NEW SCHEMES FOR RESONANT EJECTION IN RF QUADRUPOLAR ION TRAPS

In order to achieve relative ion number measurement with the best possible accuracy, a systematic investigation of ion detection by resonance ejection has been performed. The first experiment is described here. Ions stored in a radiofrequency quadrupole trap were subjected, for a short time, to a sinusoidal voltage of small amplitude, the frequency of which was variable from zero to half of the r.f. drive frequency. This pulse, of duration 10 ms, was applied to the trap electrodes in either the dipolar or quadrupolar mode. The signal formed by ions ejected out of the trap was observed at two different times which permitted differentiation of ion ejection along the z direction from combined ion ejection along the x, y and z directions. As the frequency composition of the pulse is swept through the above range, the observed ion signals show absorptions at secular frequencies ωx, ωz, doubled frequencies, and combinations of ωx and ωz. These lines show anharmonicities and space charge perturbation. The second experiment attempts to analyze the effect of an initial pulse (or tickle) of a fixed frequency with a second pulse, the frequency of which is variable as described above, in order to explore the spectrum of ion motion under the influence of this tickle. © 1990.