Extreme UV light steers electrons in a lab experiment

Shine light on an atom and its electrons respond. At the FERMI free-electron laser in Trieste, Italy, a team led by Ulli Eichmann of the Max Born Institute in Berlin has shown that it can steer that response with extreme ultraviolet light (XUV). By changing the phase of the pulses, the researchers decided whether two quantum routes inside a helium atom reinforced each other or cancelled out. The work is published in Nature Photonics.

Quantum mechanics allows a process to happen along several paths at once. An electron can reach the same final state by different routes, and those routes either add up or cancel, much like overlapping ripples on the surface of water. The researchers created two competing routes to excite a bound state in helium, using a method called stimulated Raman excitation with XUV pulses. Matthieu Génévriez, a researcher at UCLouvain and one of the authors, describes the effect in plain terms: “it's a little bit as if we could eject an electron from an atom and then bring it back home with light.”

Visible and infrared lasers have been the workhorses of this kind of probing for decades. Stimulated Raman scattering and related techniques already follow chemical reactions and electronic motion with those wavelengths. Extending them into the XUV, a region the team calls a long-standing challenge, required a light source few facilities can provide. Eichmann says the experiment shows the methods “can now be brought into the extreme ultraviolet,” opening “new possibilities for imaging and manipulating matter with unprecedented spatial resolution.”

For chemists and materials scientists, the appeal lies in the element-specific nature of the light. XUV and X-ray photons reach electrons close to atomic nuclei, where each chemical element carries its own electronic fingerprint. The team believes the concepts shown here could eventually lead to a new generation of imaging and spectroscopy tools, able to track electronic motion and chemical transformations at the smallest scales. It has not published a timeline for such instruments, and the experiment is a laboratory demonstration carried out on helium alone.