World's most powerful X-ray laser makes krypton absorb 100x more light

Researchers at the European XFEL have achieved a significant breakthrough in quantum-level light-matter interaction by increasing photoionization efficiency in highly ionized krypton by 100 times. Using the world’s most powerful X-ray laser, the team successfully created and studied a rare double core-hole state in krypton ions (Kr26+), where two inner electrons are simultaneously excited. This state, which exists only for a few femtoseconds, allows a second photon to be absorbed before the first vacancy is filled, enabling a doubly resonant excitation pathway that greatly enhances photon absorption efficiency compared to simple photoionization. The experiment leveraged relativistic effects that cause the core electron energy levels to align closely, permitting both electron excitations to be induced by a single-color X-ray beam with a narrow bandwidth. This precise control allowed the researchers to isolate and track the process clearly by working with highly charged krypton stripped down to 10 electrons. The findings deepen understanding of atomic behavior under extreme conditions and open new avenues