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Pushing the boundaries of molecular collisions

How do molecules behave when they collide? What are the fundamental processes that underlie molecular interactions? Studying collisions between individual molecules in the gas phase is one of the most fundamental methods to acquire a detailed understanding of chemical interactions. In a joint experimental and theoretical study, scientists within the Institute for Molecules and Materials (IMM) of Radboud University reveal how collisions between dipolar molecules transform from high to very low temperatures. This offers new methods for controlling these collisions using external fields, and new insights on the fundamental quantum mechanical principles underlying a molecular collision event. The results of the study have recently been published in Science.

Advancing collision experiments

So far, controlling both molecules in a collision experiment to study their collisions under well-defined conditions has been difficult. Until now. The research team succeeded to measure collisions between neutral dipolar nitric oxide (NO) and ammonia (ND3) molecules. By precisely controlling their relative motion, they could measure their collision properties at energies tunable by four orders of magnitude. Experimentally, this is a major breakthrough, as thus far only one of the collision partners could be controlled in such experiment. “When we started with these kind of experiments more than 15 years ago, controlling both collision partners was an ultimate dream. But many scientists believed that controlling both would be too difficult, including ourselves”, Professor Bas van de Meerakker says. “But we succeeded to control two beams of molecules, overlap them in space and time, and still have enough molecules in the beams to observe the collision products.“

collisions

Setup used to measure collisions between NO and ND3 molecules. The collision energy was scanned over four orders of magnitude by crossing two beams at 45 or 90°, or merging them at 0°. Right bottom: typical scattering image revealing correlated excitations in both molecules.

The collisions were studied in Van de Meerakker’s lab at the Faculty of Science in Nijmegen, using a 2.6-meter-long Stark decelerator to control the exact speed of one of the beams. Molecules in the second beam were controlled such that they were either crossed with the first beam, or bent tangentially into the first beam’s path. This method allowed the scientists to scan the collision energy, or temperature, from relatively warm at hundreds of kelvin all the way down to about 100 millikelvin, while mapping out all details of the collision event using advanced velocity map imaging techniques. “In our field, getting access to these low energies is very exciting, as these cold polar molecules have fascinating prospects for all kinds of applications in quantum physics, metrology and even quantum computation. But the collision properties of cold polar molecules are not well understood, and experimentally almost unexplored”, Van de Meerakker says.

From hot to ultracold

In the experiment, the scientists observed new and unexpected collision phenomena. At high energies of a few hundred Kelvin, there was sufficient energy for both molecules to start rotating. At energies below 100 kelvin, something very different occurred. Here, the two molecules made “U-turn-like” trajectories around each other under the influence of their interaction. While cooling down further, at energies below 10 kelvin, strong effects of the dipole-dipole interaction were observed. “The behavior we observed at the lowest energies was really unexpected and kept us puzzled for quite some time”, Van de Meerakker says. So far, experts in the field assumed that strong dipole-dipole interactions in cold molecules can only be seen in the presence of an external electric field, as these dipoles need to be induced first by the field. “But in our experiment, we collide without any field. We found that at certain energies, the molecules can polarize each other during the collision event, effectively switching on their dipole moment. This self-polarizing effect can also vanish again as a function of the collision energy, and this is exactly what we observed”. Complex and detailed calculations by Prof. Groenenboom, Prof. Van der Avoird and Dr. Karman from the Theoretical Chemistry department within IMM confirmed the experimental work. “Using the theory, we now completely understand how it all works, and have a clear picture of the mechanisms that cause these U-turn trajectories and the mutual polarization of dipoles at low energies. Moreover, this project allowed us to observe how molecular collisions gradually transform from the semi-classical world at high temperatures to the low temperature regime that is dominated by quantum mechanics”, Dr. Karman says.

Future research

The calculations predict that the mechanisms that were observed for NO-ND3 should be ubiquitous in a large class of collisions between cold dipolar molecules. The low-energy collision properties are predicted to respond extremely sensitively to external electric fields, that could be used to control and steer the collision outcome. “Now that we can study cold dipolar collisions at tunable energies well below 1 kelvin gives enormous potential for new discoveries in the future. We have only just begun.”, Van de Meerakker and Karman conclude.

Article information

Quantum state resolved molecular dipolar collisions over four decades of energy
Guoqiang Tang, Matthieu Besemer, Stach Kuijpers, Gerrit C. Groenenboom,
Ad van der Avoird, Tijs Karman, Sebastiaan Y.T. van de Meerakker
Science (2023)
DOI 10.1126/science.adf9836

Contact information

IMM Communications, Miriam Heijmerink, miriam.heijmerink [at] ru.nl (miriam[dot]heijmerink[at]ru[dot]nl)