High-speed electron gun based on optical fibers to reveal atomic movements

WASHINGTON, D.C., October 9, 2018 – One of the most enduring scientific experiments of the "Holy Grail" has been the attempt to directly observe atomic movements during structural changes. This perspective underlies the entire field of chemistry because a chemical process occurs during a transition state – the point of no return separating the reagent configuration from the product configuration.

What does this transition state look like and, given the large number of possible nuclear configurations, how does a system find a way to achieve it?

Now in the newspaper Applied Physics Letters, from AIP Publishing, researchers at the Max Planck Institute for Structure and Matter Dynamics are reporting "ultra-bright" electron sources with sufficient brightness to literally illuminate atomic movements in real time – at a time scale of 100 femtoseconds – making these sources particularly relevant for chemistry because the atomic movements occur in this window of time.

After viewing the first atomic films of phase transitions in thin layers in bulk using high energy electron bunches (100 kilovolts), the researchers wondered whether they could obtain an atomic resolution of the reactions. surface – occurring in the first monolayers of materials – to gain a better understanding of surface catalysis.

They have therefore developed a time-resolved electronic diffraction concept using optical fiber for miniaturization and the possibility of stretching the electronic pulse, then using scanning camera technology to obtain a resolution. temporal subpicosecond, a difficult feat in the energy regime with weak electrons.

"The first atomic films use a stroboscopic approach similar to an old camera 8 millimeters, frame by frame, in which a laser excitation pulse triggers the structure, and then an electronic pulse is used to illuminate the atomic positions," said the co-author. Dwayne Miller. "We thought that a continuous scanning camera could record a whole movie at once in the window defined by the electron pulse voluntarily stretched in. It solves the problem of low numbers of pixels. electrons and greatly improves the quality of the image. "

Among the myriad of possible nuclear configurations, the group discovered that the system was reduced to a few key modes that drive chemistry and that it was possible to infer a reduction in dimensionality in the state of transition or barrier crossing area. "We see it directly with the first atomic films on cycle closure, electron transfer and link breakup," Miller said.