To explore the origin of these three peaks, we fixed N cav v = 10 and analyzed the rate profile k as a function of ω c with different N mol v . When N mol v = 4, the peak centered at ω c = 900cm -1 arises, which matches the energy gap ∆ε mol 0↔3 = 902cm -1 for the transition v mol 0 ↔ v mol 3 and