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FIRSTHAND KNOWLEDGE MOLECULAR PHYSICS Molecules in Slow Motion In the microworld, slow means cold. Research on decelerated, ultracold atoms has recently yielded spectacular results – such as Bose-Einstein condensates, which have already become indispensable for quantum physics. Cold molecules should also have similar potential, but unfortunately, they are very difficult to produce. The group working with GERARD MEIJER, Director at the FRITZ HABER INSTITUTE OF THE MAX PLANCK SOCIETY in Berlin, has High-tech with sophisticated design: The Stark decelerator in Berlin (opposite page) slows molecules down. Afterwards, they reach the trap that Bas van de Meerakker is adjusting (above). P hysicists seem to have a penchant for operations at full throttle: they use huge particle accelerators to spin components of all kinds of matter to the fastest speeds possible to make them collide. The subatomic traces of such collisions have actually been providing valuable information about the building blocks of our cosmos for decades. 50 MA X P L A N C K R E S E A R C H But many physicists are also fascinated by extreme slowness. At the Fritz Haber Institute of the Max Planck Society, for example, we are working on techniques for decelerating molecules efficiently. The molecules we use come from a jet of highly diluted gas. In a “normal” warm gas, most molecules zip around so quickly that only a supersonic jet 1/2007 could keep up. Our device allows us to slow them down and actually get a proper grip on them. Technologically speaking, this is no mean feat. But the effort is worth it, because we can conduct some fascinating basic research on the slow molecules. To slow them down, we use methods developed in accelerator physics. We apply them with reverse effect, so P HOTOS : N ORBERT M ICHALKE developed an efficient system for slowing down molecules, and reports on it in this article. we transform an accelerator into a trap. The slow molecules can be stored and studied very closely, looking, for instance, at the quantum properties of their complex electron shells. Or we use them to conduct further experiments, such as slowmotion soft collisions. In physics, as soon as small particles are involved, slowness is noth- ing other than cold. High temperatures occur when the atoms or molecules in gases or liquids move quickly – or oscillate back and forth rapidly in solid matter. For example, at room temperature, nitrogen molecules whiz through the air at an average speed of 500 meters per second, and are thus faster than sound, which travels 340 meters per second. So heat is kinetic energy. If we take it out of these molecules by “slamming on the brakes,” then we automatically cool it down. In the case of atoms in dilute gases, physicists have since become very adept at such braking techniques. In the lab, they routinely achieve extremely low temperatures that would have still been pure science fiction 20 years ago. They are just a few millionths of a degree above absolute zero, which is minus 273.15 degrees Celsius. The consequences of this progress for science are very exciting, and sometimes completely unforeseen. To create ultracold atom clouds, the experimenters combine the brake effect that laser light can exert on atoms with magnetic fields that confine the cold atoms. Such magnetooptical traps have been used successfully for a good 10 years to produce, for example, the famous Bose-Einstein condensates. A Bose-Einstein condensate is a collective quantum state in which a cloud is “condensed” from certain types of atoms when they are cooled to extremely low temperatures. What fascinates physicists about this trapped quantum collective is the property that it is completely freely accessible and manipulable from practically all sides. GOLD RUSH IN QUANTUMLAND For this reason, it is much easier to research than other multi-particle quantum states: supraconductivity, for example, hides in the interior of metals, which is very difficult to access. That is why the first successful 1/2007 MA X PL A N C K R E S E A R C H 51 MOLECULAR PHYSICS PHYSICAL BRAKEBOXING ONLY WORKS WITH ATOMS First, the molecule hits the electric field of a braking element (top). Then it must climb a potential hill and slows down until it reaches the peak (center). Going downhill would cause it to accelerate again, but that is prevented by turning off the electric field in due time (bottom). The trap arrests the decelerated molecule (gray cloud) with its electric field like a pillow (left). Now the molecule is confined, with the field acting like a funnel (right). 52 MA X P L A N C K R E S E A R C H So it is not surprising that physicists no longer want to limit this exciting game to atoms. Ultracold molecules may promise even more thrilling discoveries. But it is much more difficult to cool molecules down to extremely low temperatures than it is with atoms. Unfortunately, the laser cooling that has proven so effective with atoms usually fails. To put it simply, the atoms, with their electron shells, swallow the light quanta (photons) from the approaching laser beam. Later, they emit these photons again, usually in any direction at random. On the whole, the laser beam slows down the atom like a boxer deters a charging opponent with targeted punches, while the latter – the atom – merely flails about with no effect. Unfortunately, this physical brakeboxing doesn’t usually work with molecules. They are made up of multiple atoms and have far more complex electron shells. Inside these, they convert the pulses of the impinging laser photons differently than atoms do, and they hardly slow down at all. So we had to take a different route to develop an effective molecule brake. At present, only a few techniques are suitable for this. In Berlin, we use a small particle