-shell ionization of textit{cis}- and textit{trans}-1,2-dichloroethene.
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 measurements are benchmarked against state-of-the-art theoretical cross-section calculations from the Dirac-Coulomb R-matrix method.
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