The VSC-modified rate profile scanned as a function of the cavity frequency ω c also shows a very sharp resonant feature (Fig. ), similar to the absorption spectra of the molecule outside the cavity (Fig. ). This is because both profiles originate from the same type of transition |ν L → |ν L . In optical absorption spectra, the laser frequency must match ω 0 to generate the optical excitation |ν L → |ν L , whereas in VSC-modified reactions, the cavity frequency ω c must match ω 0 to efficiently promote the |ν L → |ν L transition (under thermal condition).