Based on the two diabatic states |ν L ⟩ and |ν ′ L ⟩ in the left well (see Eqs. 38-39 in Method), we define the quantum vibration frequency of the reactant as ω 0 ≡ E ′ -E = 1172.2 cm -1 , which is directly related to the quantum transition of |ν L ⟩ → |ν ′ L ⟩ and can be determined by spectroscopy measurements (IR or transmission spectra). The Rabi splitting from the spectral measurements is related to the light-matter coupling strength as follows