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To find out the mechanism of a chemical reaction and the nature of its elementary processes it is very important to know whether the reaction occurs in the homogeneous (gas) phase only, or whether heterogeneous factors are also involved. With gas reactions such a factor usually is the reactor wall. The importance of the reactor wall, in particular of its nature and relative surface area in the kinetics of chemical gas reactions was first noticed by van’t Hoff [499].

The rate of a chemical reaction is determined by the changes in concentrations of the reactants (or of reaction products) with time. The concentration is usually measured by the number of moles of the given species per unit volume, i.e. by the value c = N/V mol/cm3 (N is the number of moles, V is the volume in cm3) or by the number of molecules per unit volume (number density) n = cNA molecule/cm3, where NA = 6.02 · 1023 molecules per mole is the Avogadro number. Sometimes the concentration is measured by the mass of the species per unit volume in grams, i.e. by the species density ρ = mn (m is the molecular mass) and also by partial pressure p = nkT = (ρ/m) kT.

Chemical reactions both in electric discharge and induced by ionizing radiation, involve ionized gas. However, the chemical role of ionization in these two cases differs. In electric discharge the mean energy of electrons is usually much lower than the ionization potential of molecules. Consequently, the ionization by electron impact in discharge, though needed for sustaining the discharge, to impart the electron energy to the gas, only makes up a small contribution to chemical conversions. The primary elementary steps of chemical importance usually involve the excitation of molecules and their dissociation to neutral fragments. These steps are considered in the following.

Unlike photochemistry that acts on a substance by using only photons of a relatively low, some 1–10 eV energy, the radiation chemistry makes use of high energy particles (102 to 107 eV). These comprise: γ-quanta, fast electrons, fast nuclei, protons, deuterons, tritons, α-particles, fission fragments and neutrons. (For a detailed description of ionizing radiation sources and of their use in radiation-chemical studies see [60, 502].)

Bimolecular exchange reactions are defined as those in which both collision partners change their chemical structure.

Calculation of the rate constants of various elementary gas reactions essentially consists of two independent parts: dynamical and statistical. Such independent treatment is based on the fact that the duration of molecular collisions (10-12 to 10-13 s) is typically much shorter than the average time between successive collisions (10-10 s under normal conditions). Thus, we can choose a time interval which is short compared to that between collisions, but is longer than the collisions time. Within this time interval we can consider the colliding system as independent of all other molecules and describe its evolution in terms of the mechanical equation of motion relating to colliding molecules only. The effect of all other molecules is taken into account only as the initial conditions for the state of the molecules before the collision. Solution of the mechanical problem (either classical or quantum-mechanical) giving the transition probabilities between the microscopic states of the colliding molecules is the first task of the calculation.