XFEL: A 100-fold efficiency boost for photo ionisation

Speed matters. When an X-ray photon excites an atom or ion, making a core electron jump onto a higher energy level, a short-lived window of opportunity opens. For just a few femtoseconds, before an electron fills the void in the lower energy level, a second photon has the chance to be absorbed by another core electron, creating a doubly excited state. Using 5,000 intense X-ray flashes per second, generated by the European XFEL, an international team of scientists has investigated such double core-hole states in highly ionised Krypton, using photons that all had nearly the same energy or colour.

For their experiments, scientists from European XFEL and the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg as well as six other institutions across Germany, Italy, Portugal, and the United States, used highly charged krypton, Kr²⁶⁺, lacking all but ten of its electrons. “This allowed us to get a clear picture of the process, eliminating unwanted contribution from other electrons”, says Thomas Baumann, Operations Leader at the SQS Scientific Instrument, where the work was carried out. The use of these krypton ions was advantageous, because for both steps in the resonant excitation of two core electrons, the required photon energy is very similar. Thus, a single-colour X-ray beam with a bandwidth of just 0.5% could be used.“This overlap in resonance energy is rooted in relativistic effects”, explains Moto Togawa, first-author of the publication in Physical Review Letters. “They result in the shift of the core electron energy levels, which allows for a perfect double resonance.” After few femtoseconds, the double excited state decays by electron emission, generating a higher charge state. Compared to simple photoionisation, the doubly resonant pathway leads to a more than 100x increased absorption of photons– greatly boosting efficiency.

According to José Crespo López-Urrutia, group leader at the MPIK, “this effect not only deepens our understanding of how light and matter interact under extreme conditions, but also opens new possibilities for high-precision X-ray measurements.” The scientists believe that their results are applicable as efficient excitation scheme to future time-resolved experiments, using the two-color mode of the SASE3 undulator at European XFEL. Furthermore, an extension to experiments with hard X-rays making use of XFELOs (X-ray Free-Electron Laser Oscillators) as developed at European XFEL could be conceived.

Original Publication: Enhanced One-Color-Two-Photon Resonant Ionization in Highly Charged Ions by Fine-Structure Effects