Unlocking secrets of cold molecular collisions
At high temperatures, molecules can be thought of as behaving like billiard balls. At very low temperatures however, quantum mechanics becomes dominant. This means that molecules no longer act like ‘classical billiard balls’, but rather as quantum waves. “During a collision, these ‘waves’ start to interfere, leading to bizarre new molecular collision phenomena that cannot occur at high temperatures,” explains FICOMOL(opens in new window) project coordinator Sebastiaan van de Meerakker from Radboud University(opens in new window) in the Netherlands. “These phenomena were predicted decades ago but have experimentally remained elusive. This is because it is extremely difficult to study molecular collisions at sufficiently low temperatures.”
Steering molecular collisions using electric fields
The FICOMOL project, supported by the European Research Council(opens in new window) (ERC), set out to overcome this challenge. “To do this, we developed methods to reach temperatures as low as ~100 millikelvin,” says van de Meerakker. “We also made the very first steps to engineer ‘control knobs’ using electric fields to steer molecular collisions.” A so-called Stark decelerator was used to change the forward velocity of molecules, and curved hexapoles to change their path. This enabled the project team to let molecules interact with each other with little relative velocity. “As temperature is essentially a measure of the relative motion of molecules, our technique enabled us to probe collisions at temperatures down to 100 millikelvin, without making use of cryogenic methods,” explains van de Meerakker. Collisions were detected using powerful lasers and then mapped two-dimensionally, revealing the velocity and recoil direction of the collided molecules.
Low-temperature scattering phenomena
These techniques enabled the project team to probe and discover new low-temperature scattering phenomena. “This work is purely fundamental, and the main motivation for this work was scientific curiosity,” says van de Meerakker. “There could however be important applications. Scientists working with cold molecules are developing methods to use individual molecules in a quantum computer, for instance. A thorough understanding of collision properties of individual molecules at low temperatures is a key prerequisite.” Other applications could include astrochemistry. “We know that the region between the stars in the universe is full of molecules that interact with each other,” notes van de Meerakker. “We can probe the chemical composition of interstellar space using space telescopes. To interpret and model these observations however, a thorough understanding of collision properties of individual molecules at low temperatures is again a key prerequisite.”
Quantum devices based on individual molecules
Future lines of research will focus on two key issues. “We reached sufficiently low temperatures to start manipulating collisions with external electric or magnetic fields,” says van de Meerakker. “In the years to come, we expect to unlock the full potential of this ‘additional control knob’.” Second, van de Meerakker and his colleagues plan to develop new methods to reduce attainable temperatures by another two to three orders of magnitude. An ERC Advanced Grant has been secured to work towards these goals over the next five years. “The hope is that we will unlock all the secrets of cold molecular collisions, at the full quantum mechanical level,” adds van de Meerakker. “This may eventually lead to new quantum devices based on individual molecules, which could benefit a range of research domains from atmospheric science to astrochemistry.”
