Research

Over the past decade, an interdisciplinary outlook at the crossroads of Cavity Quantum Electrodynamics (CQED) and Chemistry has emerged which is popularly known as “Polariton Chemistry”. The central question is – can chemical reaction in terms of rate and reaction mechanism be modified simply by placing molecules inside an optical cavity without using any external light source...? In chemistry control of chemical reaction using an external field has been a long-standing goal for decades as it would allow us to impart control over chemical reactivity, product selectivity, and energy transfer. But sometimes fast intramolecular vibrational energy redistribution (IVR) is kind of a roadblock to performing mode-specific chemistry. This idea of controlling chemical reactivity has regained its confidence with the emergence of “Vibro-Polariton Chemistry”. In recent years, there has been a huge surge of interest in performing chemical reactions exploiting the quantum nature of light. Specifically, researchers have put molecules inside optical cavities and observed an intriguing range of effects associated with the creation of hybrid light-matter (polariton) states. Inside an optical cavity, molecular excitation (electronic/vibrational) interacts with the photonic excitation of confined radiation mode which leads to the emergence of new hybrid states aka “polaritonic states” separated by Rabi splitting. This process occurs even in the ‘dark’ because it is the zero-point fluctuation of electromagnetic field confined inside a cavity, that strongly couple with the molecule. In the weak coupling regime, the interaction of light and matter is weak enough that it may be treated as a small perturbation. strong coupling is achieved when the coherent energy exchange between molecular transition (electronic/vibrational) and the cavity radiation modes (photonic excitations) is faster than any other decay process inside the cavity. The hybrid light–matter states also have unusual properties: they can be delocalized over a very large number of molecules.

It has been demonstrated by recent experimental works that the possibility of modifying the photo-isomerization reactivities, electron transfer kinetics, modifying ground-state chemical reactivity, tilting the energy landscape to a specific product, and many more. Despite several recent efforts, a clear microscopic mechanism underlying experimental observations is still yet to be understood.

Though in experiments, the collective strong coupling is achieved by placing a large number of molecules inside an optical cavity, as a first step, it is important to understand in detail how the cavity mode influences chemical reaction dynamics on a single molecular level. In a recent paper[3], authors have systematically studied the effect of the cavity on ground-state properties like equilibrium bond length, dissociation energy, etc. of single molecules coupled to a single cavity mode. We are interested to address the question of how dynamics get modified inside an optical cavity. Therefore, we take a single diatomic HF molecule coupled to a single cavity mode as our model system and study the dissociation dynamics of H−F bond both classically and quantum mechanically by tuning the frequency of the cavity mode. We observe a marked suppression of the dissociation probability, with non-linear dipole function, at cavity frequencies significantly below the fundamental transition frequency of the molecule. Whereas, with linear dipole approximation, surprisingly the dip in the dissociation probability is absent and more or less independent of cavity mode frequency. We show that this suppression in the dissociation probability can be rationalized entirely in terms of the structures in the classical phase space of the model system.

We show that the dipole function does play an important role in the dissociation dynamics. It is the non-linearity of the dipole function which is responsible for the suppression of dissociation probability. We also show by non-linear resonance analysis that at some frequencies the Fourier coefficients associated with the resonances vanish and correspondingly, the phase space structures become more regular in nature leading to the inhibition of dissociation. Back