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Bas van de Meerakker: ‘We are doing chemistry with only two molecules’

Interview with Prof. Bas van de Meerakker (Spectroscopy of Cold Molecules)

Bas van de MeerakkerLast year Bas van de Meerakker has been appointed professor at the department Spectroscopy of Cold Molecules. He is studying molecule collisions, a process that happens all around us, in environments ranging from the earth’s atmosphere to interstellar space, but is still poorly understood. Van de Meerakker is very pleased with his new position. ‘Since we have our department now, we can more than ever manage our own research and people. More important, I see this as a signal from the faculty. They support our research and want to keep it for the future.’

The work of a professor varies a lot from day to day, Van de Meerakker says. He spends days reading and (co)writing scientific articles, teaching students, attending conferences and talking to the people of the department. Time for doing research himself is increasingly sparse. ‘Only six years ago I started in a practically empty lab. We had to develop and build the machines from scratch,’ he says. ‘But quickly I had so many tasks that working in the lab was becoming difficult. Now, I don’t have time to do labwork myself. But I really want to understand all the work that is done in the lab. I want to keep that connection.’

When two molecules collide

In short, Van de Meerakker and colleagues want to know what happens when two molecules collide. ‘Fundamentally, this process is still not understood’, he says. ‘Take two oxygen molecules in our atmosphere, nobody knows what happens exactly when they interact with each other. The problem lies in the many degrees of freedom of molecules, there are infinite ways in which they can interact. For example, after a collision molecules can start vibrating or turning. Even the best theorists cannot solve this problem exactly.’

wave function of colliding nitrogen oxide (in red white) and a helium atom (yellow). (Credit: Sjoerd Vogels)As one of the first in the world the research group of Van de Meerakker is trying to tackle this problem practically. They smash molecules like nitrogen oxide, hydroxyl radicals or ammonia together. Now you might think of CERN, where new particles are being discovered by smashing ions together at enormous energies. In a way, Van de Meerakker tries to do the same. The big difference is he wants to collide neutral molecules instead of ions, and at the lowest possible energy. At low energies subtle quantum effects can be studied in great detail. To perform slow or ‘cold’ collisions four so called decelerators have been built in his laboratory.

But how can you keep track of a single molecular collision? This requires a difficult experiment with meticulous timing, Van de Meerakker says. ‘In our decelerators we shoot bundles of many molecules towards each other. When they cross there is a small chance of about one percent that two molecules within the bundles interact. When they do, they are directed towards a screen that records the impacts of the collided molecules. The points of impact tell us a lot about what happened during the collision. By repeating this experiment many times we derive a detailed image of the interactions.’

A fraction above absolute zero

The experiments of Van de Meerakker enable detailed testing of the theory for the collision of molecules. ‘Most models make use of approximations. Now we can check which approximations work best’, he says. ‘On top of that, testing of these models can tell us something about how molecules behave in chemical reactions. We collaborate with the very best theoreticians in the world in this area. Together, we can now unravel details of collision processes that were unthinkable only a few years ago.’

The energies of the collisions are measured in temperature. Now, the experiments are done at around ten kelvin, minus 263 degrees celsius. Van de Meerakker wants to go even lower. ‘Soon we will go to one kelvin, a fraction above absolute zero. That means quantum effects will start to dominate the collisions. That means the particles behave more like waves. This is where things start getting really interesting.’

Also, at lower energies the researchers are able to actively interfere with the collision by applying a magnetic field. ‘For example, you can add energy during a collision’, Van de Meerakker says. ‘Not only can we map the interaction but also manipulate it. I think we’re coming very close to the holy grail of chemists, to directly control molecule interactions. We’re doing chemistry with only two molecules! I am a physicist by the way, but we are all working exactly on the border of chemistry and physics. We see a lot of chemistry, physics and science students. They all fit very well.’

Science fiction weaponry

The shiny and polished decelerators in the lab of Spectroscopy of Cold Molecules look like a weapon out of a science fiction movie. Van de Meerakker explains they consist of 300 metal rods that apply an electric field to the molecules inside the machines. The decelerator path is typically a few metres long and the polishing is not for aesthetic reasons. ‘We are working with very high voltages’, Van de Meerakker says. ‘If we would not polish parts of the machine the 40.000 volts we apply could easily cause sparks.’

Part of one of the decelerators. (Credit: Ludwig Scharfenberg)Van de Meerakker is proud that the techniques his lab is using are mainly inventions done in Nijmegen. ‘The deceleration technique was invented here by Gerard Meijer in 1997. The laser technique for imaging was developed by David Parker’, he says. ‘Both techniques are very important for our experiments and it’s great this is a product of local science.’

Exporting machines

Next to the three electrical decelerators a magnetic version has just been built. It will guide the molecules with magnetic fields. This will open up new research possibilities. ‘We can do collisions with oxygen, a common molecule that is strongly magnetic’, Van de Meerakker says. Next to the expansion of their own lab he also would like to ‘export’ the decelerators to other research groups, which has already happened with a group in Basel. ‘We really like to see other researchers getting involved in the field. Competition is good, it keeps you sharp. And there is still so much to discover in this field. We simply cannot do everything ourselves.