How to shoot ultrafast electrons

X-rays are used to probe the atomic-level structure of complex molecules, and methods for that tasks have been developed to more and more perfection, for instance at light sources like the PETRA III storage ring at DESY. But the images generated to date have been static: pictures resolve situations in real-space, but not real-time. Free-electron lasers, a new generation of X-ray sources providing unprecedented intensity in very short light pulses, promise to solve the problem in near future, opening up whole new world of exploring matter in action. One goal of the emerging field of time-resolved imaging (TRI) is to visualize electrons as they move in atoms, complex molecules  or polyatomic systems, such as occurs for instance during bond formation and breakage. But how does an ultrashort X-ray pulse interact with electrons in motion? How do we interpret a scattering picture of a dynamic system?

Visualization of an experiment with an ultrashort X-ray pulse revealing the evolution of electrons in motion.

To answer these questions a group of theorists at the Center for Free-Electron Laser Science (CFEL) at DESY have simulated the interaction of light and dynamic matter and found out that the obtained diffraction patterns substantially deviate from the common notion of an image of the instantaneous electron density being encoded in the scattering pattern, i.e. the question of where the electrons are located at a particular instant of time, and that they encode information on the electron motion directly. Today, the team has published the results of their work in the scientific journal PNAS (Proceedings of the National Academy of Science), USA.

For simulating the scattering of X-rays in a dynamic electron system, the researchers employed two distinct models of light. They found out that one of them, a quantum electrodynamics (QED) model which describes light in terms of photons, is essential for understanding electronic motion, whereas a classical model, which describes light in terms of a wave, incorrectly predicts that the diffraction pattern relates to the instantaneous positions of the electrons. Not only is this not the case, but the interpretation of the scattering pattern as encoding an instantaneous density would mislead the analysis of experimental data.In time resolved imaging, a very short laser pulse of only attoseconds(10^-18 sec) duration initiates the electronic motion in an atomic, molecular or polyatomic system. This motion can be described as an electronic wavepacket. The wavepacket can be probed at various time intervals with an ultrashort X-ray pulse, producing a distinctive scattering pattern that changes as the electrons move. Employing the theory based on a QED model of light, scattering patterns were obtained for atomic hydrogen that reflect the quantum motion of the electron. This approach distinguishes TRI from stationary X-ray imaging and represents a conceptual advance in TRI, which awaits technology capable of producing those ultrashort X-ray pulses in order to obtain the type of scattering patterns predicted by theory.

Scientific paper: "Imaging electronic quantum motion with light"; Gopal Dixit, Oriol Vendrell, Robin Santra, "Proceedings of the National Academy of Sciences" (PNAS); DOI: 10.1073/pnas.1202226109