Speaker
Description
Using the photon-ion merged-beams technique at a synchrotron light source, we have measured relative cross sections for double and up to tenfold photoionization of La$^+$ ions in the energy range 820–1400 eV where resonances and thresholds occur that are associated with the excitation or ionization
of one M-shell electron. The normalization of the sum of the measured partial cross sections to the absolute photoabsorption cross section of La allows the transformation of the relative cross sections into absolute cross sections.
In 2017, the LIGO/Virgo collaboration detected the first gravitational-wave signal from the merger of a neutronstar
binary. Less than two seconds later, a network of telescopes detected a short gamma-ray burst, followed
by a longer optical “afterglow” powered by the radioactive decay of the neutron-rich material ejected in the
merger, i.e., a kilonova. The kilonova light-curves potentially reveal the abundances of the heavy chemical
elements that are produced in the preceding violent neutron-star merger events [1,2].
In order to be able to reliably infer elemental abundances from the astronomical observations, absolute cross sections are required
for the basic atomic processes that occur in the afterglow. So far, local thermodynamic equilibrium (LTE)
conditions have been assumed in the astrophysical modelling of kilonovae [1,2], which certainly is an
oversimplification given the highly dynamic and transient nature of the phenomenon. Any future more accurate
non-equilibrium modelling will have to rely on accurate atomic cross sections, which generally cannot be easily
calculated (if at all) to a sufficient precision for the heavy many-electron ions of interest.
The present experiments were carried out at the photon-ion merged-beams setup PIPE [3], a permanently installed end-station of the PETRA\,III photon beamline P04, using already previously established experimental procedures (see e.g. [4]). A beam of La$^+$-ions was extracted from a Penning ionization source and merged with the P04 photon beam. Relative yields of multiply charged La$^{q+}$ reaction products with $3 \leq q \leq 11$ were recorded as functions of photon energy over the energy range 800-1400 eV.
References
[1] Kasen D et al. 2017 Nature 551 80
[2] Watson D et al. 2019 Nature 574 497
[3] Schippers S and M¨uller A 2020 Atoms 8 45
[4] Schippers S et al. 2017 Astrophys. J. 849 5
