Signal modeling for ion cyclotron resonance

Abstract

A new signal model, called the rotating electric monopole signal model, is developed for ion cyclotron resonance (ICR) experiments. Unlike prior signal models, the model is applicable to all forms of ICR spectroscopy which use radio-frequency detection methods. The signal model consists of a rotating electric monopole which is shown to be equivalent for signal generation purposes to a rotating electric dipole and for a macroscopic number of ions to a rotating electric polarization. The rotating polarization induces an alternating charge in the plates of the ICR cell and an alternating current between and an alternating voltage across the plates of the ICR cell. A noise analysis is performed, and if amplifier noise is neglected, the ICR sensitivity according to the model is found to be inversely proportional to the ion mass. This is the same sensitivity dependence as is found with conventional ICR power absorption experiments. The ICR voltage signal strength is inversely proportional to ion mass if the monitoring circuit is predominantly resistive and is independent of ion mass if the circuit is predominantly capacitive. The model clearly illustrates the parallelism between signal generation in ICR spectroscopy and signal generation in other coherent radiation spectroscopies such as NMR and microwave spectroscopy.