A New Optical Fiber Electron Gun

Ultrafast streak diffraction using optical fiber-driven low-energy electron gun. (Source: C. Lee)

One of the most enduring ”Holy Grail” experi­ments in science has been attempts to directly observe atomic motions during structural changes. This prospect underpins the entire field of chemistry because a chemical process occurs during a transition state – the point of no return separating the reactant confi­guration from the product confi­guration.

What does that transition state look like and, given the enormous number of different possible nuclear confi­gurations, how does a system even find a way to make it happen? Now, researchers at the Max Planck Institute for the Structure and Dynamics of Matter are reporting ultra­bright electron sources with suffi­cient brightness to literally light up atomic motions in real time at a time scale of 100 femto­seconds, making these sources parti­cularly relevant to chemistry because atomic motions occur in that window of time.

After seeing the first atomic movies of phase transi­tions in bulk thin films using high-energy electron bunches, the researchers wondered if they could achieve atomic resolution of surface reactions – occurring within the first few monolayers of materials – to gain a better under­standing of surface catalysis. So they devised a low-energy time-resolved electron diffraction concept of using fiber optics for miniaturi­zation and the ability to stretch the electron pulse, then apply streak camera tech­nology to poten­tially obtain subpico­second temporal reso­lution.

“The first atomic movies use a strobo­scopic approach akin to an old 8-milli­meter camera, frame by frame, in which a laser exci­tation pulse triggers the structure, then an electron pulse is used to light up the atomic positions,” said Dwayne Miller. “We believed that a streak camera could get a whole movie in one shot within the window defined by the delibe­rately stretched electron pulse. It solves the problem of low electron numbers and greatly improves image quality.”

Of the myriad possible nuclear confi­gurations, the group discovered that the system collapses to just a few key modes that direct chemistry and that a reduction in dimen­sionality that occurs in the tran­sition state or barrier-crossing region can be inferred. “We see it directly with the first atomic movies of ring closing, electron transfer and bond breaking,” said Miller. (Source: AIP)

Reference: C. Lee et al.: Optical fiber-driven low energy electron gun for ultrafast streak diffraction, Appl. Phys. Lett. 113, 133502 (2018); DOI: 10.1063/1.5039737

Link: Atomically Resolved Dynamics (R. J. D. Miller), Max Planck Institute for the Structure and Dynamics of Matters, Hamburg, Germany

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