Temperature dependence of the rates of ion-molecule collisions

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 ${\mathrm{CH}}_5^{+}$ in methane, ${\mathrm{N}}_2^{+}$ 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 ${\mathrm{N}}_2^{+}$ in N₂, with dc mobility experiments. For relative ion energies of approximately 0.08 eV, the collision rate constants for ${\mathrm{CH}}_5^{+}$ 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.