Supporting the Movement of Electrons Within an Atom – Down to One Billionth of a Second

Accurate synchronization of X-rays and external laser pulses is a major obstacle in analyzing data from XFEL experiments. The researchers took advantage of the time differences between the two types of electrons, plotting tens of thousands of measurements that formed a characteristic elliptical shape in a grid. Their results showed that the position of these individual data points can be read as one-hour hands to detect the time of rapid electron movements. Credit: Daniel Haynes / Jörg Harms

Scientists get dramatically better resolution on X-ray free electron lasers with a new technique.

Intense, ultra-short X-ray pulses from free-electron X-ray (XFEL) lasers can capture images of biological structures up to the atomic scale and shed light on the fastest processes in nature with a shutter speed of just one femteconds, of one millionth of a billionth of a second.

However, at these small time scales, it is extremely difficult to synchronize the X-ray pulse that ignites a reaction in the sample with the subsequent pulse that monitors the reaction. This problem, called jitter timeing, is a major hurdle in conducting these XFEL experiments with ever-better resolution.

Now, an international team of researchers from the Department of Energy’s SLAC National Accelerator Laboratory, Max Planck Institute for Structure and Dynamics of Matter, Deutsches Elektronen-Synchrotron Laboratory (DESY) and the Paul Scherrer Institute has found a way out of this problem by measuring an essential decay process in neon gas at SLAC Coherent Linac Light Source (LCLS). The work was published in Physics of nature in January.

Shaking nervousness

Many biological systems – and some non-biological ones – are damaged when stimulated by an X-ray pulse from an XFEL. One cause of damage is a process known as Auger breakdown: The X-ray pulse emits some of the most tightly bonded electrons, called photoelectrons, from the atoms in the sample, and the weaker bonded electrons fall to take their place. This process of “relaxation” releases energy and can cause the emission of another electron, known as an Auger electron.

Intense X-ray radiation and the continuous emission of Auger electrons can quickly damage the sample. To mitigate this damage, measurements must be taken before the breakdown begins, so accurate knowledge of the breakdown time scales is valuable. However, due to time constraints, it is not usually possible to resolve such rapid breakdown processes in XFEL.

“It’s like trying to photograph the end of a race when the camera lid can be activated at any moment in the last 10 seconds,” says lead author Dan Haynes, a doctoral student at Max Planck.

To avoid the problem of nervousness, the research team came up with a very precise way to describe the Auger decay and demonstrated their method on neon gas samples.

A generation of inspiration

The new technique is based on established streaking spectroscopy methods, where the emitted electrons are accelerated or decelerated by the electric field of a “streaking” laser pulse. In this method, the XFEL pulse starts the processes and the line pulse acts as a probe to observe them. Usually, jitter time limits the resolution of this technique in XFELs.

After exposing both photoelectrons and Auger electrons to an external laser pulse, the researchers determined their final kinetic energy in each of the tens of thousands of individual measurements. Since Auger electrons are emitted later than photoelectrons, they also interact with the laser pulse coming a little later, and this steady change allowed the researchers to distinguish the two types of electrons from each other.

Despite the uncontrollable nervousness of the time between two pulses, this steady factor leads to a characteristic elliptical shape in the data analyzed when measurements are presented in a grid. The position of individual data points around the ellipse can be read as one-hour hands to detect the exact time of ultra-fast electronic movements.

Researchers hope the technique will have a wider impact in the field of ultra fast science. Moreover, the Auger decay is a major factor in the study of exotic, highly excited states of matter, which can only be investigated in XFEL.

“This technique allowed us to measure the delay with sub-femtosecond accuracy, even though the shock time during the experiment was in the one-hundredth of a second femtosecond interval,” says Haynes, “This could facilitate a new class of experiments taking advantage of the flexibility and extreme intensity. XFEL-ve. ”

References: “Clocking Auger electrons” by DC Haynes, M. Wurzer, A. Schletter, A. Al-Haddad, C. Blaga, C. Bostedt, J. Bozek, H. Bromberger, M. Bucher, A. Camper, S Carron, R. Coffee, JT Costello, LF DiMauro, Y. Ding, K. Ferguson, I. Grguraš, W. Helml, MC Hoffmann, M. Ilchen, S. Jalas, NM Kabachnik, AK Kazansky, R. Kienberger, AR Maier, T. Maxwell, T. Mazza, M. Meyer, H. Park, J. Robinson, C. Roedig, H. Schlarb, R. Singla, F. Tellkamp, ​​PA Walker, K. Zhang, G. Doumy , C. Behrens and AL Cavalieri, January 18, 2021, Physics of nature.
DOI: 10.1038 / s41567-020-01111-0

LCLS is a user facility of the DOE Science Office. This research was supported by the DOE Science Office.

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