Colliding molecules exhibit strange behaviour
Physicists of the FOM Foundation and Radboud University have investigated how molecules that collide with atoms, sometimes end up behind the atom. On certain occasions this behaviour, which is counterintuitive, is related to how the molecules rotate on their axis. The researchers published their discovery on 31 October in Nature Chemistry.
"In our lab, we investigate how molecules collide with each other and with atoms," says first author Jolijn Onvlee. She and her colleagues want to determine, for instance, the angle at which the molecules are deflected after they have collided with the atoms. Onvlee: "In those situations, we can often view the colliding particles as billiard balls. In that case, it is logically impossible that after the collision, the molecule will end up directly behind the atom or in the shadow of the atom that it has collided with." That idea agrees with our intuition. However, in practice, the physicists see that our intuition does not always grab the right end of the stick: "A couple of years ago, we saw for the first time that some molecules do end up in the shadow of the atom. And in the current research, we have discovered that there is certain angle behind the atom where molecules only end up if they rotate around their axis in a certain way after the collision."
From billiards table to wave pool
This means that the billiard ball model is not adequate. Phenomena occur that you cannot explain classically, regardless of how much effect you give to the billiard balls. "According to the laws of quantum mechanics, molecules have the properties of both particles and waves," says Onvlee. When a molecule collides with an atom, their waves collide as well and interference arises. "This gives rise to a series of new waves that propagate in various directions, including behind the atom. In that case, the molecule can in fact end up behind the atom."
Rotating molecules
The theory of this phenomenon has been known for a long time, and several years ago, this research group was the first to measure the so-called diffraction patterns of the colliding waves behind the atom. In their new publication, the researchers delve deeper into what exactly is happening, and they discovered that there are locations behind the atom where molecules can only end up if they rotate about their axis in a certain way. "We have even been able to determine at which exact angle particles collide to end up there without directly checking or measuring this," says Onvlee.
The physicists achieved this by using a relatively simple model that is also used to explain the diffraction of light and other types of wave patterns. The wave-like properties of the colliding particles play a crucial role in this phenomenon. One of the discoveries the researchers made was that the spin direction of the molecules after the collision, left- or right-handed, depends on the parity of the molecules: a quantum mechanical property that says something about how properties of the particle change when it is observed in a mirror.
"These results contribute to a better understanding of extremely complicated chemical processes, which occur in space, the atmosphere and combustion engines, for example," says Onvlee.
Additional information
The research was partly funded by the FOM-Projectruimte. First author Jolijn Onvlee won the Poster Prize last year during Physics@FOM Veldhoven 2015 for her poster on this subject. Co-author Sjoerd Vogels is a FOM employee.
Reference
Imaging quantum stereodynamics through Fraunhofer scattering of NO radicals with rare-gas atoms, Jolijn Onvlee, Sean D.S. Gordon, Sjoerd N. Vogels, Thomas Auth, Tijs Karman, Bethan Nichols, Ad van der Avoird, Gerrit C. Groenenboom, Mark Brouard & Sebastiaan Y.T. van de Meerakker, Nature Chemistry, 31 October 2016.