Eq. 14 is also an approximate IR spectra function under the homogeneous broadening limit (see Ref. , Eq. (6.67)), with the width Γ ν . The parameters used in this study give Γ ν ≈ 30.83 cm -1 , which is in line with the typical values of the molecular systems investigated in the recent VSC experiments . As such, the rate constant k2 can be evaluated as convolution between κ2 (Eq. 13) and A ν (ω -ω 0 ) (Eq. 14) as