The motion of an ion in a Penning trap can be driven by radio frequency field of appropriate multipolarity and orientation. The fields are created by applying RF voltages between the trapping electrodes or segments of them.
Dipole fields: They can be used for exciting the ion motion at the Eigen frequencies. If the initial amplitude was zero then a linear increase is observed as a function of time. This kind of excitation is for example employed to selectively remove unwanted ions from the trap or for parking ions on specific orbits.
Quadrupole fields: They excite the ion motion at sums or differences of two Eigen frequencies. Of particular importance for mass spectrometry is the excitation of the ion motion at the sum of magnetron and reduced cyclotron frequency. This sum equals the cyclotron frequency of an ion in a pure magnetic field. The response of the ion motion to quadrupole excitation is always non-linear. In the case mentioned here a periodic beating between magnetron and cyclotron motion can be obtained.
Example: Resonant transformation of a pure magnetron motion into a pure cyclotron motion via quadrupole excitation at the ion’s cyclotron frequency. (The cross indicates the axis of the trap).
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