Storing keV Negative Ions for an Hour : The Lifetime of the Metastable P 21 / 2 o level in S 32 −

Negative ions (anions) play an important role in chemical reactions in interstellar space and planetary atmospheres. Molecular anions were, for example, observed in the ionosphere of Saturn’s moon Titan, and their collisions with neutral atoms and molecules are thought to affect the production of unsaturated carbon and nitrogen chain molecules in the interstellar medium [1]. Laboratory experiments with anions are, however, difficult, as the ions are easily disturbed by background radiation or collisions with other particles. These effects can be mitigated in ion traps that have radiation shielding and extremely low pressures. But ion trapping is limited to experiments with ions at rest, while many anion interactions of interest are better studied when the ions move at high speeds. Using a new cryogenic ion-storage ring, Erik Bäckström and colleagues at Stockholm University, Sweden, have successfully stored a 10-kilo-electron-volt (keV) beam of negative sulfur ions (S−) for, on average, 1000 seconds (s). The long storage time allowed them to measure a natural decay time of 503 ± 54 s for the only excited bound state of the S− ion [2], the longest lifetime measurement of a negative ion in a stored beam. The new storage machine, and similar facilities planned around the world, will allow scientists to study negative ions with lifetimes of minutes or hours—such as the atomic and molecular ions found in planetary ionospheres and astrophysical environments.