A research team led by Frank Stienkemeier and Lukas Bruder from the Institute of Physics at the University of Freiburg observed real-time ultrafast quantum interferences, or oscillation patterns, of electrons in the atomic shells of rare gas atoms. By exciting rare gas atoms with specially prepared laser pulses and then tracking the atoms' response with a new measurement technique, the scientists were able to observe oscillations occurring within a period of about 150 attoseconds (as). Various chemical reactions, such as the breaking of bonds in molecules, are triggered by the absorption of light. In the first moment after the absorption, the distribution of the electrons in the atomic shell changes, significantly influencing the subsequent course of the reaction. Because spectroscopic technologies using visible laser pulses are not fast enough to track such processes, researchers are currently developing laser sources and spectroscopic technologies in the ultraviolet and x-ray ranges. Stienkemeier’s team has extended coherent pump-probe spectroscopy from the visible into the ultraviolet range — the spectral range between x-ray radiation and ultraviolet light. To do this, the scientists prepared a sequence of two ultrashort laser pulses in the extreme ultraviolet range at the FERMI free electron laser (FEL) in Trieste, Italy. The pulses were separated by a precisely defined time interval, with a predetermined phase relationship to one another. The first pulse, known as the pump-process, starts in the electron shell. The second pulse, the probe-process, probes the status of the electron shell at a later point. By altering the time interval and the phase relationship, the researchers could reach conclusions on the temporal development in the electron shell. “The greatest challenge was to achieve precise control over the pulse properties and to isolate the weak signals,” said Andreas Wituschek, the researcher in charge of the experimental procedure. The Freiburg physicists studied the rare gas argon, among others. In argon, the pump-pulse causes a special configuration of two electrons within the atomic shell. This configuration disintegrates, with one electron leaving the atom within a very short time and the atom itself remaining behind as an ion. For the first time, the researchers were now able to observe in real time the decay of the quantum interference as the electron left the atom. “This experiment paves the way for many new applications in the study of atomic and molecular processes after selective stimulation with high-energy radiation in the extreme ultraviolet range,” said Bruder. The research was presented in Nature Communications (www.doi.10.1038/s41467-020-14721-2).