With a novel approach to X-ray spectroscopy, researchers are now able to capture detailed, ultrafast snapshots of electron interactions.
(Nanowerk News) In a leap forward for atomic-scale imaging, researchers have introduced a novel X-ray technique that could transform our understanding of electron motion at the microscopic level. This cutting-edge method, developed by an international team of scientists, uses the unique properties of X-ray lasers to capture detailed snapshots of atomic interactions (Nature, “Super-resolution stimulated X-ray Raman spectroscopy”).
The technique, called stochastic Stimulated X-ray Raman Scattering (s-SXRS), turns noise into valuable data, offering snapshots of the electronic structures near specific atoms. This advancement sets the stage for breakthroughs in chemical analysis and materials science.
Researchers from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, along with the Max Planck Institute for Nuclear Physics and the European X-ray Free Electron Laser (European XFEL), both in Germany, developed this innovative approach to X-ray spectroscopy, achieving unprecedented detail and resolution.
“For a long time, chemists have dreamed of seeing how electrons move when they’re in excited states, as these movements are what drive chemical reactions,” said Linda Young, an Argonne Distinguished Fellow and professor at the University of Chicago. “Our technique brings us closer to realizing that dream.”
The key innovation is a super-resolution technique that greatly improves the detail in X-ray spectroscopy, a method for studying electron placement around atomic centers. This advancement helps scientists identify closely spaced energy levels in atoms, offering a clearer view of their electronic structures, which determine chemical properties.
“Think of it like upgrading from a standard-definition television to an ultra-high-definition screen,” Young explained. “We’re now able to see the fine details of electronic motion that were previously blurred or invisible.”
The practical applications of s-SXRS are wide-ranging. For example, it can provide insights into how chemical bonds form and break, offering a deeper understanding of fundamental processes relevant to chemical analysis. This knowledge is essential for developing new materials with specific electronic properties, impacting industries like electronics and nanotechnology.
s-SXRS uses intense X-ray pulses to excite electrons within atoms. As the X-rays pass through a gas, they amplify the Raman signals — a type of X-ray fingerprint that provides information about the excited electronic states of molecules — by nearly a billion-fold.
This amplified signal provides detailed information about the electronic structure of the gas on a femtosecond timescale, or one quadrillionth of a second. By analyzing the relationship between the incoming pulses and the resulting Raman signals, scientists can create a detailed energy spectrum from many individual snapshots, rather than scanning slowly across different energy levels.
“The large number of pulses in each X-ray flash not only boosts the measurement signal but also holds the key to the highest spectral resolution by averaging over many photon impacts on the detector at once,” said Thomas Pfeifer from the Max Planck Institute for Nuclear Physics.
This approach, pinpointing the center position of broad but distinct spectral spikes much more precisely than the width of the spikes, is similar to the super-resolution microscopy technique that won the 2014 Nobel Prize in chemistry, Pfeifer added.
s-SXRS also uses a statistical method, called covariance analysis, to link the incoming X-ray pulses with the emitted Raman signals. This transforms what was once considered “noise” into a valuable resource, allowing extraction of detailed information from complex data. This approach not only enhances the resolution, but also speeds up data collection, providing rapid and detailed snapshots of atomic interactions.
Researchers conducted a straightforward experiment to implement this technique. They directed an X-ray beam through a gas and used a spectrometer to collect the resulting radiation. At the European XFEL, a small, 5-millimeter gas cell designed by the Max Planck Institute for Nuclear Physics, was positioned in the path of the X-ray beam.
As an incoming X-ray light wave propagates through dense gas, it amplifies Raman signals. When analyzed with a grating, these signals provide extremely high-resolution spectra that surpass traditional instrumental limits by employing super-resolution correlation methods. (Image: Stacy Huang, Argonne National Laboratory)
The intense beam created tiny holes in the cell’s entrance and exit windows, allowing the X-rays to pass through to a grating spectrometer — a device that separates light into its different wavelengths — provided by collaborators from Uppsala University in Sweden. The European XFEL staff played a vital role in coordinating the installation and performing thorough pre-experimental testing. This ensured optimal focusing conditions, which were crucial for efficiently acquiring a large amount of data during the experiment.
“It’s remarkable how a simple X-ray experiment, combined with innovative data analysis, can reveal electronic dynamics and structure details with unprecedented clarity,” said Kai Li, a graduate student at Argonne and the University of Chicago.
Michael Meyer, group head of the Small Quantum Systems instrument at European XFEL, added, “This study is an excellent example of the capability of the European XFEL, especially of its high intensities and the recently demonstrated generation of extremely short X-ray pulses with durations of less than one femtosecond. These advancements will certainly trigger further investigations to unravel the dynamics of complex chemical reactions.”
Another crucial part of this research involved the Argonne Leadership Computing Facility (ALCF), a DOE Office of Science user facility, which provided the necessary computational power to simulate the complex interactions between X-ray pulses and matter. The team worked closely with ALCF staff to optimize their code for the Theta supercomputer and implement a software tool to help run large ensembles of calculations more efficiently. These simulations were vital for interpreting the experimental results and refining the technique.
The ALCF’s supercomputing capabilities enabled the team to conduct detailed simulations that closely matched the experimental data, providing valuable insights into how X-ray pulses behave as they travel through gases. The computations were instrumental in confirming the researchers’ understanding of how the X-ray pulses move and interact, paving the way for future investigations.
With continued advancements, s-SXRS could become a standard tool in laboratories worldwide, driving innovation across many fields.
“We’re just beginning to scratch the surface of what we can achieve with this level of detail,” Young said. “It’s an exciting time for science and technology.”
