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Simulations and experiments on the motion of collections of ions show that impulsive excitation, applied in the axial direction, forces ions into coherent motion with characteristic secular frequencies. A version of the ion trap simulation program, itsim , which accounts for the effects of ion/molecule collisions, is applied to show that this motion is remarkably stable even in the presence of bath gas. Comparisons are made with experimental data on ion motion, specifically with data obtained on ion motion using the d.c.-pump/laser-probe method, and with results of spatially- and temporally-resolved laser tomography experiments. These observations form the basis for a suggested mode of massselective and non-destructive detection of ions in which impulsive excitation, using a d.c. pulse, is followed by detection of the image current. This proposal is tested by Fourier transformation of the simulated data and by simulation of the resulting image currents. Comparisons are made with the results of earlier experiments in which axial excitation is achieved using an a.c. field. |
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