Individual and center-of-mass resonances in the motional spectrum of an electron cloud in a Penning trapP. Paasche, T. Valenzuela, D. Biswas, C. Angelescu and G. WerthEur. Phys. J. D, 18 3 (2002) 295-300DOI: https://doi.org/10.1140/epjd/e20020033