Cooling beams of negative ions

Abstract Negatively charged atomic ions have been cooled in gas-filled rf-quadrupoles from injection energies between 10 and 150 eV to final energies below 0.5 eV. By choosing the atoms or molecules of the buffer gas to have low masses as compared to the masses of the ions to be cooled, the energy transfer in ion–atom or ion–molecule collisions is minimized. For small enough initial ion energies thus only a small percentage of the ions is neutralized in the collision processes. For Cl − and I − ions cooled in a He buffer gas we have achieved absolute transmission efficiencies of more than 50%.