  We have developed a novel scheme to measure the secular motion of trapped
ions. Employing pulsed excitation and analysis of the fluorescence of laser
cooled ions, we have measured the centre-of-mass mode frequency of single as
well as entire ion crystals with a frequency precision better than 5e-4 within
an interrogation time on the order of seconds, limited only by the fluorescence
collection efficiency and the background noise. We have used this method to
measure the mass of ions and observed charge exchange collisions between
trapped calcium isotopes.
