Temperature dependence of the rates of ion-molecule collisions

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Abstract
Momentum transfer collision rate constants have been obtained from ion cyclotron resonance linewidths over a temperature range of 200–425°K. The systems studied were CH5+ in methane, N2+ in N2, H3S+ in hydrogen sulfide, and CH2F+ in methyl fluoride. Collision rates measured at room temperature are in agreement with previous ion cyclotron resonance results and, in the case of N2+ in N2, with dc mobility experiments. For relative ion energies of approximately 0.08 eV, the collision rate constants for CH5+ in CH4 and CH2F+ in CH3F decrease substantially with increasing temperature. This fact shows that the ion-neutral interaction is strongly dependent on the rotational state of the neutral molecule. The frequently used locked dipole approximation to the ion-neutral potential, V(r) = −e2α/2r4−ep/r2, is completely inadequate for describing these experiments since it does not include any orientation or rotational state dependent terms.