The motion of an electron in a quantum state of matter is tracked using X-ray pulses less than a millionth of a millionth of a second

XLEAP Powerful Low Energy X-ray Laser Pulses

A team led by SLAC has invented a method called XLEAP that generates powerful, low-power X-ray laser pulses that are only 280 attoseconds long, or billionths of a billionth of a second, and could detect for the first time the fastest motions of electrons that drive chemistry. This illustration shows how scientists use a series of magnets to convert an electron array (blue figure on the left) in SLAC’s Linac Coherent light source into a narrow current spike (blue figure on the right), which then produces extremely intense X-rays. Flash (yellow). Credit: Greg Stewart/SLAC National Accelerator Laboratory

Long X-ray pulses of less than a millionth of a billionth of a second allow the researchers to peer deep into molecules, track electrons as they spin and eventually start chemical reactions.

Scientists at the Department of Energy’s SLAC National Accelerator Laboratory devised a method for generating X-ray laser bursts lasting hundreds of attoseconds (or billionth of a billionth of a second) in 2018. This technology, known as an X-ray Laser Enhanced Attosecond Pulse Generation (XLEAP), enables researchers to Investigating how electrons racing around molecules start key processes in biology, chemistry, materials science, and other fields.

“The motion of the electron is an important process by which nature can move energy around,” says James Cryan, SLAC scientist. “A charge is created in one part of the molecule and transferred to another part of the molecule, which can trigger a chemical reaction. It’s an important piece of the puzzle when you start thinking about the photoelectric devices of artificial photosynthesis, or charge transfer within a molecule.”

Now, researchers at SLAC’s Linac Coherent Light Source (LCLS) have excited electrons in a molecule using attosecond pulses to create an excited quantum state and measure how the electrons behave in this state in previously unseen detail. The results were recently published in the journal Science.

“XLEAP allows us to look deep inside molecules and follow the movement of an electron on a natural time scale,” says SLAC scientist Agostino Marinelli, who leads the XLEAP project. “This could provide insight into many important quantum mechanics phenomena, in which electrons play a key role.”

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Attosecond pulses are the shortest that are generated in an X-ray free electron laser such as the LCLS. The unique achievement of the XLEAP project was making attosecond pulses at the correct wavelength to look inside the most important small atoms, such as carbon, nitrogen and oxygen. Like cameras with ultra-fast shutter speeds, XLEAP pulses can capture the motions of electrons and other motions in an extremely fast timescale that was previously unresolved.

X-ray pulse plotting in SLAC

In this experiment, the researchers hit nitric oxide molecules with a pulse of X-rays, knocking electrons out of their normal position into a highly excited electron cloud. They created a superfast clock using a circularly polarized laser to measure what happened next. The electron cloud faded away by spitting out fast electrons, which orbited around the laser field before landing on the detector. The location where the electrons landed on the detector helped the researchers see how the electron cloud was changing. They saw the cloud move in a unique quantum fashion over the course of a few millionths of a billionth of a second. Credit: Greg Stewart/SLAC National Accelerator Laboratory

When the X-ray pulses interact with the material, they can boost some of the more closely bound core electrons in the sample into high-energy states, known as fundamental excited states. Because they are so energetic, the basic excited states are very unstable and will usually decay very quickly by releasing energy in the form of a fast electron, known as an Auger-Meitner electron. This phenomenon has historically been known as Auger decay, but scientists have recently chosen to add the name Lise Meitner, who first observed this phenomenon, in recognition of her wide-ranging contributions to modern atomic physics.

In their study, the researchers precisely tuned the wavelength of X-rays from the LCLS to create a quantum state of matter called coherent superposition, a manifestation of the wave nature of matter. Similar to Schrödinger’s cat, which found itself dead and alive at the same time, the simultaneously excited electrons were in different states of fundamental excitation. This means that they were orbiting around the molecule along different paths at the same time.

To follow how this correlative superposition of the fundamental excited states appears over time, the researchers created an ultrafast clock known as an ‘Attoclock’, in which a rapidly rotating electric field from a circularly polarized laser pulse acts like a clockwise. The Auger-Meitner electrons emitted in the decay of the excited fundamental states were circling the circularly polarized laser pulse before landing on the detector. The location where the electron landed on the detector told the researchers what time it had been ejected from the molecule. By measuring the ejection times of several Auger-Meitner electrons, the researchers were able to build a picture of how the state of the coherent superposition changes with a time resolution of only a few hundred attoseconds.

“It’s the first time we’ve been able to track this particular phenomenon and directly measure the rate of electron emission,” says SLAC scientist and lead author Siqi Li. “Our method takes us a step beyond just seeing the process happen and allows us to spy on the complex electron behavior that’s happening in a molecule within a few millionths of a billionth of a second. It gives us a really cool way to look inside the molecule and see what’s happening on a fast timescale. very. “

World-leading capability

To pursue this experiment, the researchers are working on new measurements of more complex quantum behavior.

“In this experiment, we investigate the electronic behavior of a very simple model that you can roughly solve with a pencil and paper,” says SLAC scientist and co-lead author Taran Driver. “Now that we’ve shown that we can make these ultra-fast measurements, the next step is to look at more complex phenomena that theories haven’t yet been able to accurately describe.”

Being able to make measurements on faster and faster time scales is exciting, says Cryan, because the first things that happen in a chemical reaction may be the key to understanding what happens next.

“This research is the first time-resolved application of ultrashort X-ray pulses, bringing us one step closer to doing really cool things like watching quantum phenomena evolve in real time,” he says. “It has the promise of becoming a world-leading capability that many people will care about for years to come.”

LCLS is a user facility of the Department of Energy’s Office of Science. This research is part of a collaboration between researchers from SLAC, Stanford University,[{” attribute=””>Imperial College London and other institutions. It was supported by the Office of Science.

Reference: “Attosecond coherent electron motion in Auger-Meitner decay” by Siqi Li, Taran Driver, Philipp Rosenberger, Elio G. Champenois, Joseph Duris, Andre Al-Haddad, Vitali Averbukh, Jonathan C. T. Barnard, Nora Berrah, Christoph Bostedt, Philip H. Bucksbaum, Ryan N. Coffee, Louis F. DiMauro, Li Fang, Douglas Garratt, Averell Gatton, Zhaoheng Guo, Gregor Hartmann, Daniel Haxton, Wolfram Helml, Zhirong Huang, Aaron C. LaForge, Andrei Kamalov, Jonas Knurr, Ming-Fu Lin, Alberto A. Lutman, James P. MacArthur, Jon P. Marangos, Megan Nantel, Adi Natan, Razib Obaid, Jordan T. O’Neal, Niranjan H. Shivaram, Aviad Schori, Peter Walter, Anna Li Wang, Thomas J. A. Wolf, Zhen Zhang, Matthias F. Kling, Agostino Marinelli and James P. Cryan, 6 January 2022, Science.
DOI: 10.1126/science.abj2096