electron laser
Credit: Marilyn Chung/Berkeley Lab; retrieved from SLAC/Flickr

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

In 2009, the Linac Coherent Light Source turned on its laser to emit the most intense X-ray light the world had ever seen. By wiggling electrons between a 130-meter stretch of magnets, the machine, located near the Stanford, California campus, produces X-rays in fleeting pulses, each a trillionth of a second long. A single pulse can produce light 100 times more intense than the light you would get if all the sunlight hitting the earth were focused on a thumbnail.

LCLS was the first so-called X-ray free-electron laser or XFEL. Other countries have since built XFELs of a similar kind: in Japan in 2012, in South Korea in 2016 and in Germany in 2017. They all, like LCLS, are miles tall and cost around a billion dollars to build.

Research at the XFELs took the spotlight as scientists gathered in Orlando for this year’s week-long Division of Atomic, Molecular, and Optical Physics (DAMOP) meeting hosted by the American Physical Society.

With big lasers come big ambitions: researchers are using XFELs to better understand the behavior of single molecules and chemical reactions, which could shape fields from physics to materials science and biology.

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

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

The films contain more than just visual information. Thorsten Weber of the Lawrence Berkeley National Laboratory is studying reaction microscopy, a technique “in her teenage years,” says Weber. He uses the technique to ‘film’ a movie of a molecule breaking apart while simultaneously measuring the angles and kinetic energies of the ejected particles. With XFELs it is also possible to study ions and electrons simultaneously in a reaction, says Weber. Before XFELs, scientists studied electron behavior and ion behavior separately, since ions are over a thousand times heavier than electrons.

During a presentation at the DAMOP meeting, Weber outlined one of the challenges in using XFELs for molecular movies: time. To make a movie, a researcher fires an X-ray pulse at the molecule of interest, triggering a chemical reaction. Then a second pulse illuminates the molecule for imaging. But current XFELs only generate pulses up to a thousand times per second. This may sound quick, but the researcher has to trigger the reaction millions of times, so making a movie can take days. With so many researchers worldwide competing for time to use these machines, this pace is a challenge.

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

Weber is now working on combining the X-ray light with a UV laser. In this setup, the researchers would first shine lower-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 the X-rays would provide high image resolution.

Linda Young of Argonne National Laboratory presented work at DAMOP related to the study and control of the X-ray pulses themselves. The XFEL produces a spiky, noisy spectrum that researchers need to measure before experiments. However, this measurement is difficult because the researcher usually has to deflect the X-rays with fixed beamsplitters, which do not tolerate high intensities well. In a recent study, her team developed a way to measure the spectrum with a neon gas beamsplitter, using a technique called ghost imaging.

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

With XFELs being just over a decade old, researchers like Weber and Young are still figuring out 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 of the LCLS, is expected to be completed by the end of the year. This new XFEL will be able to generate up to a 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 ability to really search systematically for the understanding that we need for our dreaming experiments,” says Young.

Sophia Chen is an author living in Columbus, Ohio.

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