Activated components (ions, electrons, dissociated fragments) have a certain life-span, depending on the ambient conditions (solid body, liquid, gas). The life-span can be expressed by a flight distance. That is called mean free path. An energy input, supplied by impact or direct energy injection, is seen as the cause for the components‘ movement. The free path is thus to be seen as a description of the energy state that can be given in eV or as well in K. A plasma can thus be characterised by the mean free path of electrons and ions, as much as through the indication of the neutral gas temperature.
In physics and kinetic theory, the mean free path of a particle, such as a molecule, is the average distance the particle travels between collisions with other particles.
