New Fit for the O ⁺ ‐O Collision Frequency Including Fine Structure Effects

Abstract A new fit is presented for the oxygen ion‐neutral collision frequency . The fit explicitly depends on the neutral temperature , which characterizes the thermal distribution of fine structure levels of O( 3 ,1,2), and the reduced temperature , where is the ion temperature. A physically realistic fitting function adapted from the exact solution of a two‐state system is constrained to approach plausible analytic models in the limits that is high (Dalgarno's model, ) or is low (Langevin's model, –50 K). The target data for the fit are determined from the cross sections calculated by Hickman et al. (1997, https://doi.org/10.1103/PhysRevA.56.4633 ) and their extrapolations to lower and higher energies using the Langevin and Dalgarno models. The fit reproduces the target data within 1% for any and for any in the range . The values of the fitting parameters suggest that the fine structure effect is more important than the effect of the long range ion‐quadrupole and polarization potentials for K. A relationship is derived between the parameters of the Dalgarno model and the exponent of the power law dependence () at high .