Momentum transfer between ions and neutrals in molecular gas

We have performed quantum mechanical calculations of the cross-section for momentum transfer in collisions between para-H-2 molecules and HCO+ ions. A t low temperatures, the computed values of the rate coefficient are close t o the Langevin value. As the mean collision velocity increases, so does the rate coefficient, which is approximately four times the (constant) Langevi n value when the relative velocity reaches 10 km s(-1). Our results imply t hat the strength of the ion-neutral coupling has been underestimated in stu dies of C-type shock waves propagating in molecular gas.