Photoionization cross sections of the aluminumlike Si+ ion in the region of the 2 p threshold (94–137 eV) (original) (raw)
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Physical Review A, 2003
We present the absolute photoionization cross section of doubly ionized silicon as a function of photon energy. These were obtained by merging a Si 2ϩ ion beam generated in an electron cyclotron resonance source with monochromatized synchrotron radiation from an undulator. The photoion yield measurements were carried out in the photon energy range between 95 eV and 170 eV, i.e., the region corresponding to the excitation followed by the ionization ͑threshold ϳ133.8 eV) of an inner-subshell 2p electron. Resonance structure due to 2p excitation in the 2p 6 3s3p 3 P metastable state was also observed with its contribution to the total cross section not exceeding 3%. Calculation of the 2p photoionization continuum cross section as a function of photon energy was carried out using the relativistic random-phase approximation ͑RRPA͒ and agreed very well with the corresponding measurements. The resonance structure in the 3s cross section below the 2p threshold was found to be in good agreement with the multiconfiguration atomic structure calculations of Sayyad et al. ͓J. Phys. B 28, 1715 ͑1995͔͒, while the corresponding RRPA-RMQDT ͑relativistic multi-channel quantumdefect theory͒ calculations proved less successful.
The Absolute Cross Section forL‐Shell Photoionization of C+Ions from Threshold to 105 eV
The Astrophysical Journal Supplement Series, 2001
The absolute cross section for photoionization of C`ions has been measured from the ionization threshold at 24 eV to 105 eV by overlapping an ion beam with a monochromatized synchrotronradiation beam from the ASTRID undulator. The measurements, which are important for astrophysical modeling of, for example, stellar atmospheres, have been compared with R-matrix calculations from the Opacity Project and the Iron Project. The general agreement between theory and experiment is good, yet di †erences in the magnitude of the cross section of up to 50% are observed as well as some deviations concerning the resonance structure. Subject headings : atomic data È atomic processes
Photoionisation of Ca + ions in the valence- energy region 20–56eV: experiment and theory
Relative cross sections for the valence shell photoionisation (PI) of S 2 ground level and D 2 metastable Ca + ions were measured with high energy resolution by using the ion–photon merged-beams technique at the Advanced Light Source. Overview measurements were performed with a full width at half maximum bandpass of E 17 D = meV, covering the energy range 20–56eV. Details of the PI spectrum were investigated at energy resolutions reaching the level of E 3.3 D = meV. The photon energy scale was calibrated with an uncertainty of ±5meV. By comparison with previous absolute measurements the present experimental high-resolution data were normalized to an absolute cross-section scale and the fraction of metastable Ca + ions that were present in the parent ion beam was determined to be 18% ±4%. Large-scale R-matrix calculations using the Dirac Coulomb approximation and employing 594 levels in the close-coupling expansion were performed for the s p s Ca 3 3 4 S 2 6 2 1 2 + ()and s p d Ca 3 3 3 D 2 6 2 3 2,5 2 + () levels. The experimental data are compared with the results of these calculations and previous theoretical and experimental studies.
Absolute measurements of photoionization cross-sections for ions
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2005
A merged-beam setup for absolute measurements of photoionization cross-sections of ions is described. The facility is capable of recording cross-sections as low as 10 À19 cm 2 and has been used to study a large number of singly-and multiply-charged, atomic and molecular, positive and negative ions. It is based on a synchrotron radiation beam line fitted with an undulator at the storage ring ASTRID and a low-energy ($2 keV) ion beam line. Photons in the energy range 15-200 eV are merged co-linearly with the target ions over a distance of 50 cm, and the absolute photoionization cross-section is determined from the resulting photoion yield with a typical accuracy of 10%. Different types of ion sources are available, thus permitting a large number of positive and negative, atomic and molecular, singly-and multiply-charged ions to be investigated. Emphasis is put on accurate determination of the absolute cross-sections, requiring calibration of photodiode and particle detectors together with measurements of the photon-ion overlap.
Electron-impact ionization of Si^{6+} and Si^{7+} ions
Physical Review A, 1993
Crossed beams of electrons and ions have been used to measure absolute cross sections for single ionization of Si + and Si + ions from below threshold to 1400 eV. The measurements for Si and Si + agree with distorted-wave calculations for direct ionization within 20% and 5%%uo, respectively, suggesting that this mechanism is the dominant contribution to the cross section. The bulk of the measurements were performed with the more abundant Si isotope; however, due to beam impurities, measurements with Si were used to correct the 'Si data.
The Journal of Physical Chemistry A, 2000
The ionization potentials (IP) of Si 2 (X 3 Σ g -) to form the X 4 Σ gand a 2 Π u states of Si 2 + have been calculated at very high levels of ab initio molecular orbital theory (CCSD(T) with augmented correlation-consistent basis sets extrapolated to the complete basis set limit). The calculated value of the IP to form the X 4 Σ gground state of the ion is 7.913 eV as compared to an experimental value of 7.9206 eV. The a 2 Π u state is predicted to lie 0.52 eV above the X 4 Σ gground state of Si 2 + . The 1 3 ∆ u , 2 3 ∆ u , H 3 Σ u -, and K 3 Σ uexcited states of Si 2 , as well as the X 4 Σ g -, a 2 Π u , and 2 2 Π u states of Si 2 + , have been calculated at the multireference configuration interaction level. The agreement of the calculated positions of the states with the known experimental values is quite good (better than 0.1 eV). The calculated wave functions for the excited states of Si 2 show significant multireference character. This is especially true for the H state which has been used as an intermediate state in photoionization experiments. The multireference character of the H state readily allows the connection of this state to the ground X 4 Σ gelectronic state of Si 2 + via a one electron photoionization process.
Photoionization for the ground state of Al VII from threshold to the K shell
Physical Review A, 2001
The photoionization cross sections of N-like Al VII are calculated from the first ionization threshold to 140 Ry using the R-matrix method. 14 target states ͑including 4 K-shell excited states͒ of Al VIII are included in the close-coupling calculation. The detailed structure and resonances of the cross sections across the L-shell and K-shell thresholds is described. The resonance energies and widths of some of the autoionization states are determined by analyzing the resonance structures. The results show that the autoionization width of the K-shell excited state of a 1s electron being excited into 2p orbital is the largest among all of the autoionization states. It is also larger than Doppler and Stark widths under a typical plasma condition. This result shows that the autoionization widths will have essential influence in simulating the x-ray transmission spectrum of Al plasma.
L-photoionization of atomic aluminium: production of Al ii , Al iii and Al iv UV lines
Astronomy and Astrophysics, 2002
Aluminium atoms undergoing soft X-ray irradiation produce multiply-ionized ions in excited states through innershell photoionization followed by radiationless transitions, i.e. Auger cascades. Photoionization of 2s-, 2p-and 3s subshells are considered for the 80 eV to 1 keV incident photon energy range. Photoionization cross sections, shake probabilities and Auger and Coster-Kronig rates are evaluated in various approximations. Al, Al and Al UV lines are directly produced independently of local temperature and electron density. Monopole shake up/off and conjugate shake up excitations are important over a large energy interval beyond the L thresholds.
The Astrophysical Journal, 1996
We present a complete set of analytic fits to the non-relativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity Project theoretical cross sections interpolated and smoothed over resonances. At higher energies, the fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections.