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