Lighting up tiny movie sets with the world’s most intense X-rays


Credit: Marilyn Chung/Berkeley Lab; excerpt from SLAC/Flickr

The LCLS-II, an upgrade of the LCLS, the world’s first free-electron hard X-ray laser, is being built in California. Here, scientists work with part of the LCLS-II electron gun.

In 2009, the Linac’s Coherent Light Source turned on its laser to emit the most intense X-ray light the world had ever seen. By waving electrons between a 130-meter strip of magnets, the machine, which sits near the Stanford campus in California, produces X-rays in fleeting pulses, lasting every quadrillionth of a second. A single pulse can create light 100 times more intense than the light you would get if all the sunlight hitting the Earth were focused on one thumbnail.

The LCLS was the first of what are called X-ray free electron lasers, or XFELs. Other countries have since built XFELs of the same ilk: Japan in 2012, South Korea in 2016 and Germany in 2017. All, like LCLS, are miles long and cost around $1 billion to build.

When scientists gathered in Orlando at this year’s week-long meeting of the Division of Atomic, Molecular, and Optical Physics (DAMOP), hosted by the American Physical Society, research at XFEL had plenty of time. under the projectors.

With big lasers comes big ambitions: researchers are using XFELs to better understand single-molecule behavior and chemical reactions, which could shape fields ranging from physics to materials science and biology.

Because they can penetrate dense materials, these high-intensity X-rays can see inside and even alter the microscopic structure of objects that are opaque to optical light. For example, researchers have used XFEL pulses of light to create and study plasmas, with the goal of better understanding planets and stars.

The short wavelength of X-rays also allows high resolution imaging. The short pulses of X-rays work like an extremely fast camera shutter: they trigger chemical reactions, then take “snapshots” of electrons moving around molecules, creating what scientists call “molecular films”. Some researchers have used this technique to study photosynthesis at the atomic level.

Movies contain more than just visual information. Thorsten Weber of Lawrence Berkeley National Laboratory is studying reaction microscopy, a technique in “his teenage years,” Weber says. He uses the technique to “film” a film of a molecule falling apart while simultaneously measuring the angles and kinetic energies of the ejected particles. XFELs also make it possible to simultaneously study ions and electrons in a reaction, Weber explains. Before XFELs, scientists studied the behavior of electrons and the behavior of ions separately, because ions are more than a thousand times heavier than electrons.

In a presentation at the DAMOP meeting, Weber highlighted one of the challenges of using XFELs for molecular films: time. To make a movie, a researcher sends a pulse of X-rays to the molecule of interest, triggering a chemical reaction. Then, a second pulse illuminates the molecule for imaging. But current XFELs only pulse up to thousands of times per second. It may seem quick, but the researcher has to trigger the reaction millions of times, so it can take days to make a movie. With so many researchers around the world competing for time to use these machines, this pace is a challenge.

But what if the X-ray that triggers the chemical reaction and the X-ray that illuminates it could be fired in the same pulse? Weber presented a method to keep time in this case, to track when the movement takes place. The technique would reduce the time a laser researcher needs to make a movie.

Now Weber is working on combining X-ray light with an ultraviolet laser. In this setup, researchers would first shine low-energy UV light on a molecule before imaging it with X-rays. The initial UV illumination would more closely mimic how sunlight interacts with organisms, while X-rays would provide high imaging resolution.

Linda Young of Argonne National Laboratory presented work at DAMOP related to the study and control of X-ray pulses themselves. The XFEL produces a sharp, noisy spectrum that researchers need to measure before experiments. However, this measurement is difficult, as it usually requires the researcher to deflect the X-rays with strong beam splitters that do not tolerate high intensities well. In a recent study, his team developed a way to measure the spectrum with a neon gas beam splitter using a technique called ghost imaging.

Young’s team also used the XFEL facility in Germany to study interactions between X-rays and neon gas. When an X-ray pulse hits neon, it emits light, and this light in turn changes the spectrum of the X-ray pulse. This outgoing spectrum reveals information about the electronic structure of neon atoms. While neon has a simple structure, Young says these studies will help them study more complex molecules in the future. She also plans to study the effects of the interaction of X-rays and neon on the shape of the pulse over time.

As XFELs are just over a decade old, researchers like Weber and Young are still researching all the ways to use them, and they’ll soon have a new toy to look forward to. Construction of the LCLS-II, an upgrade to the LCLS, is expected to be completed by the end of the year. This new XFEL will be capable of producing up to one million pulses per second, compared to the 120 pulses per second of its predecessor.

For researchers, having more machines will make a big difference. “It gives us the opportunity to really systematically go in search of the understanding needed for our dream experiences,” says Young.

Sophia Chen is a writer based in Columbus, Ohio.


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