Simulated ion trajectory and induced signal in ion cyclotron resonance ion traps. effect of ion initial axial position on ion coherence, induced signal, and radial or z ejection in a cubic trap

AbstractThe effects of ion initial axial position on coherence of ion motion, induced ion cyclotron resonance (ICR) signal, and radial and z ejection have been evaluated by numerical simulation for a cubic Fourier transform-ion cyclotron resonance ion trap. For a given initial ion cyclotron phase and radius, ions of different initial z position are shown to be excited to significantly different ion cyclotron radii (and ultimately radially ejected at significantly different excitation amplitude-duration products). Ion initial z displacement from the trap midplane affects observed ICR signal magnitude in two ways: (1) for the same postexcitation cyclotron radius, an ion with larger initial z displacement induces a smaller ICR signal and (2) an ion with larger initial z displacement is excited to a smaller cyclotron radius. We also evaluate the induced ICR signal as a function of excitation amplitude-duration product for spatially uniform or Gaussian ion initial z distributions. In general, if the excitation waveform contains components at frequency, 2ωz or ω+ + 2ωz in which ωz is the axial (“trapping”) oscillation frequency, then ejection occurs axially. However, the resulting excitation amplitude-duration product for such axial ejection is significantly higher (factor of ∼4) than that required for radial ejection (at ω+) for ions of small initial radius. The present results offer the first explanation of how, even if the ion is initially at rest on the z axis (i.e., zero excitation electric field amplitude on the z axis), z ejection (axial ejection) may nevertheless occur if the excitation waveform contains frequency components at ω+ + 2ωz and/or 2ωz. Namely, our simulations reveal that off-resonant excitation pushes ions away from the z axis, after which the ions are exposed to z excitation and eventual z ejection