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January 6, 2015

Catching the fleeting motions of electrons

The trHAXPES instrument installed in its hutch at SACLA

Nearly everything that takes place in our material world—the interactions between atoms which lead to the formation of molecules and solids, their electrical and magnetic properties, their chemical reactions and vibrations, the generation of light—are all governed by the movements of electrons within materials. In order to understand electronic properties, it is necessary to know what energy shells the electrons occupy and how fast they move as they undergo changes. Thus, taking ultra-fast snapshots of electrons as they actually operate in materials and devices can provide us with a deeper knowledge of how the world around us works.

Now, in a paper recently published in the New Journal of Physics, a Japanese-German research collaboration led by the RIKEN SPring-8 Center and the University of Kiel in Germany has developed a new technique, called time-resolved hard X-ray photoelectron spectroscopy (trHAXPES), which promises to open new opportunities for studying ultrafast electron dynamics in complex materials, at buried interfaces, and in electronic devices in actual operation. Essentially, extremely short (femtosecond) pulses of optical light from a table-top laser are pumped into a material to excite the electrons. The excited electrons are then probed at extremely short time scales using hard X-ray light from the SACLA laser, a powerful X-ray free electron laser that went online in 2011. The released photoelectrons, measured in rapid succession, provide a stroboscopic movie of how the electrons relax back into equilibrium.

This method has been successfully used in the ultraviolet and extreme ultraviolet range before, but now it can be used with hard X-rays, which have greater penetrating power, thus making it possible to see deeper into materials.