A cryogenic apparatus with a rotatable substrate for matrix isolation spectroscopy of BaF in neon crystals down to 3.2 K

To understand the work described here, one must first grasp a simple but powerful idea in physics: to see the true nature of a molecule, you must stop it from moving. In the natural world, atoms and molecules are in constant, frantic motion, colliding and spinning so rapidly that their internal signals blur together. Scientists use a technique called matrix isolation to solve this. They trap a specific molecule inside a solid block of a harmless, inert gas, like neon, and freeze that block to temperatures colder than deep space. In this frozen, rigid cage, the molecule is held perfectly still, allowing researchers to study its behavior with extreme precision. This method is particularly valuable for hunting down a ghostly property of the electron called the electric dipole moment. If this property exists, it would shake our understanding of the universe's fundamental laws, but it is so tiny that it requires the most still, quiet environment possible to detect.

A team of researchers has now built a new machine designed specifically to create these ultra-cold, frozen cages for a molecule called barium monofluoride. This molecule is a prime candidate for the search because its heavy barium atom amplifies the tiny effects scientists are looking for. The researchers constructed a sophisticated vacuum chamber that acts as a laboratory for growing these crystals. Inside, they use a high-powered laser to blast a solid target of barium fluoride, creating a cloud of atoms and molecules. This cloud is then swept along by a stream of cold gas—either neon or a special form of hydrogen—into a chamber where it lands on a sapphire plate. This plate is cooled by a mechanical refrigerator to a staggering 3.2 Kelvin, a temperature just above absolute zero. At this extreme cold, the gas molecules freeze instantly, forming a solid crystal that traps the barium monofluoride inside.

What makes this new apparatus unique is its ability to rotate the frozen sample without breaking the vacuum or warming it up. Once the crystal has grown, the researchers can spin the plate to face different instruments. They can shine a laser through the crystal to see how much light it absorbs, or they can turn it to face a mirror that collects the light the crystal glows with. This flexibility allows them to study the same single crystal in multiple ways, ensuring that their measurements are consistent and not skewed by differences between separate samples. Using this setup, the team successfully grew crystals of neon doped with barium monofluoride and confirmed that the molecules were trapped and stable. When they shone a specific laser light on the crystal, the molecules absorbed the energy and then re-emitted it as a glow, or fluorescence. By analyzing this glow, they identified the specific energy states of the molecules, confirming that the neon crystal environment behaves much like the gas phase, which is exactly what is needed for precise measurements.

The researchers also tested the system with a different type of gas, a special form of hydrogen known as para-hydrogen. While neon worked well, hydrogen is theoretically even better for these experiments because it is lighter and interacts less with the trapped molecules. The team successfully produced barium monofluoride using hydrogen as the cooling gas, marking a significant first step toward growing crystals in this superior medium. However, the study also revealed a challenge that must be managed in future experiments. When the laser light was too bright or the crystal was extremely cold, the glowing signal from the molecules would fade away quickly. The researchers determined this was not a failure of the molecules themselves, but a temporary effect where the intense light or local heating caused the molecules to stop glowing for a moment. They found that this fading effect became stronger as the temperature dropped and the laser power increased.

In addition to the barium monofluoride, the team noticed that their laser also produced neutral barium atoms, which were also trapped in the crystal and glowed with their own distinct color. This confirmed that the laser blasting process creates a mix of different species, all of which end up in the frozen block. By measuring the brightness of the glow, the scientists estimated the number of barium monofluoride molecules trapped in the crystal, finding a density on the order of 5 times 10 to the power of 13 molecules per square centimeter. This new machine, with its rotatable sample and ability to reach temperatures as low as 3.2 Kelvin, provides a robust platform for future studies. It proves that scientists can grow these delicate crystals, control their environment, and begin the precise work of probing the electron's electric dipole moment, bringing us closer to answering one of the deepest questions in physics.