Laser-induced fluorescence for ion tomography in a Penning trap

Laser-induced ion fluorescence of laser-desorbed Ba⁺ ions provides a measure of the relative number of ions near the center of the Penning trap of a Fourier transform ion cyclotron resonance mass spectrometer. Here, we report the detection of Penning-trapped ions by ion fluorescence, subject to radially outward ion cloud expansion (because of ion-neutral collisions), radially inward ion cloud compression (because of quadrupolar axialization), and the effects of buffer gas pressure and electrostatic trapping potential on those processes. At high pressure and high trapping voltage, radial ejection is far more rapid than axial ejection; quadrupolar axialization increases the number of ions near the center of the trap as well as the length of time that ions may be trapped; higher pressure results in faster magnetron radial expansion; and the choice of azimuthal quadrupolar excitation waveform significantly affects the efficacy of axialization. Based on these results, we suggest that directly detected laser-induced ion fluorescence provides a general new tool for mapping the ion distribution and its time evolution in response to various excitatory and damping effects.