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.
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.
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