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Cyclotron frequency shifts arising from polarization forces


The cyclotron frequency of a charged particle in a uniform magnetic field B is related to its mass m and charge q by the relationship ωc = qB/m. This simple relationship forms the basis for sensitive mass comparisons using ion cyclotron resonance mass spectroscopy, with applications ranging from the identification of biomolecules1 and the study of chemical reaction rates2 to determinations of the fine structure constant of atomic spectra3. Here we report the observation of a deviation from the cyclotron frequency relationship for polarizable particles: in high-accuracy measurements of a single CO+ ion, a dipole induced in the orbiting ion shifts the measured cyclotron frequency. We use this cyclotron frequency shift to measure non-destructively the quantum state of the CO+ ion. The effect also provides a means to determine to a few per cent the body-frame dipole moment of CO+, thus establishing a method for measuring dipole moments of molecular ions for which few comparably accurate measurements exist4,5,6. The general perturbation that we describe here affects the most precise mass comparisons attainable today7,8, with applications including direct tests of Einstein's mass–energy relationship9 and charge-parity-time reversal symmetry10, and possibly the weighing of chemical bonds7.

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Figure 1: An exaggerated microscopic picture of the polarization force shift of the cyclotron frequency.
Figure 2: The cyclotron frequency ratio R = ωc[N2+]/ωc[CO+].


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We thank E.G. Myers, G.J. Sussman, J. Wisdom and W. Ketterle for useful discussions. This work was supported by the National Science Foundation and a National Institutes of Standards and Technology Precision Measurement Grant. S.R. acknowledges support from the Fonds pour la Formation de Chercheurs et l'Aide à la Recherche.

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Correspondence to James K. Thompson.

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Thompson, J., Rainville, S. & Pritchard, D. Cyclotron frequency shifts arising from polarization forces. Nature 430, 58–61 (2004).

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