accelerator that we operate in reverse as a particle decelerator. We are now able to use it to cool molecules down to a few thousandths of a degree above absolute zero. Our device is called the “Stark decelerator,” in memory of Johannes Stark. The German Nobel laureate in physics in 1919 discovered the effect that our anti-accelerator exploits to elegantly solve a further problem: accelerators work only on electrically charged particles. This 1/2007 also applies, in principle, to the decelerators derived from them. But we want to slow down electrically neutral molecules because they are of greater interest to us. And this is where the Stark effect helps. Normally, physicists feed their accelerators with atoms, whose incomplete electron configuration creates an electric charge, or with electrically non-neutral elementary particles. Due to their charge, they sense the accelerator’s electric field. They surf along on this field as if on a wave, continuously gaining speed in the vacuum tube – or losing it in the decelerator. However, the Stark effect allows us to also draw in neutral molecules with strong electric fields. This is because, in many molecules, the spatial distribution of the electric charges is somewhat irregular. These “polar” molecules behave like tiny antennas that react to the electric field of the Stark decelerator. A RIDE ON THE NANO-CANNONBALL However, their interaction with the electric field is much weaker than in the case of electrically charged particles. Depending on the molecule, it can be just one ten-billionth of the normal value. Nevertheless, with our Stark decelerator, we are able to get the molecules completely under control. The actual molecule brake here is only slightly more than one meter long. So compared with the usually enormous particle accelerators, which, however, work at a much higher energy level, our experiment is very manageable. We developed it at the FOM Institute for Plasma Physics Rijnhuizen in Nieuwegein, The Netherlands. Since late 2003, we have continued this work at the Fritz Haber Institute, which offers ideal research conditions. To better understand how the Stark decelerator works, let’s conduct a little thought experiment. We pick out a molecule from the gas stream and sit on it, much like Baron Münchhausen on his cannonball. We now jet around on our nano-projectile at a D IAGRAMS : F RITZ H ABER I NSTITUTE creation of Bose-Einstein condensates in 1995 practically triggered a “gold rush.” Pioneers Eric Cornell, Carl Wieman and Wolfgang Ketterle were even awarded the Physics Nobel Prize for it in 2001 (MaxPlanckResearch 1/2002, page 62 ff.). few hundred meters per second toward the entrance of the Stark decelerator. We see it from the front like a kind of metallic hash mark into whose central opening we shoot. In doing so, an increasing force gently brakes our molecular cannonball. Inside the hash mark, we see that its metal rods are arranged one behind another. At that moment, the braking action suddenly ceases. Our cannonball speedometer tells us that we have lost approximately 1 percent of our speed. The next metal cross is racing toward us, where the process repeats itself. In the end, after a good one hundred such crosses, we are nearly standing still. To catch the molecule at just the right moment when flying through the next deceleration element, the controls of the Stark decelerator send short pulses of high-voltage electricity through all of the elements in succession. The pulses jump along this cascade like a running light that grows continuously slower. Now, if a polar molecule hits a brake element at the right moment, then the briefly increasing electric field acts like a small hill of pure energy. The molecule becomes slower – as if it had to roll up a virtual hill to reach the “energy peak” at the center of the element. However, when flying out, it would roll down the other side of the virtual hill. It would be just as fast at the end as it was at the beginning, and nothing would be gained. To prevent this, the field shuts down as soon as the molecule reaches the energy peak. The hill disappears right out from under the molecule’s feet, as it were, and it can’t accelerate again. Now the question remains how we get a molecule to accurately hit a pulse applied at the entrance of the Stark decelerator. Very easily: our gas beam is likewise pulsed. It consists of molecule packets whose arrival we coordinate with the decelerator’s voltage pulses. Consequently, at the end of a braking cycle, we end up with not just one molecule, but an entire group of molecules at the desired speed. We can continu- P HOTO : N ORBERT M ICHALKE FIRSTHAND KNOWLEDGE Molecular physicists in action: While Steven Hoekstra, Bas van de Meerakker (right front) and Joop Gilijamse (center) work on the experiment, Ludwig Scharfenberg (left) is busy building the two new, intersecting Stark decelerators. ously adjust this final speed between about 700 and just a few meters per second. The lowest speed currently attainable corresponds to a temperature of about ten thousandths of a degree above absolute zero. Our technique has helped us successfully decelerate various types of molecules: carbon monoxide (CO), diatomic hydroxyl (OH) and ND3, which consists of nitrogen (N) and three heavy hydrogen atoms (deuterium, D). TRAPPED IN THE ENERGY FUNNEL To experiment further with the slow molecules, our lab has various apparatuses that we can connect behind the Stark decelerator. One is a small storage ring – another device borrowed from particle physics. We can park the prepared molecules in an orbit in this ring before we use them in further experiments. Unlike in particle physics, our storage ring is not kilometers in size, but a mere 80 centimeters around. Unfortunately, the molecules in a packet never have exactly the same speed. Thus, with