For the first time, scientists have succeeded in controlling molecules so precisely that the quantum effects of a collision between molecules can be measured. The results will be published in Science on November 23rd. The ar [bw] ticle also provides a theoretical model of the interaction between the OH (hydroxyl) and NO (nitrogen monoxide) molecules used. Molecule collisions were previously thought to be too complex to model.
Collisions between molecules play an essential role in the atmosphere, in combustion processes, and even in interstellar space. Even so, very little is known about exactly what happens when two molecules collide with each other, even in the case of simple molecules consisting of just two atoms. Physicists have up to now been unable to control molecules sufficiently to allow them to undergo a controlled collision with each other.
This has now been made possible, using a decelerator. A decelerator is a piece of equipment that gains full control over the motion of molecules by switching electrical fields on and off. It controls both the forward motion and the speed with which the molecules rotate about their own axis. In this case, the OH radical was used. If a collision then takes place (with similarly controlled NO radicals), the molecules may, as a result, rotate at a different speed. The experiment published in Science shows with great precision the extent of rotation that takes place at a particular collision energy.
The method was developed by a team from the Institute for Molecules and Materials (IMM) at Radboud University Nijmegen, the Fritz Haber Institute in Berlin and the Institute of Atomic and Molecular Sciences in Taipei. The experiment was carried out in Berlin, under the leadership of molecular physicists Bas van de Meerakker and Gerard Meijer, currently President of the Executive Board of Radboud University Nijmegen. Van de Meerakker, the main author of the article, has now returned to the Netherlands to set up his own group with the help of an NWO Vidi grant, and his Nijmegen laboratory also has a decelerator.
Van de Meerakker and Meijer achieved another world first in 2006 when they managed to get fully-controlled molecules to collide with atoms. However, collision experiments between two molecules are much more complicated.In the experiment, just 1 in about 100,000 molecules changed its rotation speed. Also, according to quantum mechanics theory, not all rotation speeds are possible. A comparison with the quantum calculations made by Gerrit Groenenboom - professor of Theoretical Chemistry in Nijmegen - provided revealed surprising results: the extent of rotation that takes place appears to already be determined when the molecules are still quite far apart.
The researchers are already working on the next step: as well as the change in rotation, they would also like to measure the change in direction of the colliding molecules. ‘This would provide even more precise information about the interaction between molecules. And that's the kind of data that theoretical physicists really want to get their hands on,' explains Van de Meerakker. ‘Astrophysicists are also very interested in the directional effects of radical collisions. OH is found in interstellar space, where it is so cold that the quantum effects that we measure here using the decelerator could take place.'