A simulation study of ion kinetic energies during resonant excitation in a stretched ion trap

The trajectories of ions confined individually in a commercial quadrupolar ion trap of stretched geometry and subjected to resonant excitation have been calculated. During resonant excitation, the average ion kinetic energy increases when the ion secular frequency motion is in phase with the resonant excitation and decreases when the phases are opposed. The temporal variation of ion kinetic energy when subjected to resonant excitation exhibits two cyclical forms, one with low ion kinetic energy and one with relatively high ion kinetic energy. In the low ion kinetic energy cyclical form is characterized by smoothly rounded profiles at time intervals of ca. 2 ms; the high ion kinetic energy cyclical form is characterized by sharply pointed crests at time intervals of ca. 1 ms. In the low ion kinetic energy cyclical form, the maximum instantaneous ion kinetic energy attained is ca. 16 eV and corresponds to axial excursions of up to 3 mm. In the high ion kinetic energy cyclical form, the maximum instantaneous ion kinetic energy attained increases in value to in excess of 70 eV, with an axial excursion of 6.2 mm, as the resonant excitation frequency is increased. At a critical frequency, epsilon(c), there is a discontinuous change from the high to the low ion kinetic energy cyclical form. The maximum instantaneous ion kinetic energy, KE(max), varies as a simple quadratic function of the axial excursion of the ion, so that ion kinetic energy is derived from the main storage field within the ion trap. The asymmetric form observed in the experimental resonance absorption curve, obtained as a function of resonant frequency, has been reproduced qualitatively and the asymmetry can be ascribed to the transition between the two cyclical forms of the temporal variation of ion kinetic energy.