At the U.S. Department of Energy’s SLAC National Accelerator Laboratory, scientists have developed a new method to view the movements of electrons using intense X-ray laser bursts. These laser bursts last for only 280 attoseconds, that is, billionths of a billionth of a second.
Known as X-ray laser-enhanced attosecond pulse generation (XLEAP), the technology represents a major advancement that was being investigated by researchers for decades.
The XLEAP technology offers new opportunities to perform groundbreaking research on how electrons moving quickly around molecules trigger important processes in materials science, chemistry, biology, etc. The researchers have described their technique in an article published in the Nature Photonics journal.
Until now, we could precisely observe the motions of atomic nuclei, but the much faster electron motions that actually drive chemical reactions were blurred out. With this advance, we’ll be able to use an X-ray laser to see how electrons move around and how that sets the stage for the chemistry that follows. It pushes the frontiers of ultrafast science.
James Cryan, Study Lead Author and Scientist, SLAC National Accelerator Laboratory
Cryan is also an investigator with the Stanford PULSE Institute—a joint institute of Stanford University and SLAC National Accelerator Laboratory.
Research on these timescales can possibly show, for instance, how the absorption of light at the time of photosynthesis almost instantly drives the electrons around and then triggers a series of relatively slower events that eventually produce oxygen.
“With XLEAP we can create X-ray pulses with just the right energy that are more than a million times brighter than attosecond pulses of similar energy before,” stated Agostino Marinelli, a SLAC scientist, XLEAP project lead, and also one of the lead authors of the paper. “It’ll let us do so many things people have always wanted to do with an X-ray laser—and now also on attosecond timescales.”
A Leap for Ultrafast X-Ray Science
A single attosecond is a remarkably short period of time—that is, two attoseconds is to a second as one second is to the universe’s age. In the recent past, researchers have made significant progress in producing attosecond X-ray pulses. But these X-ray pulses were either too feeble or they lacked the correct energy to home in on the electrons’ rapid movements.
In the last three years, Marinelli and his collaborators have been trying to interpret how an X-ray laser technique that was proposed 14 years ago could be utilized to produce pulses that have the right characteristics—an attempt that led to the XLEAP technology.
Experiments were performed just before the team started to work on a crucial upgrade of the Linac Coherent Lightsource (LCLS) X-ray laser of SLAC. In these experiments, the XLEAP group demonstrated that accurately timed pairs of attosecond X-ray pulses can be produced, and that these pulses can set the electrons in motion and subsequently capture those movements. Snapshots like these can be strung together into stop-action movies.
According to Linda Young, an expert in X-ray science at DOE’s Argonne National Laboratory as well as the University of Chicago, “XLEAP is a truly great advance. Its attosecond X-ray pulses of unprecedented intensity and flexibility are a breakthrough tool to observe and control electron motion at individual atomic sites in complex systems.” Young was not involved in the research.
LCLS is an example of X-ray lasers that usually produce light flashes that last only for femtoseconds or a few millionths of a billionth of a second. This process begins with producing an electron beam, and the electrons are subsequently bundled into short bunches and transported via a linear particle accelerator, where they eventually gain energy.
These electrons then travel at virtually the speed of light and pass via a magnet called an undulator. Within this undulator, some of the electrons’ energy is changed into X-ray bursts.
If the electron bunches are brighter and shorter, the X-ray bursts produced by them will also be shorter. Therefore, compressing the electrons into increasingly smaller bunches with high-peak brightness is one method for making attosecond X-ray pulses. XLEAP offers an ingenious way to achieve just that.
Making Attosecond X-Ray Laser Pulses
The researchers at LCLS subsequently inserted two sets of magnets in front of the undulator; with the help of this undulator, they molded each bunch of electrons into the desired shape—a narrow and powerful spike comprising electrons that have a wide range of energies.
When we send these spikes, which have pulse lengths of about a femtosecond, through the undulator, they produce X-ray pulses that are much shorter than that.
Joseph Duris, Study Co-First Author and Staff Scientist, SLAC National Accelerator Laboratory
Duris added that the pulses are also very strong, with some of them achieving a peak power of half a terawatt.
In order to quantify these extremely short X-ray pulses, a unique device was developed in which the X-rays pass via a gas, remove some of its electrons, and eventually produce an electron cloud.
An infrared laser emits circularly polarized light that communicates with the cloud and gives a kick to the electrons. Due to the specific polarization of the light, some of the electrons begin to move faster when compared to others.
The technique works similar to another idea implemented at LCLS, which maps time onto angles like the arms of a clock. It allows us to measure the distribution of the electron speeds and directions, and from that we can calculate the X-ray pulse length.
Siqi Li, PhD, Study Co-First Author, Stanford University
The XLEAP researchers are now planning to further enhance their technique, which may result in pulses that are more powerful and also potentially shorter. In addition, the team is preparing for LCLS-II, the LCLS upgrade that will fire up to a million X-ray pulses every second—that is, 8,000 times quicker than before.
Such an approach will enable the scientists to perform experiments that they have been dreaming for a long time, for example, analyses of separate molecules as well as their behavior on the fastest timescales of nature.
The XLEAP research team included scientists from Stanford University; SLAC National Accelerator Laboratory; Imperial College, United Kingdom; DOE’s Argonne National Laboratory; and Max Planck Institute for Quantum Optics, Ludwig-Maximilians University Munich, Kassel University, Technical University Dortmund, and Technical University Munich in Germany.
A major part of this study was funded by the DOE Office of Science and via DOE’s Laboratory Directed Research and Development (LDRD) program. LCLS is a DOE Office of Science user facility.