Web Date: January 24, 2007
Synchrotron For Neutral Molecules
Conventional synchrotrons can be used to study only high-energy charged particles. Physicists in Germany have now constructed a new type of synchrotron that can be used to investigate low-energy neutral molecules.
The device, known as a molecular synchrotron, could be used to study collisions between neutral molecules. It was developed by molecular physicist Gerard Meijer, a director at the Fritz-Haber Institute of the Max Planck Society, in Berlin, and his coworkers (Nat. Phys., DOI: 10.1038/nphys513).
Conventional synchrotrons, some of which are massive machines with circumferences of hundreds of meters, rely on electric and magnetic fields to accelerate the particles around the synchrotron ring. The molecular synchrotron, which has a circumference of just 80 cm, uses electric fields to confine packets of low-energy neutral molecules and move them around the ring.
The molecular synchrotron consists of two hexapoles—hexagonal tubes made of six electrodes—inside of which molecules can be stored under vacuum. The hexapoles are bent into semicircles and used to form a circle broken by two 2-mm gaps. To test the device, Meijer's team injected deuterated ammonia molecules, ND3, into one of the hexapoles.
Some molecules drift farther ahead in the hexapole, while others lag behind, Meijer explains. Because the molecules have an electric dipole moment, their movement can be synchronized by applying different voltages to the electrodes and switching the voltages as they move through the gaps. The switching accelerates slow molecules, decelerates fast ones, and therefore bunches them up into packets of molecules with synchronized position and velocity.
The team showed that the packets are about 3 mm in length and remain so, even after completing 40 round trips.
"Inside the ring, the neutral molecules in the circulating packet are all in the same internal quantum state, and they all have the same orientation in space," Meijer says. "As they all move with the same velocity, the temperature of the packet, which is determined by the velocity spread, is very low—around 1 mK."
A laser directed between the gaps is used to detect the molecules stored in the device. The laser ionizes the molecules, which are then extracted from the ring and counted by an ion detector.
The Berlin physicists also showed that it is possible to bunch and store two, and in principle more, packets of molecules in the ring. Their aim is to send packets in opposite directions around the ring so that some of the molecules collide. The authors point out that the sensitivity for detecting collisions increases by orders of magnitude as the numbers of packets and round trips increase. For instance, in a ring containing 10 packets revolving in both directions, a packet having completed 100 round trips will have had 2,000 encounters with other packets, they note.
The work is exquisite, says Dan Stamper-Kurn, an associate professor of physics who leads a research group on ultracold atomic physics at the University of California, Berkeley. "Like in particle physics, even if the collision probability per intersection of the two packets is low, by passing these packets through each other repeatedly, one can probably obtain a useful collision signal," he comments. "The study of very low energy reactive scattering is bound to be fascinating, representing the beginnings of exploring chemistry at zero temperature. The development by Meijer's group brings us yet closer to this goal."
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