(updated for 2013)
Short range forces involving “neutral” particles (neutral-ion, neutral-neutral) are inherently quantum-mechanical. Neutral-neutral interactions are very weak until electron clouds overlap (cm). We can therefore treat these particles as hard spheres. The collisional cross section for two species is a circle of radius r1 + r2, since that is the closest two particles can get without touching.
What does that collision rate imply? Consider the mean free path:
This is about 100 AU in typical ISM conditions ()
In gas at temperature T, the mean particle velocity is given by the 3-d kinetic energy: , or
, where is the mass of the neutral particle. The mean free path and velocity allows us to define a collision timescale:
- For (n,T) = (), the collision time is 500 years
- For (n,T) = (), the collision time is 1.7 months
- For (n,T) = (), the collision time is 45 years
So we see that density matters much more than temperature in determining the frequency of neutral-neutral collisions.