Abstract
Here we present a fundamental study on how the ground-state chemical reactivity of a molecule can be modified in a QED scenario, i.e., when it is placed inside a cavity and there is strong coupling between the cavity field and vibrational modes within the molecule. We work with a model system for the molecule (Shin-Metiu model) in which nuclear, electronic and photonic degrees of freedom are treated on the same footing. This simplified model allows the comparison of exact quantum reaction rate calculations with predictions emerging from transition state theory based on the cavity Born-Oppenheimer approach. We demonstrate that QED effects are indeed able to significantly modify activation barriers in chemical reactions and, as a consequence, reaction rates. The critical physical parameter controlling this effect is the permanent dipole of the molecule and how this magnitude changes along the reaction coordinate. We show that the effective coupling can lead to significant single-molecule energy shifts in an experimentally available nanoparticle-on-mirror cavity. We then apply the validated theory to a realistic case (internal rotation in the 1,2-dichloroethane molecule), showing how reactions can be inhibited or catalyzed depending on the profile of the molecular dipole. Furthermore, we discuss the absence of resonance effects in this process, which can be understood through its connection to Casimir-Polder forces. Finally, we treat the case of many-molecule strong coupling, and find collective modifications of reaction rates if the molecular permanent dipole moments are oriented with respected to the cavity field. This demonstrates that collective coupling can also provide a mechanism for modifying ground-state chemical reactivity of an ensemble of molecules coupled to a cavity mode.
Highlights
The field of cavity quantum electrodynamics (CQED) has proved that the quantum nature of light can be exploited for many interesting applications that involve the modifications of material properties in one way or another [1,2]
While we do not explicitly treat the situation in recent experiments on the modification of ground-state reactions by vibrational strong coupling, we believe that our results indicate that the resonance-dependent effects cannot be explained by a straightforward modification of ground-state reaction energy barriers at thermal equilibrium, as these would be captured by transition state theory (TST) within the cavity Born-Oppenheimer approximation (CBOA) in a many-mode, many-molecule setting
By treating a simple model system, the Shin-Metiu model, we were able to show through full quantum rate calculations on the singlemolecule level that ground-state thermally driven reaction rates can be significantly modified under strong light-matter coupling
Summary
The field of (nonrelativistic) cavity quantum electrodynamics (CQED) has proved that the quantum nature of light can be exploited for many interesting applications that involve the modifications of material properties in one way or another [1,2]. Most of the research on polaritonic chemistry with organic molecules deals with electronic strong coupling This phenomenon leads to many interesting effects such as collective protection of polaritons and changes in chemical reactivity [19,20], cavity-induced nonadiabatic phenomena [21,28,34], and the opening of novel reaction pathways in photochemistry [23]. V that, to a good approximation, perturbation theory can be used to predict cavity-induced chemical changes in terms of baremolecule ground-state properties and allows us to make explicit connections to electrostatic, van der Waals, and Casimir-Polder interactions We mention that the latest experimental studies indicate that solvent effects might be responsible and/or relevant for the experimentally observed resonance-dependent ground-state chemical reactivity [35,36]
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