New methods unveil the true quantum nature of molecular collisions
Whilst molecules colliding may undergo a chemical reaction, more often they only transfer energy to each other: they end up rotating a little faster or moving a little slower. This seemingly simple event plays a key role in an amazingly wide variety of processes: the formation of stars and planets, combustion, the heating balance of the atmosphere, and even processes in certain quantum computers. We already have a good understanding of such collisions at high energy levels. Low energy collisions, however, are a completely different matter: only quantum mechanics can describe the scattering process. Appropriate methods were missing until the MOLBIL (Molecular Billiards in Slow Motion) project came up with a way to image the true quantum nature of molecular collisions. Together with his team, Prof. Dr. Sebastiaan van de Meerakker has developed methods to completely control the motion of molecules before collision, so as to ensure that they collide under extremely well-defined conditions. Such control allows for taking a zoomed-in picture of the process, revealing collision mechanisms that would otherwise remain hidden. “The first task is to carefully control the velocity of the molecules, so as to get a result that we can interpret more easily,” says Prof. Dr van de Meerakker, coordinator of MOLBIL. “This is somewhat like a car manufacturer conducting a crash test: you don't learn much from looking at the car wreck if you don’t know how fast the car was driving at the time of impact”. The ‘Stark decelerator’, which controls velocity, is what makes MOLBIL results unique as Prof. Dr van de Meerakker explains: “Not only can we control the absolute velocity of the molecules, but we also control the velocity spread of an ensemble of molecules that all take part in the collision process. The former is important for controlling or scanning the collision energy, while the latter is important to scan the collision energy in high resolution (collision energy uncertainty).” This method allowed the team to observe scattering phenomena that had been predicted theoretically decades ago but had yet to be observed experimentally. These include quantum diffraction oscillations, low-energy scattering resonances and product-pair correlations. As the Stark decelerator only works for ‘electric’ molecules (those having an electric dipole moment), and not for magnetic ones, the team also used the Zeeman decelerator. “Using a series of electromagnets, we can get full control over magnetic molecules, allowing us to study a whole new group of atoms and molecules. Zeeman decelerators are not new, but we have developed a new concept that is particularly optimized for molecular collision experiments,” says Prof. Dr van de Meerakker. All in all, MOLBIL findings provide data that will challenge the preconceptions of theorists trying to solve the equations of quantum mechanics. Whilst this won’t lead to market products anytime soon, Prof. Dr van de Meerakker is confident that project learnings will benefit many different scientific fields, such as research on quantum gases, astrophysics and meteorology. And although the project is now completed, Prof. Dr van de Meerakker intends to pursue his research. “The collision energies that we have achieved so far correspond to collisions between molecules found in gas at a temperature of about 10 kelvin (and more). That is already a very low energy, but now we have a plan to modify the machine to reach even lower temperatures. Whilst this may sound like a small change, it has in fact big implications,” Prof. Dr van de Meerakker concludes.
Keywords
MOLBIL, molecular collisions, molecules, low energy, quantum mechanics
