High Precision K-Shell Photoabsorption Cross Sections for Atomic Oxygen: Experiment and Theory

Abstract

Photoabsorption of atomic oxygen in the energy region below the 1s −1 threshold in x-ray spectroscopy from Chandra and XMM-Newton is observed in a variety of x-ray binary spectra. Photoabsorption cross sections determined from an R-matrix method with pseudo-states (RMPS) and new, high precision measurements from the Advanced Light Source (ALS) are presented. High-resolution spectroscopy with E/∆E ≈ 4,250 ± 400 was obtained for photon energies from 520 eV to 555 eV at an energy resolution of 124 ± 12 meV FWHM. K-shell photoabsorption cross-section measurements were made with a re-analysis of previous experimental data on atomic oxygen at the ALS. Natural linewidths Γ are extracted for the 1s −1 2s 2 2p 4 ( 4 P)np 3 P • and 1s −1 2s 2 2p 4 ( 2 P)np 3 P • Rydberg resonances series and compared with theoretical predictions. Accurate cross sections and linewidths are obtained for applications in x-ray astronomy. Excellent agreement between theory and the ALS measurements is shown which will have profound implications for the modelling of x-ray spectra and spectral diagnostics.

Prominent role of multielectron processes inK-shell double and triple photodetachment of oxygen anions

“…Indeed, the sum of our cross sections for double and triple detachment of O − at 535 eV amounts to 0.43 ± 0.07 Mb and agrees well with the cross section for nonresonant K-shell absorption in neutral oxygen of 0.5 Mb [20]. Similar values were measured for photoabsorption of O + and O 2+ [21].…”

“…Two K-shell ionization thresholds were identified and the corresponding threshold energies were determined. At higher energies, the sum of the double and triple detachment cross sections agrees with the measured absorption cross sections for neutral oxygen [20] as well as for O + and O 2+ [21]. This suggests that the production of neutral oxygen by nonresonant K-shell detachment of O − plays only a minor role.…”

K-shell photoionization ofO+andO2+ions: Experiment and theory

“…They noted for O and O + a displacement of the 1s → 2p calculated lines with respect to the Chandra observations. The K-shell photoionization cross section of the neutral oxygen atom measured by Stolte et al [25,26] at the Advanced Light Source (ALS) gives a position for this line almost 0.8 eV lower than the energy deduced from Chandra observations [24], a difference distinctly outside the quoted uncertainties of both measurements. Similar disagreement was found between our previous measurements at SOLEIL and the Chandra and XMM Newton observations for the O 3+ ion [9].…”

“…Marked discrepancies were recently noted between the energies measured by synchrotron radiation based experiments for neutral oxygen [26] and the O 3+ ion [9] and x-ray satellite observations [5,24]. The energies of the 1s → 2p transitions for the first members of the oxygen isonuclear series as determined by Chandra observations and SR experiments are summarized in Table VI.…”

“…The photon beam is the monochromatized synchrotron radiation from the PLEIADES beam line [25,26]. In the photon energy range considered here, a permanent magnet Apple II undulator with 80-mm period is used.…”