When ions of slightly different mass and the same charge are stored in a radio-frequency electric quadrupole trap, a collective oscillation is observed rather than the independent motions usually assumed. A high-temperature model of ion clouds, each characterized by ions of one mass, shows that the ions of each cloud move as a unit and exhibit a correlated motion with the other clouds when a strong coupling condition is satisfied. This phenomenon is expected to arise whenever more than one ion is trapped and might be observable in neutral atom traps as well.

• Received 15 July 1987

DOI:https://doi.org/10.1103/PhysRevA.36.3451