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Abstract

When extracting ions from a quadrupole ion trap into a time-of-flight (TOF) mass spectrometer, oscillations are observed in the intensity and arrival time of the mass-selected ions as a function of the time delay between laser induced ion creation and extraction. The oscillation frequency is inversely proportional to the ion mass, suggesting that the effect is caused by the secular motion of the ions. Based on trajectory simulations, we show that the oscillations are likely due to an asymmetric distribution of initial positions and velocities in phase space, which is caused by the partial overlap between ion cloud and laser beam. Subsequently, this non-perfectly matched phase-space distribution evolves coherently. This is shown to give rise to oscillations in the ion signal at twice the secular frequency, in agreement with observations.