Many of the deepest mysteries of science lie hidden at the microscopic scale. To uncover these mysteries, researchers from around the world are gathering at the U.S. Department of Energy's Stanford Linear Accelerator Center (SLAC) National Laboratory to explore using its Linear Coherent Light Source (LCLS).
The LCLS functions like a giant microscope, emitting ultra-bright X-ray pulses and directing them to various precision scientific instruments. Scientists use it to capture the instantaneous movement of atoms, track the real-time dynamics of chemical reactions, uncover the unique properties of materials, and gain insights into the fundamental mechanisms of life. After over a decade of successful operation, the LCLS has completed a critical upgrade known as LCLS-II. The upgraded system increases the repetition rate of X-ray pulses from 120 times per second to an astonishing 1 million times per second, a nearly tenfold increase. This leap forward is giving rise to a new generation of experimental equipment and research methods, enabling scientists to tackle cutting-edge scientific questions that were once considered out of reach.
Capturing effective photons: a leap from days to moments
Among the various research instruments, the qRIXS and chemRIXS spectrometers utilize resonant inelastic X-ray scattering (RIXS) technology. This technology works by illuminating a sample with X-ray pulses, exciting its inner-shell electrons; when the electrons return to their stable state, they release energy in the form of photons. By analyzing these emitted photons, researchers can reconstruct the intermediate processes of the reaction and precisely probe the electronic properties of quantum materials.
Georgi Dakovski, chief scientist at SLAC and head of the qRIXS instrument, explains that RIXS is a measurement technique with extremely low signal yield. In experiments, the vast majority of incident X-ray photons are absorbed or scattered by the sample and never reach the detector. On average, only one out of every billion incident photons produces an effective signal that can be successfully detected. Georgi Dakovski states: At the original pulse frequency of the LCLS, capturing even the slightest effective photon was an art form, as we had to wait a long time to accumulate sufficient data."
However, the LCLS now produces X-ray pulses at a rate 100 to 10,000 times higher per second. RIXS measurements that once took days to complete can now be obtained in minutes or even seconds.
Georgi Dakovski said: "This improvement has brought about remarkable changes. Not only has the speed of data acquisition significantly increased, but the clarity is also unprecedented. We can now observe in real-time how materials transform over time, track the transmission of energy within materials, and monitor the interactions between atomic components. We can 'film' these dynamic processes frame by frame, and all of this is thanks to the LCLS's significantly enhanced X-ray pulse frequency."
Georgi Dakovski stands next to the qRIXS instrument
This spring, following the completion of upgrades, the qRIXS instrument made its debut. This is a massive device equipped with a 12-foot-long spectrometer capable of rotating 110 degrees, utilizing RIXS technology to study the quantum dynamics of solid-state crystalline materials. Its large size enables scientists to analyze materials at extremely high resolution from multiple angles, but it also requires a large input of X-rays to obtain high-quality data. These capabilities have long been a pressing need for the LCLS user community, but due to the extremely high photon requirements, they have only now become feasible.
Researchers are now using qRIXS to study materials such as high-temperature superconductors, which can transmit electricity with zero energy loss. A deeper understanding of the underlying quantum phenomena could drive the development of more efficient quantum computers, improve magnetic resonance imaging (MRI) equipment for medical use, and enable the realization of potential lossless power transmission networks on a large scale.
Kristjan Kunnus with the chemRIXS instrument
While qRIXS is primarily used for quantum materials research, chemRIXS is specifically designed to analyze the chemical properties of liquid samples, ranging from ultra-pure water to chemical solvents. chemRIXS provides researchers with detailed insights into chemical processes, such as the intermediate steps of photosynthesis, which could potentially lead to the development of artificial photosynthesis systems in the future.
chemRIXS was installed in 2021 and has been operating on the LCLS beamline for several years, accumulating a large amount of data. Kristjan Kunnus, a SLAC scientist and the principal investigator for the chemRIXS instrument, stated that the significant increase in X-ray intensity brought by LCLS-II has greatly expanded the research potential of the device. He said, "Previously, we could not study low-concentration solvates and had to use higher-concentration samples, which did not fully reflect the chemical processes under real-world conditions. Now, we can analyze the dilute samples that are truly important in chemical applications and still obtain high-quality data, which was simply impossible in the past."
Capturing molecular movies: Tracking chemical reactions at the trillionth of a second
At the Time-Resolved Atomic, Molecular, and Photonic Sciences (TMO) endstation, multiple new instruments are leveraging the upgraded capabilities of LCLS-II to study how electrons initiate various processes in biology, chemistry, and materials science. One of these is the Multi-Resolution "Cookie Box" (MRCO) instrument, whose core is a ring array of 16 electron detectors designed to fully leverage the LCLS's higher repetition rate. By combining this advanced system with the LCLS's ultrafast laser pulses, researchers can precisely pinpoint the moment electrons escape from molecules and measure the energy spectrum and angular distribution of the escaping electrons with extremely high precision. These measurements enable scientists to resolve the transfer of charge and energy within molecular systems at natural timescales as short as one trillionth of a second. Ultimately, such research not only tests the limits of quantum theory but also provides crucial insights for designing more efficient catalysts and fuels.
Razib Obaid, a SLAC scientist and head of the MRCO instrument, stated: We are no longer constrained by the narrow 'observation window' of the past; this upgrade has expanded the scientific boundaries we can explore in each experiment."
One of the new members of the TMO terminal station is the Dynamic Reaction Microscope (DREAM). As the name suggests, DREAM is a powerful reaction microscope that enables researchers to observe the state of individual molecules during chemical transformations. The instrument focuses an X-ray beam on a single molecule, gradually stripping away its electrons until the molecule "explodes," with all chemical bonds completely broken. The resulting fragments are then detected and used to reconstruct a high-resolution structural map of the molecule. By accumulating millions of such images, researchers can ultimately construct a molecular-level "film" of the chemical reaction.
James Cryan, a senior scientist at SLAC and head of the TMO instrument, stated, "This equipment allows us to understand phenomena at the most fundamental level, such as how photochemical processes like vision, solar energy conversion, and photosynthesis unfold, how DNA transfers energy when absorbing light, and how electrons move from one side of a molecule to the other."
This breakthrough technology relies entirely on the LCLS's high-speed pulse frequency. To fully capture a single molecular reaction, researchers need to take images from nearly a million different angles, which means millions of X-ray exposures. In 2020, the team built a prototype on the existing beamline for capability verification. They spent a week collecting data but could only generate a single frame of the molecular film.
James Cryan said, "Under the original conditions, it might have taken years to fully resolve a single reaction. Now, with DREAM operating on the upgraded LCLS beamline, we can observe these processes in a completely new way. This upgrade is a turning point, making previously impossible research a reality."
The significant increase in data collection capacity at LCLS has not only spawned new research methods but also generated massive amounts of data for training foundational AI models. These AI models can help researchers collect data more efficiently to explore new materials and provide real-time assistance to operators during beamline adjustments. Matthias Kling, LCLS Research and Development Director, stated, "The deep integration of this AI technology will undoubtedly reshape the research landscape and accelerate the pace of scientific discovery."
With enhanced performance and a new instrumentation system, the LCLS-II upgrade has significantly expanded the scope of LCLS research. Researchers are currently analyzing data from the first experiments and plan to conduct more experiments this year. The scientific discoveries enabled by these advanced facilities are expected to further deepen humanity's understanding of the fundamental processes that shape the world.
