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