Negative Ions in Cold Storage

Abstract

A cooled ring stores high-speed negative ions for more than 1000 seconds and enables new studies of atomic and molecular ions that are important in interstellar and atmospheric chemistry. Negative ions (anions) play an important role in chem-ical reactions in interstellar space and planetary atmo-spheres. Molecular anions were, for example, observed in the ionosphere of Saturn's moon Titan, and their col-lisions with neutral atoms and molecules are thought to affect the production of unsaturated carbon and nitro-gen chain molecules in the interstellar medium [1]. Lab-oratory 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 interac-tions 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. The formation of an anion (binding of an electron to a neutral atom) and a cation (removal of an electron) in-volve different forces. To form an anion, a neutral atom binds an excess electron because the " intruder " electron polarizes the atom's electronic shell. This distortion gen-erates a dipolar electric field that, in turn, attracts the electron. In general, the stability of a negative ion de-pends on the electron cloud's charge distribution and, in particular, on electron–electron correlation effects. These interactions are small compared to the direct electrostatic