The collision efficiency, or de-energization efficiency, is defined by: \[k_{\text{-1}} = B_{\text{c}}\ k_{\text{-1}}^{\text{sc}}\] where k-1 is the rate constant for a particular substance M when it brings about the de-energization process and k-1sc is the corresponding rate constant for a reference molecule M r that de-energizes A* on every collision; that is, the reference molecule M r undergoes strong collisions, and by definition has a collision efficiency Bc of unity. The species A* is usually in a vibrationally-excited state, and A has energy less than that required for reaction to occur.