A time-of-flight method for studying the properties of an ion cloud stored in an rf trap

Abstract

We report here on an experiment undertaken to obtain information on the spatial and velocity distributions of ions within a radio frequency quadrupole trap. A time-of-flight technique was used in which the progression of the rf confinement voltage was suddenly arrested, thus allowing the ions to escape axially. The measured profile of the ion time of flight is related to the ion distributions in position and velocity at the moment when the confinement voltage ceases to change (the time or phase of ejection). We have developed a general procedure for calculating the time-of-flight profile for any given spatial and velocity distribution of the ions within the trap. Although, in general, the solution involves a numerical integration, we find that, for the case of Gaussian distributions, this calculation can be carried out analytically. Shifts in the time-of-flight profile due to changes in the phase of ejection and in the mean ion energy are calculated. These results are compared with experimental measurements made on trapped Xe + ions in the presence of helium buffer gas.