Scientists report a theoretical mechanism by which putting molecules inside a tuned optical cavity can change their London dispersion forces. London dispersion forces are a type of van der Waals attraction that fall off quickly with distance (roughly as 1/r^6). The authors show that when a cavity mode is tuned to match a molecular vibration — a situation called vibrational strong coupling — those dispersion forces can be changed in a resonant way. For the simple case of two molecules, this change can lead to faster reaction rates in their simulations.

To reach this conclusion the researchers worked from a quantum-mechanical description of the molecules’ electronic and vibrational states and then included the cavity mode. They kept the model simple: two electronic levels per molecule and one vibrational coordinate. By resolving vibrational states inside the electronic levels, they derived vibrationally-resolved dispersion energies and a corresponding C6 coefficient that measures the strength of the 1/r^6 interaction. Using the Tavis–Cummings picture for a small number of vibrational excitations, they identified “bright” and “dark” collective vibrational states and the mixed light–matter states called polaritons (lower and upper). The bright and polaritonic states showed enhanced C6 values near resonance, while dark states showed suppressed C6. As a concrete example from the paper, one ground-state value of C6 was about 1.65 atomic units and a vibrationally excited value was about 2.35 atomic units in the authors’ model.

Next, the team used a mixed quantum–classical dynamics scheme to see how the modified dispersion forces influence chemical kinetics. In the case of two molecules strongly coupled to the cavity mode, their dynamics showed a resonant enhancement of reaction rates. They report that this rate enhancement appears across different regimes of solvent friction, meaning the effect in their model does not vanish when the environment damps molecular motion more strongly.