Frozen Lightning: Capturing the Invisible Dance of Atoms with Switzerland's X-ray Laser
How the SwissFEL is making movies of processes faster than a blink of an eye and smaller than a wavelength of light.
Did You Know?
A femtosecond is to a second what a second is to about 31.7 million years.
Imagine trying to film a hummingbird's wings in perfect detail. You'd need an incredibly fast and powerful camera to freeze the motion, and a fantastic zoom lens to see every feather. Now, imagine that instead of a hummingbird, you want to film the fundamental dance of atoms and electrons—processes that are the foundation of life, technology, and the universe itself. These events happen in femtoseconds (millionths of a billionth of a second) and on nanometre scales (billionths of a meter). This was once impossible. Not anymore.
Welcome to the world of ultrafast nanoscale phenomena, and to the machine built to capture it: the SwissFEL (Swiss Free-Electron Laser). This magnificent instrument, located at the Paul Scherrer Institute (PSI), is like a strobe light and an ultra-high-resolution microscope combined into one, using the power of X-rays to make the invisible visible. It is opening novel science opportunities, allowing us to witness the birth of chemical bonds, the secrets of magnetic data storage, and the inner workings of novel materials.
The Ultimate Challenge: Time and Space
To understand why the SwissFEL is so revolutionary, we need to appreciate the two-fold challenge scientists face:
The Time Problem
The processes that dictate how a material conducts electricity, how a drug binds to a protein, or how a solar cell converts sunlight, happen impossibly fast. We're talking about femtoseconds. To put this in perspective: a femtosecond is to a second what a second is to about 31.7 million years.
The Space Problem
These ultrafast processes involve the movement of atoms and electrons. Atoms are incredibly small, typically around 0.1 to 0.5 nanometres in diameter. This is far smaller than the wavelength of visible light, which is why ordinary microscopes can't see them.
For decades, scientists could take static, atomic-resolution snapshots using machines like synchrotrons or electron microscopes. Others could study ultrafast timing with laser pulses. The SwissFEL is the first tool that does both simultaneously, providing "molecular movies" with the perfect combination of high spatial and temporal resolution.
How the SwissFEL Works: A Super-Microscope and a Super-Strobe Light
The SwissFEL is a kilometre-long, state-of-the-art machine that works on a brilliant principle:
Create the "Film Reel"
It first generates a beam of electrons and accelerates them to nearly the speed of light.
Make Them Dance
These high-speed electrons are then sent through a special array of magnets called an undulator. The magnets force the electrons to slalom back and forth. This dance causes them to emit incredibly intense, laser-like pulses of X-ray light.
The Ultimate Flash
Each pulse is incredibly short (femtoseconds) and incredibly bright (a billion times brighter than traditional X-ray sources). This flash is so powerful that it can reveal the position of atoms, and so short that it can freeze their motion.
Make a Movie
Scientists fire a pulse from an optical laser to start a reaction (e.g., heating a material, triggering a chemical change) and then fire the SwissFEL's X-ray pulse at a precisely controlled delay to probe it. By repeating this with different delays, they compile a sequence of frames, creating a stop-motion movie of the atomic world.
The SwissFEL facility at Paul Scherrer Institute. Credit: PSI
A Front-Row Seat to a Phase Transition: The Magnetite Experiment
One of the most stunning demonstrations of the SwissFEL's power was an experiment that filmed a phase transition in the mineral magnetite (Fe₃O₄).
The Mystery
Magnetite is a famous magnetic material. At low temperatures, it is an insulator (doesn't conduct electricity), but when heated above a certain point, it undergoes a ultrafast transition to become a conductor. How do the atoms and electrons rearrange to make this happen? The theory was decades old, but no one had ever seen it happen in real time.
The Experimental Procedure: Step-by-Step
The team grew a perfect, tiny crystal of magnetite and cooled it down to its insulating state.
At a meticulously controlled delay—from femtoseconds to picoseconds (thousandths of a billionth of a second) after the initial laser pulse—the team fired an ultra-short, ultra-bright X-ray pulse from the SwissFEL at the crystal.
By repeating this process millions of times and changing the delay between the trigger laser and the probe X-ray, the scientists assembled a frame-by-frame movie of the atomic rearrangement.
They hit the crystal with an extremely short pulse from a powerful infrared laser. This pulse delivered a precise amount of energy, instantly heating the crystal and triggering the insulator-to-metal phase transition.
The X-ray pulse scattered off the crystal's atoms and was captured by a high-speed detector. The pattern of this scattering (a diffraction pattern) contains information about the precise positions of the atoms at that exact moment in time.
Results and Analysis: The Twist No One Expected
The "movie" revealed a shocking plot twist. It was long assumed that the atoms moved first, and the electrons followed, changing the material's conductivity.
The SwissFEL data showed the exact opposite.
The X-ray diffraction patterns proved that the electrons rearranged themselves almost instantly (in under 50 femtoseconds) to form the metallic state. The atoms, however, took over 10 times longer (hundreds of femtoseconds) to shift to their new positions. The electronic structure changes first, and the atomic lattice slowly follows.
Scientific Importance
This turned the textbook explanation on its head. Understanding this sequence is crucial for designing new materials for ultrafast electronics, such as optical switches that could form the backbone of future computers operating at speeds far beyond today's limits.
Data from the Magnetite Phase Transition
| Parameter | Value | Description |
|---|---|---|
| X-ray Wavelength | 0.62 Å (Angstroms) | Allows resolution of atomic distances. |
| Pulse Duration | ~30 fs | Short enough to "freeze" atomic motion. |
| Time Delay Step | 10 fs | The interval between "frames" in the movie. |
| Photon Energy | 20 keV | Provides high penetration to probe the bulk material. |
| Event | Time After Laser Pulse | Observation Method |
|---|---|---|
| Electronic Rearrangement | < 50 fs | Change in X-ray diffraction pattern symmetry. |
| Onset of Atomic Movement | ~300 fs | Shift in specific atomic peak positions. |
| Full Lattice Transformation | ~1.5 ps (1500 fs) | Complete transition to new diffraction pattern. |
| Research Reagent / Material | Function in the Experiment |
|---|---|
| High-Quality Single Crystal | Serves as the pristine sample to study, with a known, ordered atomic structure. |
| Optical Laser (Trigger Pulse) | Initiates the reaction with a precise energy kick, starting the clock. |
| SwissFEL X-ray Pulse (Probe) | The ultimate flashbulb, taking an instantaneous snapshot of atomic positions. |
| Ultrafast X-ray Detector | Captures the diffraction pattern with high efficiency and speed before the sample is destroyed. |
| Cryostat | Cools the sample to the desired initial temperature (e.g., for the insulating state). |
| Precision Delay Stage | Physically adjusts the path length of the laser pulse to control the timing delay with femtosecond accuracy. |
The Future is Ultrafast
The magnetite experiment is just one example. Researchers at the SwissFEL are now making movies of how proteins change shape when activated by light, how vortices in superconductors move, and how catalysts accelerate chemical reactions. Each "movie" provides not just a confirmation of theory, but often a surprise, leading to deeper questions and new ideas.
The ability to see the nanoscale world in motion is transforming our fundamental understanding of nature. The SwissFEL, this marvel of Swiss engineering, is providing the front-row seats, giving us a glimpse into the hidden dance that governs everything around us. The age of ultrafast nanoscale science has just begun, and its potential is as vast as the atomic world is small.