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New Radiation Method Helps Record Ultrafast Motions of Electrons in Chemical Reactions

At the Department of Energy’s SLAC National Accelerator Laboratory, scientists have made a new, potential breakthrough for the laboratory’s high-speed “electron camera” that could enable them to “film” minuscule, ultrafast movements of electrons and protons in chemical reactions—reactions that have never been visualized to date.

SLAC’s Emma Snively and Mohamed Othman at the lab’s high-speed “electron camera,” an instrument for ultrafast electron diffraction (MeV-UED). Image Credit: Jacqueline Orrell/SLAC National Accelerator Laboratory.

It is believed that these “movies” may ultimately help researchers for drug development to fight disease, generate state-of-the-art materials with novel properties, formulate more efficient chemical processes, and much more.

Instead of using the standard radio-frequency radiation, the latest method leverages a form of light known as terahertz radiation to exploit the beams of electrons used by the instrument.

This not only allows scientists to control the speed at which the images are captured by the camera but also enables them to decrease a disturbing effect known as timing jitter. This effect prevents scientists from precisely capturing the timeline of how molecules or atoms change.

The technique may also result in tinier particle accelerators: Since the wavelengths of terahertz radiation are roughly a hundred times smaller compared to those of radio waves, instruments utilizing terahertz radiation are likely to be more compact. The scientists published the study results in the Physical Review Letters journal on February 4th, 2020.

A Speedy Camera

The ultrafast electron diffraction (MeV-UED) instrument, or “electron camera,” developed by SLAC utilizes high-energy beams of electrons that travel almost at the speed of light to capture an array of snapshots—fundamentally a movie—of action within and between molecules.

For instance, this has been utilized to capture a movie of how a ring-shaped molecule disintegrates upon exposure to light and to analyze atom-level procedures in melting tungsten that could potentially inform the designs of nuclear reactors.

This method works by shooting electron bunches at a target object and then recording the way the electrons scatter when they communicate with the target’s atoms. These bunches of electrons define the electron camera’s shutter speed. If the bunches are shorter, they can capture the motions more quickly in a vivid image.

It’s as if the target is frozen in time for a moment,” stated SLAC’s Emma Snively, who led the latest research.

Because of that reason, researchers prefer to make the entire electrons in a bunch to strike a target as close to concurrently as possible. To achieve this, the researchers give a little boost of energy to the electrons at the back, so that they catch up to the ones in the lead.

To date, scientists have utilized radio waves to transmit this energy; however, the latest method devised by the SLAC researchers at the MeV-UED facility employs light at terahertz frequencies instead.

Why Terahertz?

A major benefit of utilizing terahertz radiation lies in how the experiment shortens the beams of electron bunches. Researchers in the MeV-UED facility shoot a laser at a copper electrode to remove electrons and produce beams of electron bunches. Until recently, the team has been generally using radio waves to make these bunches of electrons shorter.

But the radio waves also increase the bunch of every electron to a  faintly different energy, and hence individual bunches differ in how rapidly they reach their target object. This timing difference is known as jitter, and it decreased the teams’ abilities to analyze rapid processes and precisely timestamp the way a target alters with time.

To get around this, the terahertz technique splits the laser beam into two. While one beam strikes the copper electrode and produces bunches of electrons as before, the other beam produces the terahertz radiation pulses for reducing these bunches of electrons. Since they were created by the same beam of a laser beam, terahertz pulses and electron bunches are currently synchronized with one another, decreasing the timing jitter between these bunches of electrons.

Down to the Femtosecond

According to the researchers, a major breakthrough for this study was the development of a particle accelerator cavity, known as the compressor. This meticulously machined hunk of metal is sufficiently compact to be placed in the palm of a hand. Terahertz pulses within the device reduce these bunches of electrons and give them an effective and targeted push.

Consequently, the researchers could compress these bunches of electrons, and hence they last only quadrillionths of a second, or a few tens of femtoseconds. That is not as much compression as currently achieved by traditional radio-frequency techniques, but according to the scientists, the potential to concurrently reduce the jitter renders the terahertz technique promising.

Moreover, the more compact compressors enabled by the terahertz technique would mean lower cost as opposed to the radio-frequency technology.

Typical radio-frequency compression schemes produce shorter bunches but very high jitter. If you produce a compressed bunch and also reduce the jitter, then you'll be able to catch very fast processes that we’ve never been able to observe before.

Mohamed Othman, Researcher, SLAC National Accelerator Laboratory

Ultimately, the aim is to compress the beams of electron bunches down to about a femtosecond, stated the scientists. This could subsequently allow researchers to view the remarkably fast timescales of atomic behavior in important chemical reactions such as individual protons transferring between atoms or hydrogen bonds breaking, for instance, that are yet to be fully understood.

At the same time that we are investigating the physics of how these electron beams interact with these intense terahertz waves, we're also really building a tool that other scientists can use immediately to explore materials and molecules in a way that wasn't possible before. I think that's one of the most rewarding aspects of this research.

Emilio Nanni, Researcher, SLAC National Accelerator Laboratory

Nanni headed the project another SLAC researcher, with Renkai Li.

The study was financed by the DOE’s Office of Science. The MeV-UED instrument is part of SLAC’s Linac Coherent Light Source—a DOE Office of Science user facility.


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