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 @O04322@ for a particular substance \(\text{M}\) when it brings about the de-energization process and \(k_{-1}^{\text{sc}}\) is the corresponding @O04322@ for a reference molecule \(\text{M}_{\text{r}}\) that de-energizes \(\text{A}^{*}\) on every collision; that is, the reference molecule \(\text{M}_{\text{r}}\) undergoes @S06059@, and by definition has a collision efficiency \(B_{\text{c}}\) of unity. The species \(\text{A}^{*}\) is usually in a vibrationally-@E02257@, and \(\text{A}\) has energy less than that required for reaction to occur.