Resonance Theory of Vibrational Polariton Chemistry at the Normal Incidence

We present a theory that explains the reso- nance effect of the vibrational strong coupling (VSC) modified reaction rate constant at the normal incidence of a Fabry-Pérot (FP) cavity. This analytic theory is based on a mechanistic hypothesis that cavity modes promote the transition from the ground state to the vibrational excited state of the reactant, which is the rate-limiting step of the reaction. This mechanism for a single molecule coupled to a single-mode cavity has been confirmed by numerically exact simulations in our recent work in J. Chem. Phys. 159, 084104 (2023). Us- ing Fermi’s golden rule (FGR), we formulate this rate constant for many molecules coupled to many cavity modes inside a FP microcavity. The theory clearly ex- plains the resonance condition for the observed VSC effect and provides a theoretical explanation of why only at the normal incident angle there is the reso- nance effect, whereas for an oblique incidence, there is no apparent VSC effect for the rate constant even though both cases generate Rabi splitting and forming polariton states.