Auto- Ionization

Contrary to established opinion, atoms or molecules can ionize each other through collisions even if their translational energy is smaller than the ionization energy. This is because bound electrons can collide with each other when two atoms come together and one of these may gain enough energy in the process to become ionized, leaving the other with correspondingly less energy in the atom (this is a purely classical process and does not affect the quantum mechanical states the electrons occupy; one has to remember that the quantum mechanical wave function for a given energy is finite everywhere in space and allows the electron therefore to have any classical energy).
This process should be strongly temperature dependent, having its highest efficiency if the corresponding velocity of the approaching atoms is equal to the velocity of the bound electrons (i.e. about 10⁸ cm/sec). For smaller velocities the electron orbits will have time to adjust themselves mutually to the field of the other atom and ionizing collisions will become less likely.
The proposed process could be the explanation for the relatively high plasma density of the nighttime F- region of the earth's ionosphere. This would lead to an effective cross section of 10^-20 cm² in this case (which is characterized by atom velocities of 10⁵cm/sec).
In general this mechanism should result in a significant degree of ionization even in the absence of any UV- radiation sources, which should be highly relevant for some astrophysical problems like star formation