Resonance in supercooled colliding molecules observed for the first time

The findings suggest that scientists could one day steer and control certain chemical reactions.

IF a singer hits the right pitch, they can shatter a wine glass. The reason is resonance. While the glass may vibrate slightly in response to most acoustic tones, a pitch that resonates with its own natural frequency can send its vibrations into overdrive and cause it to shatter. Resonance also occurs at the microscale of atoms and molecules. A chemical reaction occurs partly owing to specific conditions that resonate with particles in a way that drives them to combine chemically.

In a new study published in Nature, physicists at MIT reported that they had for the first time observed resonance in colliding molecules that were supercooled to temperatures just above absolute zero, the lowest energy state (−273.15 ^oC). In their original experiment, the physicists applied a magnetic field that they varied over a 1,000 gauss range. They found that a cloud of supercooled sodium-lithium (Na-Li) molecules suddenly disappeared, 100 times faster than normal, within an extremely tiny window, at about 25 milligauss. That is equivalent to comparing the width of a human hair to a metre-long stick. The rapid disappearance suggests that the magnetic field tuned the particles into a resonance, making them to react more quickly than they would have otherwise. “This is the very first time a resonance between two ultracold molecules has ever been seen,” said Wolfgang Ketterle, the first author of the study. In a molecular assemblage, collisions occur constantly. Particles bounce off each other or stick together in a brief yet crucial state known as an “intermediate complex” that then sets off a reaction to transform the particles into a new chemical structure.

“When two molecules collide, most of the time they don’t make it to that intermediate state,” said Alan Jamison, co-author of the study. “But when they’re in resonance, the rate of going to that state goes up dramatically.” Ketterle added: “The intermediate complex is the mystery behind all of chemistry.” “Usually, the reactants and the products of a chemical reaction are known but not how one leads to the other. Knowing something about the resonance of molecules can give us a fingerprint of this mysterious middle state.” The findings suggest that scientists could one day steer and control certain chemical reactions.