The yield of a total plasma-chemical process is due to synergistic contributions of numerous different elementary reactions taking place simultaneously in a discharge system. The sequence of transformations of initial chemical substances and electric energy into products and thermal energy is usually referred to as the mechanism of the plasma-chemical process. Elementary reaction rates are determined by the micro-kinetic characteristics of individual reactive collisions (like, for example, reaction cross sections or elementary reaction probabilities) as well as by relevant kinetic distribution functions (like the electron energy distribution function [EEDF], or population function of excited molecular states). The elementary reaction rate is actually a result of integration of the reaction cross section or probability over the relevant distribution function and characterizes the energy or excitation state of reactants. We will focus in this chapter mostly on the micro-kinetics of the elementary reactions – on their cross sections and probabilities – assuming, if necessary, conventional Maxwellian or Boltzmann distribution functions. More sophisticated non-Maxwellian and non-Boltzmann kinetic distribution functions typical for strongly non-equilibrium discharge conditions, like the Druyvesteyn EEDF for electrons or Treanor distribution for vibrationally excited molecules, are to be considered in the next chapter.
Plasma is an ionized gas. The key process in plasma is ionization, which means conversion of neutral atoms or molecules into electrons and positive ions. Thus, ionization is the first elementary plasma-chemical processes to be considered.