Nonlinear effects in the detection of stored ions

Ions stored in a radio-frequency quadrupole trap were subjected to pulses of a small oscillating potential. The frequency of oscillation within each pulse (dipolar or quadrupolar) was variable up to one-half of the rf drive frequency. Ions that were ejected axially from the trap were detected with an electron multiplier. Ion signal intensity was observed as a function of the small oscillating potential frequency for pulsed excitation in each of two time domains in the duty cycle. Thus it was possible to see either the ejection in the z direction, or the ejection in the x or y direction and the z direction. According to the nature of the ejection pulse, the signal shows lines at secular frequencies, at doubled secular frequencies, and other lines that can be sharp and numerous. The influence of space charge is clearly shown. The observed couplings are discussed by taking into account anharmonicity and relaxation mechanisms.