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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].)

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.