Precision Penning Trap Mass Spectrometry

Polarizability Shifts to the Cyclotron Frequency

A trapped ion undergoing cyclotron motion experiences an electric field in its rest frame. If the ion is electrically polarizable, there is hence a tiny additional (Stark) potential energy, producing a radial force. The leads to a small shift in the ion's cyclotron frequency known as a polarizability shift. The fractional shift is given by

Δf_c / f_c = − α_XX B²/m

where α_XX is the polarizability in the direction transverse to the magnetic field. For atomic ions this is usually negligible. However, for polar molecular ions, i.e., ions with body-frame dipolar moments, the cyclotron frequency shifts can be of order 10^-9, as was initially demonstrated at MIT . This can complicate mass measurements using polar molecular ions. Conversely, the shifts to the cyclotron frequency can be used to measure polarizabilities and dipole moments of polar molecular ions, such as diatomic hydrides.