Ion cyclotron resonance
Ion cyclotron resonance is a phenomenon related to the movement of ions in a magnetic field. It is used for accelerating ions in a cyclotron, and for measuring the masses of an ionized analyte in mass spectrometry, particularly with Fourier transform ion cyclotron resonance mass spectrometers. It can also be used to follow the kinetics of chemical reactions in a dilute gas mixture, provided these involve charged species.
Definition of the resonant frequency
An ion in a static and uniform magnetic field will move in a circle due to the Lorentz force. The angular frequency (ω = 2π f) of this cyclotron motion for a given magnetic field strength B is given by
where z is the number of positive or negative charges of the ion, e is the elementary charge and m is the mass of the ion. An electric excitation signal having a frequency f will therefore resonate with ions having a mass-to-charge ratio m/z given by
The circular motion may be superimposed with a uniform axial motion, resulting in a helix, or with a uniform motion perpendicular to the field, e.g., in the presence of an electrical or gravitational field, resulting in a cycloid.
Ion cyclotron resonant heating in the solar wind
On March 8, 2013, NASA released an article according to which ion cyclotron waves were identified by its solar probe spacecraft called WIND as the main cause for the heating of the solar wind as it rises from the sun's surface. Before this discovery, it was unclear why the solar wind particles would heat up, instead of cool down, when speeding away from the sun's surface.
- In SI units, the elementary charge e has the value 1.602×10-19 C, the mass of the ion m is often given in atomic mass unit or dalton 1 u = 1 Da ≈ 1.660538782(83) × 10−27 kg, the magnetic field B is measured in teslas, and the angular frequency ω is measured in radians per second.
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