More Ultracool Molecules

JILA scientists Deborah Jin and Jun Ye have done something remarkable: They’ve created an entirely new form of matter! They’ve taken two different incredibly cold atoms at three hundred billionths of a degree above absolute zero and made them into molecules. These molecules contain one atom of rubidium (Rb) and one atom of potassium (K). They are too cold to exist naturally anywhere in the Universe.

So far, no other scientists in the world can make molecules like these. That’s because it’s really hard to do. In fact, it took Dr. Jin and Dr. Ye five years to figure out how to make them in their laboratory.

Since they learned how to make the really cold KRb molecules, Dr. Jin and Dr. Ye have joined forces with Dr. John Bohn to understand how these molecules interact with each other. All three of them are working together to create a new field of science called cold-molecule chemistry.

Cold molecule chemistry is interesting because ultracool molecules are in what’s called their lowest quantum mechanical state. That means they spin and shake at exactly the same way, with the lowest possible energy. The other amazing thing about molecules like this is that they don’t exactly look like one ball attached to another ball, as shown in the picture here. Instead, they spread out like waves inside traps made of laser light. And, since the molecules are prepared to be identical to each other, there are powerful consequences in the strange world of quantum mechanics.

Even though they are so cold they are barely moving, molecules can still crash into each other. When they do, the bonds that hold them together can break apart, and form new molecules.

“What controls ultracold collisions is the nature of the molecules themselves,” says Dr. Ye. “It depends on whether the molecules are fermions or bosons.”

Bosons are atoms or molecules that are very friendly. At very cold temperatures, bosons will pile up in the same place and form either a “superatom” or a “supermolecule.” This tendency to pile up means that when bosons get close to each other, they collide and form new chemical bonds or break old ones.

In contrast, identical fermions are quite independently minded and simply cannot share the same piece of real estate, even at very cold temperatures. So when these standoffish molecules approach each other, they can only get so close before they start circling around each other, separated by an invisible force field. But, even so, some pairs of these kinds of molecules can still manage to slowly form new chemical bonds.

How do they do it? Well, chemistry ruled by quantum mechanics allows some strange things to happen. Dr. Jin explains, “At some point while fermions are dancing around each other, some molecules tunnel right through the force field between them and make or break chemical bonds.” In fact, when Dr. Jin and Dr. Ye made some fermions that weren’t exactly alike, they collided head on with each other as much as a hundred times faster than the identical fermions did.

Who would have thought things could happen so fast at temperatures just three hundred billionths of a degree above absolute zero? It happens because really cold molecules are like waves. The waves can overlap with each other even when the molecules are very far apart.

“When molecules are waves, their sphere of influence is much, much bigger than when they are particles,” says Dr. Ye. He says once he and Dr. Jin make KRb molecules even colder, they might even get to see chemistry caused by the interaction of a whole gas of molecules acting together. That would be ultracool, for sure! - Julie Phillips

Vocabulary

Absolute Zero: lowest possible temperature in the Universe.

Atom: smallest amount of an element

Bosons: particles that can pile up in the same quantum state

Chemical Bond: force that holds atoms together in molecules

Chemistry: the study of matter and the way in which different kinds of matter interact with each other.

Fermions: particles whose spin won’t allow them to pile up in the same quantum state.

Laser: machine that produces a beam of concentrated light of a single color.

Molecule: matter made of two or more atoms.

Quantum Mechanics: the behavior of matter and energy at distances the size of atoms or smaller.