The effect of an optical cavity on diabatic tunneling in an ensemble of symmetric double-well systems

The vacuum field of an optical cavity can potentially modify chemical reactivity and other dynamical properties via vibrational strong coupling (VSC). This intriguing finding has inspired numerous studies, but the underlying mechanisms remain unresolved. While many theoretical efforts focus on solvent or nuclear fluctuations, the tunneling overlap in non-adiabatic processes is usually assumed unperturbed by the cavity field. This paper presents a rigorous calculation of the tunneling splitting and associated ground-state shift resulting from the non-adiabatic coupling between two degenerate, harmonic diabatic surfaces in the ground vibrational state manifold under VSC. Based on this calculation, the tunneling splitting is suppressed by the cavity field for a single-molecule or a few-molecule system, but this cavity-induced effect is neither resonant nor cooperative and vanishes in the thermodynamic limit. This prediction demonstrates the many facets of VSC-induced phenomena and sheds new light on cavity-modified non-adiabatic processes, including charge transfer, Förster resonance energy transfer, energy relaxation, and conical intersection.