Visualisation of the collisions between NO and helium (left) and NO and neon (right). The top images show the research results of Van de Meerakker and his colleagues; the bottom images show the visualised theoretical predictions of the collisions. Red indicates a high probability of change of direction; blue indicates a low probability. The small peaks show the diffraction oscillations
The project supervisor, Bas van de Meerakker, was also the first to achieve controlled collisions between two molecules and produced images of collision resonances during his stay as visiting professor in Bordeaux. While carrying out this research, Van de Meerakker and his colleagues detected diffraction oscillations – fluctuations in the angular distribution of the molecules – following a collision. These diffraction oscillations provide a unique image of the collision process. ‘The changes in direction and, in particular, the oscillation structure that we observed, have long been predicted in theory. Even though we had hoped to be able to visualise this theory in high resolution using our decelerator, the fact that we actually achieved it is really quite something,’ said Van de Meerakker (see Figure 1).
Ultimate demonstration of wave nature
‘Diffraction oscillations are the ultimate demonstration of the wave nature of molecules and the collision process itself’, said Van de Meerakker. ‘For example, the diffraction of light is a well-known phenomenon that results from interference between light waves. Just like light, atoms and molecules are in fact waves, and can therefore reinforce or cancel out one another as they collide. This results in the oscillation structure that we saw. These data provide us with even more detailed information about the interaction between molecules and, ultimately, help us learn more about nature.’
The publication of their article in Nature Chemistry means that the molecular physicists are yet another step closer to the ultimate molecular collision experiment, in which all variables are controlled. Van de Meerakker’s next step will therefore be to use not one but two Stark decelerators to control both beams of colliding molecules.
The expertise for this research project came from three different departments within the Institute for Molecules and Materials (IMM) at Radboud University Nijmegen. The quantum mechanics theory came from the group run by Gerrit Groenenboom, Professor of Theoretical Chemistry. The velocity map imaging technique was developed by David Parker, Professor of Experimental Physics. The expertise relating to operation of the decelerator came from Van de Meerakker’s Cold and Controlled Collisions Group.