Jacqueline Orrell/SLAC
SLAC staff scientist Alexander Reid was the first user of the lab's electron camera for ultrafast electron diffraction (MeV-UED) since it became available to the international community as part of the LCLS facility. Here he handles an interchangeable card used to hold samples during UED experiments.

Electron Camera Captures Small, Fast Actions

July 15, 2019
It will be a world-class tool for ultrafast science and available to scientists all over the world.

It’s taken a few years, but engineers at DoE’s SLAC National Accelerator Laboratory have finally completed a new tool that will let them visualize physical and chemical processes with outstanding clarity: an ultra-high-speed “electron camera” capable of tracking atomic motions in a broad range of materials in real time. The lab has also made this tool available to researchers worldwide.
The tool is an instrument for ultrafast electron diffraction (MeV-UED). It uses a beam of highly energetic electrons to probe matter and is especially useful for understanding atomic processes occurring on timescales as short as about 100 femtoseconds (millionths of a billionth of a second). These rapid snapshots provide completely new insights into processes in nature and technology, benefitting applications in biology, chemistry, materials science and other fields.

The first proposal-driven experimental run of the MeV-UED instrument is scheduled through December of this year and will deliver those powerful electron beams to 16 user groups from over 30 institutions. Experiments will initially focus on materials science and hot, dense states of matter.

MeV-UED complements the lab’s suite of world-leading methods for studies of ultrafast science, including SLAC’s flagship X-ray laser, the Linac Coherent Light Source (LCLS). Using the whole breadth of these methods will let  scientists explore different, yet equally important, aspects of speedy processes.

This schematic shows how  SLAC’s new device or electron camera works. It will let researchers study motions that take place in less than 100 quadrillionths of a second. A pulsed electron beam is created by shining laser pulses on a metal photocathode. The beam is accelerated by a radiofrequency field and focused by a magnetic lens. Then it travels through the sample and scatters off the its atomic nuclei and electrons, creating a diffraction image on a detector. Changes in these diffraction images over time are used to reconstruct ultrafast motions of the sample’s interior structure. (Courtesy: Greg Stewart/SLAC)

“In response to a DOE workshop on the future of electron scattering and diffraction in February 2014, SLAC launched an ultrafast electron diffraction initiative with the goal to develop a world-leading instrument whose capabilities would complement those of LCLS,” says Xijie Wang, director of the MeV-UED instrument. “Making our cutting-edge technique available to the broad scientific community and supporting SLAC’s program in ultrafast science is an exciting milestone for us.”

The MeV-UED instrument has been incorporated into the LCLS facility, adding to the experimental stations that use X-rays.
Wang and his team have been perfecting the technology since the program’s start in 2014. Along the way, MeV-UED research has led to a considerable number of discoveries in materials for solar cells and data storage; provided unprecedented movies of molecules vibrating and breaking apart; looked at radiation damage in materials for nuclear fusion reactors; and uncovered exotic fluctuating material properties that could be used in molecular switches.

“Over the past four years, we have demonstrated that MeV-UED can lead to a paradigm shift in ultrafast electron diffraction, in part due to its versatility to probe a broad range of solid and gaseous samples,” Wang says. “The high energy of the electrons has transformed ultrafast electron diffraction from a qualitative science to a quantitative one, and our experiments are now employed to validate theoretical predictions and push new theoretical developments.”

This animation explains how researchers use high-energy electrons at SLAC to study faster-than-ever motions of atoms and molecules relevant to important material properties and chemical processes. (Courtesy: Greg Stewart/SLAC)

The team’s latest R&D is devoted to exploring science in liquid states, the natural environment for many biochemical processes, so scientists will soon be able to focus on some of the most gripping details of biology and chemistry.

The new instrument’s full potential becomes even clearer when combined with the lab’s X-ray laser. With LCLS, scientists can track molecular changes that occur extremely quickly, within just a few femtoseconds. With MeV-UED, they can create crisp images of molecules with unparalleled atomic resolution during these quick reactions. The extraordinary resolution in space and in time help develop a complete picture of speedy fundamental processes.

This is exemplified by two studies of a chemical reaction in which ring-shaped molecules break open in response to light, a process that plays an important role in making vitamin D in our bodies.
A few years ago, researchers made a molecular movie using LCLS, which provided the very first glimpses into the workings of the reaction. A more recent study, using MeV-UED, added additional high-resolution details.

“Together, LCLS and MeV-UED form a one-stop X-ray photon and electron factory with a symbiotic relationship, and they address the broad needs of our scientific community,” says LCLS scientist Mike Minitti.
Over the past years, while Wang’s team built their instrument from the ground up, some outside groups were invited to perform research projects with MeV-UED and the SLAC team.
Now, SLAC has opened access to the instrument to virtually everyone. Researchers can submit proposals for experiments, which are then evaluated by a committee of experts, ranked and, if successful, given time to conduct the experiment. That’s the same way LCLS and other X-ray light sources handle access to their instruments.

One of the first experiments will look at magnetic phenomena on the nanoscale in materials such as iron-platinum, a novel but complex material that is relevant for cloud-based data memory and could improve the efficiency and reliability of data storage. But before the material can be widely used, researchers first need to understand its fundamental magnetic behavior.
LCLS will give the researchers good measurements of how magnetism changes on fast timescales. Then UED will let them see the material’s atomic structure and how it reacts to changing magnetism. Putting these two measurements together provides a full picture of what the whole system is doing.

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