Mutual repulsion normally pushes positive ions apart. But according to research presented in July 30th issue of Physical Review Letters, repulsion may actually help a group of trapped ions cling together. The phenomenon allowed researchers at the Weizmann Institute of Science in Rehovot, Israel, to make extremely accurate mass measurements on the ion group and could also point the way to new, inexpensive mass spectrometers.

The ions were stuck inside a trap containing two electrostatic "mirrors." Depending on the ions masses, they bounced between the mirrors at different speeds: a group of light ions bounced to and fro faster than a group of heavy ones. The researchers hoped to use the frequency of the ion cloud's oscillation back and forth to measure individual ion masses, but they expected to make only limited measurements. They predicted that the ions mutual repulsions, along with differences in velocities and trajectories, would cause the cloud to spread apart after a few oscillations.

That's exactly what happened, until one day they fiddled with the mirrors' voltages and found they could make the ions stick together indefinitely. "Our first reaction was, 'That's impossible,'" Daniel Zajfman, the leader of the group told Physical Review Focus. "Then we had to scratch our heads and try to understand it."

Zajfman and his colleagues now have a theory of what's causing the ions to stay tightly packed. As the cloud travels down the length of trap, it spreads out because individual ions are moving at different velocities. At the end of the trap, electric fields focus the cloud into the electrostatic mirror. Inside the mirror, the fast and slow ions come together and are then pushed apart by their mutual repulsion. During that squish and shove, the fast ions speed up the slow ones and vice versa. So when the cloud comes out of the mirror, all the ions are moving at nearly the same speed. The net effect is that the cloud is repacked each time it reaches the end of the trap.

Because the cloud sticks together over many trips, the team was able to get extremely accurate measurements of its frequency and hence its mass. Zajfman and his colleagues believe that their ion trap can perform as well as the worlds most sophisticated multimillion-dollar mass spectrometers.