Ion Cyclotron Resonance Power Absorption: Collision Frequencies for CO2+, N2+, and H3+ Ions in Their Parent Gases

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Abstract
The complete solution for the equation of motion of an ion in the ICR cell is shown to give results for the instantaneous power absorption in excellent agreement with experiment at all pressures. The instantaneous power absorption at resonance initially increases linearly with time, and at high pressures levels off to a constant value at saturation where the energy gained by ions from the rf electric field is equal to the energy dissipated in collisions. An expression is also derived for the average kinetic energy of an ion at saturation in the steady-state limit. Pulsed ICR techniques are used to obtain the instantaneous power absorption curves for N2+, CO2+, and H3+ ions in their parent gases as a function of pressure, from which are calculated the momentum transfer rate constants k, and the dependence of the rate constants on ion kinetic energy. At 293°K, k(N2+)=k(CO2+)=0.67×10−9 cm3 molecule−1· sec−1, and k(H3+)=1.09×10−9 cm3 molecule−1· sec−1. For N2+ ions in N2 and CO2+ ions in CO2, the rate constants are both significantly greater than that predicted by polarization theory and both rate constants increase significantly with increasing ion kinetic energy. This behavior is most likely a consequence of long-range resonant charge transfer outside the orbiting impact parameter.