ABSTRACT
Partially ionized plasmas of gas mixtures like air are complex systems. One may think of a plasma as a large collection of different particles that interact among each other and with external fields: ground-state and excited atoms and molecules, positive and negative ions, electrons, possibly dust. Also radiation-in the ray limit-has particle properties. C'Ne will, however, refer by 'particle' only to matter. Photons are sufficiently different to justify separate treatment.)
• Heavy particles or baryons are species which have at least one nucleon (proton or neutron). They are either atomic (one nucleus) or molecular (several nuclei). Air, for example, consists of78% N2 (molecular nitrogen), 21 % O2 (molecular oxygen), 0.9% Ar (argon), and traces of CO2 (carbon dioxide), H20 (water), 0 3 (ozone), He (helium), Kr (krypton), Xe (xenon) etc. Neutrals carry no charge, q = 0, positive ions (cations) with charge q = Ze can be singly (Z = 1) or multiply (Z> 1) ionized. In electronegative gases (for example oxygen and nitrogen), negative ions (anions) can also exist, mostly singly charged (q = -e, Z = -1). Species are denoted by the 'sum formula' (e.g. H30+) which suffices for most purposes. (Isomer effects-sensitivities to structural differences of molecules having the same sum formula-are, for example, analyzed by Deutsch et at [3].) Later in this text it will be useful to view the sum formula as an integer vector (R) = (Rz , RH , RHe , ... , Ru) of charge number and elementary content. H30+, e.g., denotes (H30 +) = (1,3,0,0,0,0,0,0,1,0,0,0, ... ,0). Neglecting electron contributions, the mass of a heavy particle is mR = L~=H Rnmn ::;::j Aa, where A is the total number of nucleons of the nuclei. Heavy particles are non-relativistic, i.e. at a given velocity v their momentum is p = mvand their kinetic energy E = ! mv2• Except for fully
ionized cations, heavy particles have also internal structure and may therefore exist in different energy states Ci' (For a schematic energy level diagram, see figure 3.1.) Atoms or atomic ions have only electronic excitations, with a typical scale of some eV. Molecules or molecular ions have also vibrational and rotational excitations, with energies of a few meV (rotation) or a few tens of me V (vibration). The energies of different species can be compared by accounting the standard enthalpy of formation D..H'f (e.g. [5]) .
