Attosecond X-ray Pulses Reveal Dancing Electrons

Look inside molecules with the world’s most powerful free electron laser

3 min read

black circles with blue and green light beams going through

A highly energetic electron beam travels through superconducting cavities to create billions of X-ray flashes per second.

SLAC National Accelerator Laboratory

An attosecond is a billionth of a billionth of a second. There are more attoseconds in a second than there are seconds in the current age of the universe.

But it is now possible to produce X-ray pulses whose time scales can be measured in attoseconds. Pulses this short give scientists the ability to take snapshots of subatomic particles in the wild.

Earlier this year, scientists made headlines by using this method to image electrons moving in liquid water. As the technology continues to advance, scientists may be able to use it for watching electrons move in many other kinds of molecules—boosting chemistry, biology, and any other scientific fields that study how molecules behave.

The tool of choice is an X-ray free electron laser (XFEL) such as the recently upgraded Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory in California, which violently undulates electrons to make them emit intense X-ray beams. Pack these beams into short enough pulses, and you have your imaging medium.

Pulses this short give scientists the ability to take snapshots of subatomic particles in the wild.

To use an FEL for electron watching, scientists typically use a technique called the pump-probe method. Scientists first “pump” a target by exciting it with one pulse, then “probe” it with a second pulse that allows scientists to observe the target. If the second pulse arrives within a short enough duration, scientists can watch subatomic particles in their quantum glory with plenty of time to spare before the X-rays can damage the target.

Over the past decade, laser scientists have steadily achieved shorter and shorter pulses. In a recently published paper, LCLS’s experimenters reduced the time to just 270 attoseconds. “That was just a nice, simple experiment that could benchmark our pump-probe capability,” says Agostino Marinelli, a particle accelerator physicist at SLAC.

“As we push to the attosecond, what we’re really trying to see is how electrons are moving around inside of molecular systems,” said James Cryan, a senior scientist at SLAC. “How do the electrons move? How do they interact with each other?”

LCLS is not the only FEL in the world—there are also the FERMI in Italy and the European XFEL in Germany, both of which are hosting attosecond pulse experiments too—but it has the highest capabilities of its kind. At LCLS, that pump-probe technique was what Marinelli, Cryan, and their colleagues used on attosecond time scales in order to image electrons moving through liquid water.

aerial view of large building with roads and trees aroundThe upgraded LCLS includes about 1 kilometer of superconducting accelerator cavities and a cryoplant that cools the accelerator to just a few kelvins.SLAC National Accelerator Laboratory

More than just a basic science stunt, that experiment helped researchers study what actually happens when ionizing radiation travels through liquid water. In fact, the experiment was part of a U.S. Department of Energy program that aims to better understand the behavior of nuclear waste stored in water. Next, SLAC scientists want to dissolve other substances in water and observe how that alters electron behavior.

Other kinds of molecules are ripe fodder for FEL probing. Take benzene: a fairly simple molecule based on six carbon atoms arranged in a hexagonal ring. Scientists can modify the ring by augmenting one or more of the carbon atoms with functional groups of other atoms. They can then excite one functional group, say, and watch the ripple effects spread across the rest of the molecule.

Attosecond pulses make experiments like this possible in the first place. Eventually, Cryan says, scientists may be able to advance the technology to understand how electrons behave within far more complex molecules, like proteins.

It was not long ago that the ability to generate attosecond pulses was novel enough to win its creators the 2023 Nobel Prize in physics. Now, this still-young field is starting to flourish as a means of research, says Marinelli. “Every time you open a new line of research, it then bifurcates,” he says. “It generates many new lines of research, and that’s what’s happening now.”

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