Collisions with Neutral Particles

Abstract

Neutral particles can, to a reasonable approximation, be assumed to have a certain geometrical cross section. The particles do not appear to be rigid however, i.e., the cross section always depends on the velocity. For neutral particles this dependence is determined solely by invariant properties of the particles and cannot be stated in a general way. Collisions can be either elastic or inelastic. In elastic collisions only an exchange of kinetic energies takes place; in inelastic collisions the kinetic energy of the colliding particles is partially or wholly changed into another form of energy. This energy transformation process can include excitation, ionization, or charge exchange. In excitation the kinetic energy is spent in shifting electrons in one or more of the atoms or molecules participating in the collision into higher quantum energy levels. This energy is subsequently released by radiation the the form of light. If the available kinetic energy exceeds the energy of the highest quantum level ionization becomes possible. Finally, if an atom collides with an ion of its own species, there is a high probability that charge exchanges will take place. This is a very important process inplasma physics. The ion extracts an electron from the atom, and the ion becomes an ion (Fig. 40). It would appear that nothing has changed. The process is reminiscent of the story, told by Koz’moi Prutkov, about the soldiers, Schmidt and Schultz, who were late in returning from leave and decided to confuse their commander. Schmidt said he was Schultz and Schultz said he was Schmidt. In a plasma “name exchange” (i.e., charge exchange) is very important.