Space-charge effects on Fourier transform ion cyclotron resonance signals: Experimental observations and three-dimensional trajectory simulations

Space-charge effects were studied by monitoring Fourier transform ion cyclotron resonance spectra while scanning the laser wavelength near the origin of a two-photon resonant 3s ← n Rydberg transition of acetaldehyde. The rotational contour of the origin band permits the experimental control of space-charge density. Both the frequency shift and the inhomogeneous line broadening were observed as a function of space-charge density. Three-dimensional ion trajectories in the presence of Coulomb interactions between ions were simulated under the quadratic and exact trapping potentials. The simulated Fourier transform ion cyclotron resonance spectra were obtained from the image-charge signals induced by a uniform field of chirp or impulse excitation. Comparisons of experiments with three-dimensional simulations reveal that the inhomogeneous line broadening observed in experiments is most likely due to both large-amplitude oscillations of ions and Coulomb interactions between different m/q ions.