Using a method known as ultrafast electron diffraction (UED), a scientific instrument at the Department of Energy’s SLAC National Accelerator Laboratory, located in Menlo Park, CA, reveals nature’s high-speed processes, including phase changes and the motions of electrons and atomic nuclei within molecules.

Renkai Li

The “electron camera” uses a pulsed electron beam to study motions in materials that take place in less than 100 quadrillionths of a second. The beam travels through a sample and creates diffraction images. Changes in the diffraction images are then used to reconstruct the ultrafast motions of the sample’s inner structure.

Imaging Technology spoke with SLAC National Accelerator Laboratory scientist Renkai Li about what makes the electron camera unique.

Imaging Technology: What does the electron camera look like?

Renkai Li: The machine’s beam line is five meters long. The electron beams travel in a pipe of a few-inches diameter. There is an electron source and a few chambers for beam diagnostics, the sample, and the detector. There is also the power source and an ultrafast laser.

Imaging Technology: How does the technology reveal motions of electrons and atomic nuclei?

Renkai Li: We send the electron beams through a very thin sample – nearly 100 nanometers for solids, or below one micrometer. Electrons scatter from the material and form a diffraction pattern. The diffraction patterns can be used to determine the structure of materials. Each electron pulse takes a very fast snapshot during the change of the material. We take many of these snapshots at different times, so we have a full history of the structure evolution. It reveals the positions of atoms and how fast they move.

Imaging Technology: Why are these observations so important?

Renkai Li: People are interested in the fast process, which is of fundamental importance on how things work at atomic length and time scales. This is only made possible by the most advanced instruments, including UED.

SLAC scientist Renkai Li makes adjustments to the ultrafast electron diffraction equipment in preparation for an experiment. The circular element, shown above, is the sample chamber. (Image Credit: SLAC National Accelerator Laboratory)

Imaging Technology: What kinds of ultrafast processes are we now able to see because of this type of technology?

Renkai Li: One of the classical examples is the melting of the structure. When you heat [a sample] with a laser, it will transfer from solid to liquid. It can melt. We want to understand how fast it will melt and how the structure changes when it is melting.

The other example: You can excite the atomic movement. You change from one configuration to the other, and you will see the shape of the lattice will change. That’s called a phase transition.

Imaging Technology: Does the camera complement the X-ray laser?

Renkai Li: X-rays and electrons interact with materials in different ways. X-rays mostly only interact with electrons in the material. The electron beam is sensitive to electrons and also ions. The other difference is the electron is very sensitive. We can look at very thin material and even a smaller layer or gas phase.

It’s more sensitive, and there is much less radiation damage. With X-rays, the deposited energy is so high; it can damage your sample, even a single shot. With the electron beam, you can look at a thin sample, and you can probe again and again without damage.

An electron gun (left), initially designed for SLAC’s X-ray laser Linac Coherent Light Source (LCLS), generates electrons that researchers send through materials (right) to study their structures and motions on the atomic level. (Image Credit: SLAC National Accelerator Laboratory)

X-ray free-electron laser facilities are also usually very large and expensive, at kilometer and billion-dollar scales. UEDs are very compact -- a few meters long and much cheaper. This UED will be much more affordable. One university can support such an instrument. If you have your own machine, you can do measurements more carefully.

Imaging Technology: What are your plans now with the device?

Renkai Li: Right now, we’re working in two directions. We provide a lot of time for users; people can bring their sample and basically measure how their sample evolved up to certain excitations. Then, they have a very nice science story to tell. The other part is we’re trying to improve the performance of the machine, in terms of spatial resolution and temporal resolution. We want to see smaller and faster changes in the structure.

Imaging Technology: Does that involve making the electron beam smaller?

Renkai Li: Exactly. We want to make it smaller in six dimensions. If we make the electron beam smaller, we can measure finer changes and faster processes.

Imaging Technology: What is the most innovative and new part of the design itself?

Renkai Li: The most unique part is the quality of the electron source. This source was developed for accelerators, for high-quality electron beams. Then, people realized they could also be used directly to measure materials. The electron source is a completely different source compared to electron guns used in an electron microscope, which allows us to have much shorter and brighter electron beams.

Imaging Technology: What do you think is most exciting about this kind of camera?

Renkai Li: I think it’s a unique position. We can see the merge of two communities: the ultrafast community and electron microscopy community. For people in the electron microscope community, in situ measurements are important. People are trying to measure the samples in their natural environment, and they want to see the changes of the structures, in liquid, for example -- a real interaction or reaction.

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This article first appeared in the December, 2015 issue of Imaging Technology Magazine.

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