every revolution around the ring, the packet drifts further and further apart. To increase the “parking time,” we built into the ring a segment that uses specially formed fields to push the molecule packet back together with each revolution. This works so well that we can now also store multiple packets in succession in the ring. However, the molecules still require a certain, not-too-low residual speed to remain in orbit. For ND3, for instance, this is around 100 meters per second. So if we want to store very slow, cold molecules, we need another technique. To this end, we use a special molecular trap. Using an electric field, it captures the decelerated molecule as if in a pillow. As soon as the molecule is properly confined, the trap switches its field to take on the form of an energy funnel. The molecule is then trapped in this funnel and can only wobble back and forth at a few meters per second. When this happens, its temperature can drop to about one fifty-thousandth of a degree above absolute zero. To date, we have caught ND3 and the OH radical with the trap. The OH is of particular interest to us: in principle, it is a water molecule (H2O) that has lost a hydrogen atom (H). This makes it highly chemically reactive. OH plays an important role in 1/2007 MA X PL A N C K R E S E A R C H 53 54 MA X P L A N C K R E S E A R C H DR. BAS VAN DE MEERAKKER, 29, is head of a research group at the Fritz Haber Institute of the Max Planck Society in Berlin. He researches cold free radicals and experiments with Stark decelerated molecular beams. He completed his doctoral studies at the University of Nijmegen in The Netherlands and was a visiting scientist at Sandia National Laboratories in Livermore, USA. 1/2007 P HOTO : P RIVATE PROF. GERARD MEIJER, 44, is one of the pioneers of the Stark deceleration technique. He became a professor in Nijmegen in 1995 and Director of the FOM Institute for Plasma Physics Rijnhuizen in Nieuwegein in 2000. There, he launched the research program on cold molecules and conducted physical experiments on molecules in the gas phase with the FELIX free electron laser. He has been Director of the Department of Molecular Physics at the Fritz Haber Institute of the Max Planck Society since 2003. P HOTO : P RIVATE P HOTO : P RIVATE only high-energy molecular collisions had previously been accessible for their experiments. Such brutal collisions conceal more subtle processes that occur while the molecules are approaching one another. With our decelerated molecules, we could now play with the finesse of a master pool player. During our tests, the xenon atoms always maintained the same speed. At the same time, Steven Hoekstra makes some adjustments to the first part of we used the Stark decelthe vacuum machine that produces the beam of OH molecules. erator to vary the speed of the OH molecules between 30 and 700 meters per second. We then observed nature, for example in cosmic gas that, at certain threshold values, the clouds and in the chemistry of the OH molecules reacted differently to Earth’s atmosphere, as well as in the collisions with the xenon atoms: combustion processes. they no longer bounced off elastically OH also recently helped us conduct like billiard balls, but rather absorbed another fundamental experiment a portion of the impact energy. (J. Gilijamse et al., Science 2006, vol. 313, 1617). For this, we intersected MOLECULAR BILLIARDS the path of the decelerated OH molWITH GREAT FINESSE ecules after they left the Stark decelerator with a beam of xenon atoms. That is what theoretical chemist The OH molecules look roughly like Gerrit Groenenboom from Nijmedumbbells with a list. They can theregen had predicted. According to his fore rotate at various speeds. This roquantum theoretical calculations, tation speed can change only in inat these threshold values, the total crements, as it is quantized according energy of the collision is just to the laws of quantum physics. Alenough for the striking xenon to though this is old hat for physicists, stimulate the next higher energy quantum of the OH rotation. We obtained excellent confirmation of Groenenboom’s predictions. We clearly understand the physics of such impacts quite well. However, in this collision experiment, one of the partners in the impact was still an atom, and a hot one at that. In the future, we want to have two molecules that are both truly cold collide gently. To do this, we must also send the intersecting molecular beam through a second Stark decelerator. We hope that these slow-motion molecular collisions will give us finely detailed information about the quantum structure of their electron shells. From this we hope to learn, for example, how stable molecular complexes, which play an important role in nature, are created. With two Stark decelerators, we can also study how chemical reactions between the cold molecules proceed. We know that their low energy will then just barely suffice for a reaction. Physical chemistry predicts that, in this case, quantum effects should dominate the behavior of the reaction partners. But no one can yet predict precisely what will happen. Perhaps there will be a reward for the long years of development work – one that is as spectacular as the discoveries made with ultracold atomic gases. But we do not yet know where the discovery of molecular slowness will take us. We are just setting off into this unknown world. ● ROLAND WENGEN45, is a physicist, science journalist and editor of the magazine PHYSIK IN UNSERER ZEIT. For him, research is an adventure and the most important driver of modern culture. He believes that as many people as possible should be able to participate in it through popular-scientific texts. To this end, the illustrator in him also likes to resort to pen and paper. MAYR, P HOTOS : N ORBERT M ICHALKE FIRSTHAND KNOWLEDGE