Photoexcitation and photoionization spectroscopy of very highly charged ions
Final Report Abstract
Within the present project interactions of photons with highly charged ions, particularly of the astrophysically most relevant heavy element, iron, has been investigated. Photoexcitation with subsequent stabilization of the intermediate, core-excited states by emission of photons and electrons was studied using third- and fourth-generation light sources. The extremely narrow bandpass widths of xray beams available at beamline P01 of the PETRA III synchrotron radiation facility together with very careful calibration of the wavelength measurements using the available crystal spectrometer and reference samples of absorbers that had been calibrated by the German Physikalisch-Technische Bundesanstalt it was possible to perform extremely accurate measurements of the transition energies for the most prominent lines in highly charged iron ions up to charge state Fe24+ produced and trapped in FLASH-EBIT. In separate experiments with helium-like Kr34+ a precison measurement of the 1 S0 − 1 P1 and 1 S0 − 3 P1 transition energies yielded 13 114.47(14) eV and 13 026.15(14) eV, respectively. Similarly accurate line energies were measured for these same transitions in helium-like Fe24+. With an uncertainty of only 11 ppm, these numbers confirm the present state-of-the-art theory values and thus contradict a recent speculation that experiment and quantum-electrodynamics calculations for these lines in He-like ions diverge with increasing atomic number. By precision spectroscopy of fluorescence radiation and photo-ion production it was possible to infer absolute cross sections for total photoabsorption and photoionization of iron ions Fe20+ , Fe21+ , Fe22+ , and Fe23+ at energies around some of the most prominent transitions in the x-ray energy regime. Besides the resonance energies, natural widths and partial decay rates, for radiative and Auger decays, as well a oscillator strengths could be determined. At the Linear Coherent Light Source (LCLS) in Stanford neon-like Fe16+ ions stored in FLASH- EBIT were bombarded with x-ray laser pulses at photon energies between 809 and 829 eV and the fluorescence radiation was monitored as a function of time. In the investigated energy range characteristic lines of Fe16+ and Fe15+ can be excited. The line intensity ratio of two lines in Fe16+ , termed 3D and 3C, is used for characterizing astrophysical environments. However, there have been discrepancies in the interpretation of the line ratio and hence, ambiguities in the parameters derived from astrophysical observations. Incomplete modeling or the atomic physics basis could be reasons for the existing problems. By the experiment at the LCLS a new ratio of the oscillator strengths of the lines 3C and 3D was determined and turned out to be below all the existing theoretical predictions. The new measurement has stirred up a flurry of new theoretical approaches. The publication of this experiment as a Letter to Nature appeared at the end of 2012. ISI Web of Science lists already 50 citations four and a half years later. The present project has produced first class experimental results on spectroscopic properties of very highly charged ions of iron and krypton. The accuracy of wavelength measurements provides a critical test of state-of-the-art atomic structure theory. Different from other recent spectroscopic measurements no discrepancy of theory and experiment has been found in the present work. EBIT has proven again to be a very versatile tool for investigating atomic properties and collision dynamics of highly charged ions relevant in astrophysics.
Publications
- An unexpectedly low oscillator strength as the origin of the Fe XVII emission problem. Nature 492 (2012) 225
S. Bernitt, G. V. Brown, J. K. Rudolph, R. Steinbrügge, A. Graf, M. Leutenegger, S. W. Epp, S. Eberle, K. Kubiček, V. Mäckel, M. C. Simon, E. Träbert, E. W. Magee, C. Beilmann, N. Hell, S. Schippers, A. Müller, S. M. Kahn, A. Surzhykov, Z. Harman, C. H. Keitel, J. Clementson, F. S. Porter, W. Schlotter, J. J. Turner, J. Ullrich, P. Beiersdorfer, and J. R. Crespo López-Urrutia
(See online at https://doi.org/10.1038/nature11627) - X-ray resonant photoexcitation: Linewidths and energies of Kα transitions in highly charged Fe ions. Phys. Rev. Lett.111 (2013) 103002
J. K. Rudolph, S. Bernitt, S. W. Epp, R. Steinbrügge, C. Beilmann, G. V. Brown, S. Eberle, A. Graf, Z. Harman, N. Hell, M. Leutenegger, A. Müller, K. Schlage, H.-C. Wille, H. Yavaş, J. Ullrich, and J. R. Crespo López-Urrutia
(See online at https://doi.org/10.1103/PhysRevLett.111.103002) - Absolute measurement of radiative and Auger rates of K-shell-vacancy states in highly charged Fe ions. Phys. Rev. A 91 (2015) 032502
R. Steinbrügge, S. Bernitt, S. W. Epp, J. K. Rudolph, C. Beilmann, H. Bekker, S. Eberle, A. Müller, O. O. Versolato, H.-C. Wille, H. Yavaş J. Ullrich, and J. R. Crespo López-Urrutia
(See online at https://doi.org/10.1103/PhysRevA.91.032502) - Precision studies of deep-inner-shell photoabsorption by atomic ions. Phys. Scr. 90 (2015) 054004
A. Müller
(See online at https://doi.org/10.1088/0031-8949/90/5/054004) - Single-photon excitation of Kα in heliumlike Kr34+ : Results supporting quantum electrodynamics predictions. Phys. Rev. A 92 (2015) 020502(R)
S. W. Epp, R. Steinbrügge, S. Bernitt, J. K. Rudolph, C. Beilmann, H. Bekker, A. Müller, O. O. Versolato, H.-C. Wille, H. Yavaş, J. Ullrich, and J. R. Crespo López-Urrutia
(See online at https://dx.doi.org/10.1103/PhysRevA.92.020502)
