Cold Hypersonic Gas Jets for Precision Laser Spectroscopy of Radioactive Molecules

Deep within the heart of matter, where protons and neutrons bind together to form atomic nuclei, lies a subtle asymmetry that could rewrite our understanding of the universe. Physicists have long suspected that the fundamental forces governing these particles might not be perfectly symmetrical, a violation known as charge-parity violation. If such a violation exists, it would manifest as a tiny, permanent separation of positive and negative charge within the nucleus itself, a property called the nuclear Schiff moment. Detecting this minute effect is incredibly difficult because the electrons surrounding the nucleus usually shield it from view. However, scientists have theorized that if they can place these nuclei inside certain molecules, the internal electric fields of the molecule could amplify this hidden signal, making it visible to sensitive instruments. This search is not just about finding a new force; it is a probe for physics that lies beyond our current standard model, potentially explaining why the universe is made of matter rather than antimatter. To succeed, researchers must study these molecules while they are nearly motionless, cooling them to temperatures just a few degrees above absolute zero to strip away the thermal noise that would otherwise drown out the signal.

A team of researchers has now taken a decisive step toward this goal by demonstrating a new way to create and study these cold, radioactive molecules. Working at a facility in Belgium, the scientists focused on molecules made of thorium and oxygen, specifically versions containing rare, short-lived isotopes of thorium. Their challenge was to take a target of solid material, turn it into a gas of molecules, and then cool that gas down so rapidly that the molecules would stop jiggling and spinning wildly, settling into a calm, predictable state. To achieve this, they employed a technique involving a specialized nozzle, shaped like a de Laval nozzle, which accelerates gas to supersonic speeds. As the gas rushes through this nozzle and expands into a vacuum, it cools dramatically, much like how a spray of compressed air feels cold when it hits your skin, but on a scale where the molecules themselves lose almost all their internal heat.

The researchers began by creating a stream of thorium oxide molecules. They used a high-powered laser to blast a tiny spot on a target made of thorium, vaporizing the material and sending a cloud of hot atoms and molecules into a chamber filled with argon gas. This cloud was then swept along by a steady flow of argon toward the nozzle. As the mixture passed through the nozzle, the rapid expansion cooled the molecules. The team then used a second set of lasers to probe the molecules, shining light at specific frequencies to see how they absorbed energy. By carefully tuning these lasers and measuring the resulting signals, they could determine the temperature of the molecules in three different ways: how fast they were moving forward, how fast they were spinning, and how much they were vibrating.

The results were striking. The molecules were cooled to a translational temperature of about 12.5 Kelvin, which is roughly 12.5 degrees above absolute zero. Even more impressively, their spinning motion, or rotation, was cooled to just 7.2 Kelvin. This extreme cold meant that the molecules were not tumbling chaotically but were instead aligned in a very orderly fashion, allowing the researchers to see their spectral lines with unprecedented clarity. While they could not measure the exact temperature of the molecules' vibrations, they found that the molecules were not vibrating in excited states, placing an upper limit on that temperature at 250 Kelvin. This level of cooling is a significant improvement over previous methods, which often left molecules too hot and chaotic to study in detail.

With the molecules now calm and cold, the team was able to measure how the energy levels of the molecules shifted when they used different isotopes of thorium. This shift, known as the isotope shift, contains information about the size and shape of the nucleus. By comparing their measurements with theoretical calculations, the researchers found that the molecules were incredibly sensitive to changes in the nuclear charge radius. They determined a field shift factor that was significantly larger than what is seen in single atoms, confirming that molecules can indeed act as powerful amplifiers for these nuclear effects. The experimental data matched well with complex computer simulations performed by the team, which used advanced quantum mechanics to predict how the electrons and nuclei would interact. This agreement between the real-world measurements and the theoretical models gives scientists confidence that their tools are working correctly and that they are on the right track.

This work is more than just a technical success; it opens a new door for studying the heaviest elements in the periodic table. The ability to produce and cool radioactive molecules like thorium oxide means that scientists can now apply these precise laser techniques to other short-lived isotopes that were previously too difficult to handle. The researchers plan to use this method to study even more exotic molecules, potentially including those containing nobelium or lawrencium, elements that exist for only moments before decaying. By mastering the art of cooling these fleeting molecules, the team has laid the groundwork for a new generation of experiments that could finally reveal the subtle violations of symmetry hidden deep within the atomic nucleus, bringing us closer to understanding the fundamental rules that govern our universe.