Vibrational Energy Redistribution and Polaritonic Fermi Resonances in the Strong Coupling Regime.

Intramolecular vibrational energy redistribution (IVR) plays a significant role in cavity-modified chemical reaction rates. As such, understanding the fundamental mechanisms by which the cavity modifies the IVR pathways is a fundamental step toward engineering the effect of the confined electromagnetic modes on the outcome of chemical processes. Here we consider an ensemble of M two-mode molecules with intramolecular anharmonic couplings interacting with an infrared cavity mode and consider their quantum dynamics and infrared spectra. Polaritonic Fermi resonances involving fundamental and overtone states of the polaritonic subsystem mediate efficient energy transfer pathways between otherwise off-resonant molecular states. These pathways are of collective nature, yet enabled by the intramolecular anharmonic couplings. Hence, through polaritonic Fermi resonances, cavity excitation can efficiently spread toward low-frequency modes while becoming delocalized over several molecules.

Many-photon excitation of organic molecules in a cavity-Superradiance as a measure of coherence.

Coherent excitation of a molecular ensemble coupled to a common radiation mode can lead to the collective emission of radiation known as superradiance. This collective emission only occurs if there is an entanglement between the molecules in their ground and excited state and can, therefore, serve as a macroscopic measure of coherence in the ensemble. Reported here are wave packet propagations for various pyrazine models of increasing complexity and molecular ensembles thereof. We show that ensemble coherence upon photoexcitation can prevail up to relatively long time scales although the effect can diminish quickly with increasing ensemble size. Coherence can also build up over time and even reemerge after the molecules have passed through a conical intersection. The effect of the pump pulse characteristics on the collective response of the molecular ensemble is also studied. A broadband pulse imprints a large amount of initial coherence to the system, as compared to a longer pulse with a smaller spread in the frequency domain. However, the differential effects arising from a different pulse duration and coherent bandwidth become less prominent if the emission of light from the ensemble takes place after a non-adiabatic decay process.

Modifying the Nonradiative Decay Dynamics through Conical Intersections via Collective Coupling to a Cavity Mode.

The coupling of a molecular ensemble to the confined electromagnetic modes of a microcavity can strongly modify the photophysics and photochemistry of the molecules upon photoexcitation. We investigate here how collective coupling effects lead to modifications of the mechanisms and rates of photochemical processes, in particular, photodissociation and nonradiative decay in NaI and pyrazine, respectively. We show that, after direct excitation into the lower polaritonic states, the lower-energy light-matter hybrid states, the dynamics of the molecular ensemble coupled to light is very similar to the dynamics of the corresponding isolated molecules. Conversely, excitation into the upper polaritonic states results in more complex dynamics that involve as a first step the population transfer toward the manifold of intermediate dark states. These dynamics differ substantially from those of the isolated molecules and may result in measurable time delays for nonradiative decay or excited-state reaction mechanisms. Similarly, we describe how addition of a buffer of nonreactive molecules coupled to the cavity mode can be used to delay the onset of the photochemical processes of the reactive part of the ensemble, where the buffer medium is more effective in inhibiting the reactive process than only reactive molecules in the cavity.