The collision efficiency, or de-energization efficiency, is defined by:
where is the rate constant for a particular substance when it brings about the de-energization process
and is the corresponding rate constant for a reference molecule that de-energizes on every collision; that is, the reference molecule undergoes strong collisions, and by definition has a collision efficiency of unity. The species is usually in a vibrationally-excited state, and has energy less than that required for reaction to occur.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 159