Action! Attosecond X-rays Produce Ultrafast Movies

Researchers use an X-ray free-electron laser to film electrons using finely tuned pairs of attosecond flashes.

Two synchronized flashes of ultrafast X-rays interact with a molecule. The electrons ionized from the molecule are detected with an energy spectrometer, providing observation of the interaction between the two particles at the attosecond timescale. — Image courtesy of SLAC National Accelerator Laboratory.

The Science

The ultrafast motion of electrons is the fundamental mechanism for chemical reactions. These reactions are at the core of many natural processes, including photosynthesis and radiation damage to DNA. Observing the motion of electrons at the atomic scale requires precisely synchronized flashes of X-ray radiation shorter than a femtosecond, or one quadrillionth of a second. In this experiment, researchers demonstrated the generation of synchronized pairs of attosecond X-ray flashes. They used these flashes to observe the ultrafast interaction of electron pairs from an ionized (electrically charged) molecule.

The Impact

This experiment showcases a crucial technique for understanding electrons' motion: attosecond pump/attosecond probe spectroscopy. This technique can create and probe non-equilibrium states of matter on the natural timescale for the motion of electrons. This is enabled by the unique properties of the X-ray free-electron laser. This type of laser can provide pulse intensities that are higher than conventional attosecond light sources by more than six orders of magnitude. This work paves the way for a more fundamental understanding of electronic processes. This could benefit technologies from photovoltaic panels to quantum computers.

Summary

Scientists at the SLAC National Accelerator Laboratory have developed a new technique to generate synchronized pairs of attosecond X-ray pulses from an X-ray free-electron laser at the Linac Coherent Light Source (LCLS). The LCLS is a Department of Energy Office of Science user facility. Researchers generated attosecond X-ray pulses by accurately shaping the LCLS electron bunch in time so that a short, high-density region dominates the X-ray emission. The scientists employed the same electrons in two X-ray free-electron lasers (XFELs) producing two pulses with synchronization at the attosecond level. To demonstrate this new capability, the researchers measured the delay between the two pulses with an attosecond clock. The researchers then used this delay to characterize the group velocity of the X-ray laser, as well as the relative arrival timing jitter of the two pulses. Finally, the team used these pulses in the first X-ray attosecond pump/attosecond probe experiment. Using time-resolved photoelectron spectroscopy, the team observed the ultrafast interaction between electron pairs extracted from a molecule by the two X-ray flashes, a process termed post-collision interaction. Beyond this initial demonstration, this new technique has been employed in several user experiments at the LCLS, demonstrating the impact of accelerator research on the X-ray user community.

Contact

Ago Marinelli
SLAC National Accelerator Laboratory
marinelli@slac.stanford.edu

James Cryan
SLAC National Accelerator Laboratory
jcryan@slac.stanford.edu

Funding

This research was supported by the Department of Energy (DOE) Office of Science, Basic Energy Sciences program’s Scientific User Facility Division and Chemical Sciences, Geosciences, and Biosciences Division. The research used resources at the Linac Coherent Light Source, a DOE Office of Science user facility. Funding was also provided by the Engineering and Physical Sciences Research Council of the United Kingdom and the Swiss National Science Foundation.