The word synchrotron conjures up images of gargantuan circular machines that accelerate subatomic particles to within an iota of light speed. But a new type of synchrotron fits on a dinner plate and can pull off a trick that its behemoth brethren cannot: The new ring accelerates electrically uncharged molecules--albeit to a pokey 87 meters per second. The clever device could help chemists and physicists decipher the interactions between colliding molecules.
A conventional synchrotron exploits the fact that charged particles curve in a magnetic field. Within the machine, a magnetic field keeps bunches of particles circulating through a long tunnel-like vacuum chamber, like so many boxcars on a circular train track. Each lap, electric fields give each bunch a push so that it gains energy. This technique works for charged particles ranging from wispy electrons to hefty lead nuclei. But it won't work for neutral atoms and molecules, which zip straight through a uniform magnetic field.
Forgoing magnetism, physicists Cynthia Heiner and Gerard Meijer of the Fritz Haber Institute in Berlin and colleagues employed spatially varying electric fields to latch onto neutral molecules. If a molecule is electrically polarized--meaning that positive charge tends to accumulate at one end and negative charge at the other--then it may feel a tiny tug toward places where the electric field is weakest. Taking advantage of this fact, Meijer 6 years ago devised a ring of six electrodes that produced a doughnut of electric fields with a line of zero field running around its middle. That "storage ring" trapped electrically polarized ammonia molecules.
The problem with the device was that the molecules spread out to fill the entire ring. To solve that problem, Heiner and Meijer had to find a way to keep the particles tightly bunched. First, they split the ring to form two c-shaped halves. They vary the voltages on the two halves so that when a bunch of ammonia molecules flies across the gap between them, the first molecules run into an electric field, which slows them down. The voltages on the electrodes then quickly switch to push the stragglers across the gap so they speed up to the others, they report in the January issue of Nature Physics. "We can keep a bunch of molecules together essentially indefinitely," Meijer says. Because they all move at nearly the exact same speed, the molecules in the 3-millimeter-long bunch have a temperature of less than a thousandth of a kelvin.
The advance is "a very nice step" in the larger enterprise of manipulating very cold gases of molecules, says Daniel Neumark, a chemist at the University of California, Berkeley. Such gases could allow researchers to perform exquisitely precise spectroscopy experiments or probe molecular collisions, which at low temperatures must be described as the collision of quantum waves. He adds that the dainty synchrotron also has a "very high aesthetic appeal."