09/26/2023
Charting the quantum landscape of hollow atoms
Researchers have identified exotic quantum states of heavy atoms missing up to six electrons in their core electron shells. The breakthrough was made possible by the unique capability offered by the European XFEL of scanning the X-ray energy over a wide range and by the application of a powerful computer program for predicting X-ray-driven quantum behaviour.
The results promise new insights into extreme light-matter interactions, which could help to improve the imaging of molecules or resolve unexplained observations in astrophysics.
When an atom is hit by a particle of light, or photon, of sufficient energy, it can be ionized - i.e. an electron is knocked out of the atom, leaving behind a positively charged ion with a hole in its electron shell. The X-ray pulses generated by the European XFEL are so intense - up to 1013 photons per pulse - and have such short durations - a few tens of femtoseconds - that the atoms or molecules they hit can absorb more than one photon at the same time, in what is called "multiphoton interaction".
In this way, it should even be possible to create atoms with multiple holes in their core electron shells. In these innermost layers of the electron shell, the electrons are very tightly bound to the atomic nucleus and, unlike the valence electrons further out, they don't participate in chemical bonding. Since holes in an inner shell are filled again very quickly, it is extremely difficult and requires extreme conditions to release several core electrons at the same time.
Understanding these extreme light-matter interactions on the level of individual atoms enables scientists to gain fundamental insights into the structure and dynamics of matter. For example, it can help to further develop new experimental X-ray free-electron laser techniques, allowing researchers to image individual biological molecules or to watch and control atomic motions in chemical reactions.
In a joint theoretical and experimental study carried out at the SQS instrument of the European XFEL, a team of scientists has now investigated the dependence of multiphoton light-matter interactions on the energy of the X-ray photons for the first time. Using a technique called resonant ion spectroscopy, the team mapped out the short-lived electronic structures that occurred during the complex charge-up processes induced by illuminating isolated heavy atoms with the intense X-ray pulses.
"Thanks to its special magnet structures used to generate the X-rays, which are adjustable with extremely high precision, the European XFEL allowed us to perform continuous scans of the photon energy while retaining a very high and stable photon flux at the sample," explains Rebecca Boll from European XFEL, the principal investigator of the experiment. "This made it possible to investigate the photon energy dependence of X-ray multiphoton absorption for the first time over a wide range of energies."
The resulting resonance spectra contained a multitude of quantum structures in the "landscape" formed by the photon energy and the charge state. They could be assigned to electronic transitions in corresponding ionic states with the help of state-of-the-art theoretical calculations. "The charge states show how many electrons the atoms have lost after absorbing many X-ray photons," says Sang-Kil Son from DESY, who led the theoretical part of the work. "By comparing the predictions from our calculations with the experimental data, we gain insight into the nature of the accessed quantum states."
At specific photon energies and charge states, the spectra were found to be determined by massively hollow atoms featuring as many as six simultaneous holes in their core electron shells. "To our knowledge, such multiple-core-hole states have only been discussed in theory so far and never been traced experimentally," Rebecca Boll adds.
The study demonstrates that highly charged ions in exotic quantum states can be created and probed simultaneously with intense X-ray pulses, thus opening up new avenues for X-ray-based techniques. For example, the unusual atomic species discovered in the study could also be formed through collisions in outer space, making them potential candidates to explain unidentified X-ray emission lines in astrophysics.
The experiment was led by a team from European XFEL and the Center for Free-Electron Laser Science (CFEL) at DESY and included researchers from Universität Hamburg, DESY, and Kansas State University. The results were published in Nature Communications.
Source: European XFEL GmbH