A remarkable consequence of quantum mechanics is that electrons can display interference effects. This interference is similar to waves interacting in the ocean or the electromagnetic waves that carry radio signals. Scientists were able to observe the quantum mechanical motion of electrons in an excited molecule thanks to a device called an "attoclock." This device measures the motion of electrons with a precision of hundreds of attoseconds (1 billionth of 1 billionth of a second!). This measurement provides insights on how the coherent oscillation of charges inside a molecule displays interference effects at attosecond time scales.
The motion of electrons happens on such fast time scales that their measurement can only be performed with extremely short flashes of light (typically shorter than one femtosecond). Until now, sub-femtosecond measurements were only possible using extreme ultraviolet sources produced by laser systems. Researchers need a source that extends these short pulses to the X-ray domain in order to enable measurements that can distinguish electron motion between different atoms in a molecule. This new experimental method will enable the study of electron dynamics in complicated molecules. This will advance our understanding of molecular physics and quantum chemistry.
The recent development of attosecond X-ray free-electron lasers has opened new avenues for ultrafast science. In this experiment, researchers used the ultrafast X-ray pulses from the Linac Coherent Light Source, a Department of Energy (DOE) user facility at SLAC National Accelerator Laboratory, to create a coherent superposition of excited states in nitric oxide. These excited states are short-lived and can decay through the Auger-Meitner process, where the excitation energy is released by ejecting a fast electron.
The researchers measured the Auger-Meitner decay process in the time-domain using an attoclock, a device that is capable of measuring the arrival time of electrons with attosecond precision. The researchers observed that the time-dependence of the decay is not a simple exponential function, but it contains ultrafast oscillations. These oscillations are a signature of coherent electron dynamics, specifically the quantum beat between two coherently excited quantum states. This represents the first atomic site-specific observation of coherent electron motion in a molecule, and the first time-domain experiment with attosecond resolution using an X-ray free-electron laser.
This research was supported by the Department of Energy Office of Science, Basic Energy Science and Chemical Sciences, Geosciences, and Biosciences. Other funding sources included the Laboratory Directed Research and Development Program of SLAC National Accelerator Laboratory; the German Research Foundation, the Federal Ministry of Education and Research, and the Max Planck Society, Germany; the Engineering and Physical Sciences Research Council of United Kingdom; the Swiss National Science Foundation and National Center of Competence in Research–Molecular Ultrafast Science and Technology; and the U.S. National Science Foundation. This research used resources of the Linac Coherent Light Source, a DOE Office of Science user facility.