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Shining Light on the Microscopic Resonant Mechanism Responsible for Cavity-Mediated Chemical Reactivity
TL;DR: In this article, the authors leverage a combination of first-principles techniques, foremost quantum-electrodynamical density functional theory, applied to the recent experimental realization by Thomas et al. to unveil the microscopic mechanism behind the experimentally observed reduced reaction-rate under cavity induced resonant vibrational strong light-matter coupling.
Abstract: Strong light-matter interaction in cavity environments has emerged as a promising approach to control chemical reactions in a non-intrusive and efficient manner. The underlying mechanism that distinguishes between steering, accelerating, or decelerating a chemical reaction has, however, remained unclear, hampering progress in this frontier area of research. In this work, we leverage a combination of first-principles techniques, foremost quantum-electrodynamical density functional theory, applied to the recent experimental realization by Thomas et al. [1] to unveil the microscopic mechanism behind the experimentally observed reduced reaction-rate under cavity induced resonant vibrational strong light-matter coupling. We find that the cavity mode functions as a mediator between different vibrational eigenmodes, transferring vibrational excitation and anharmonicity, correlating vibrations, and ultimately strengthening the chemical bond of interest. Importantly, the resonant feature observed in experiment, theoretically elusive so far, naturally arises in our investigations. Our theoretical predictions shine new light on cavity induced polaritonic chemistry, providing a crucial control strategy in state-of-the-art photocatalysis and energy conversion, pointing the way towards generalized quantum optical control of chemical processes.
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TL;DR: In this article, a quantum transition state theory (TSTT) was used to examine the coherent nature of adiabatic reactions in cavities and derive the cavity-induced changes in eigenfrequencies, zero-point energy, and quantum tunneling.
Abstract: The electromagnetic field in an optical cavity can dramatically modify and even control chemical reactivity via vibrational strong coupling (VSC). Since the typical vibration and cavity frequencies are considerably larger than thermal energy, it is essential to adopt a quantum description of cavity-catalyzed adiabatic chemical reactions. Using quantum transition state theory (TST), we examine the coherent nature of adiabatic reactions in cavities and derive the cavity-induced changes in eigenfrequencies, zero-point energy, and quantum tunneling. The resulting quantum TST calculation allows us to explain and predict the resonance effect (i.e., maximal kinetic modification via tuning the cavity frequency), collective effect (i.e., linear scaling with the molecular density), and selectivity (i.e., cavity-induced control of the branching ratio). The TST calculation is further supported by perturbative analysis of polariton normal modes, which not only provides physical insights to cavity-catalyzed chemical reactions but also presents a general approach to treat other VSC phenomena.
39 citations
TL;DR: In this article, the authors provide a theoretical explanation of the basic principle of how cavity frequency can be tuned to achieve mode-selective reactivities, showing that the dynamics of the radiation mode leads to a cavity frequency-dependent dynamical caging effect of a reaction coordinate, resulting in suppression of the rate constant.
Abstract: Recent experiments have demonstrated remarkable mode-selective reactivities by coupling molecular vibrations with a quantized radiation field inside an optical cavity. The fundamental mechanism behind such effects, on the other hand, remains elusive. In this work, we provide a theoretical explanation of the basic principle of how cavity frequency can be tuned to achieve mode-selective reactivities. We find that the dynamics of the radiation mode leads to a cavity frequency-dependent dynamical caging effect of a reaction coordinate, resulting in suppression of the rate constant. In the presence of competitive reactions, it is possible to preferentially cage a reaction coordinate when the barrier frequencies of competing reactions are different, resulting in a selective slow down of a given reaction. Our theoretical results illustrate the cavity-induced mode-selective chemistry through polaritonic vibrational strong couplings, revealing the fundamental mechanism for changing chemical selectivities through cavity quantum electrodynamics.
23 citations
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TL;DR: In this article, the first fully consistent molecular orbital theory for quantum electrodynamics environments was proposed and used to predict cavity induced modifications of molecular reactivity and pinpoint classes of systems with significant cavity effects.
Abstract: Coupling between molecules and vacuum photon fields inside an optical cavity has proven to be an effective way to engineer molecular properties, in particular reactivity. To ease the rationalization of cavity induced effects we introduce an ab initio method leading to the first fully consistent molecular orbital theory for quantum electrodynamics environments. Our framework is non-perturbative and explains modifications of the electronic structure due to the interaction with the photon field. We show that the newly developed orbital theory can be used to predict cavity induced modifications of molecular reactivity and pinpoint classes of systems with significant cavity effects. We also investigate cavity-induced modifications of molecular reactivity in the vibrational strong coupling regime.
11 citations
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TL;DR: In this article, the authors performed quantum mechanical and transition state theory rate calculations for a thermal model reaction, the inversion of ammonia along the umbrella mode, in presence of a single cavity mode of varying frequency and coupling strength.
Abstract: It has been experimentally demonstrated that reaction rates for molecules embedded in microfluidic optical cavities are altered (decelerated) when compared to rates observed under "ordinary" reaction conditions. Precise mechanisms of how the strong coupling of an optical cavity mode to molecular vibrations affect the reactivity and how resonance behavior emerges are still under dispute. In the present work, we perform quantum mechanical and transition state theory rate calculations for a thermal model reaction, the inversion of ammonia along the umbrella mode, in presence of a single cavity mode of varying frequency and coupling strength. Using a Pauli-Fierz Hamiltonian including dipole self-energies, two-dimensional cavity Born-Oppenheimer potential energy surfaces (cPES) are derived. It is found that while classical activation energies for inversion are unaffected by the cavity mode, reaction rates in cavities are nevertheless decelerated in qualitative agreement with experiments, due to two quantum effects: The stiffening of quantized modes perpendicular to the reaction path at the transition state, which reduces the number of thermally accessible reaction channels, and the broadening of the barrier region which suppresses tunneling. We also find that these two effects are very robust in a fluctuating environment, which causes statistical variations of potential parameters such as the barrier height. Further, by solving the time-dependent Schrodinger equation in the vibrational strong coupling (VSC) regime we find, in qualitative agreement with experimental and earlier theoretical work, a resonance behavior. The latter manifests as reduced reaction probability when the cavity frequency $\omega_c$ is tuned in resonance with a reactant molecular frequency. The effect is due to dynamical localization of the vibropolaritonic wavepacket in the reactant well.
9 citations
References
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TL;DR: A simple derivation of a simple GGA is presented, in which all parameters (other than those in LSD) are fundamental constants, and only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked.
Abstract: Generalized gradient approximations (GGA’s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. [S0031-9007(96)01479-2] PACS numbers: 71.15.Mb, 71.45.Gm Kohn-Sham density functional theory [1,2] is widely used for self-consistent-field electronic structure calculations of the ground-state properties of atoms, molecules, and solids. In this theory, only the exchange-correlation energy EXC › EX 1 EC as a functional of the electron spin densities n"srd and n#srd must be approximated. The most popular functionals have a form appropriate for slowly varying densities: the local spin density (LSD) approximation Z d 3 rn e unif
146,533 citations
TL;DR: The observation of the strong-coupling regime between the excitonic transition of a single GaAs quantum dot and a discrete optical mode of a microdisk microcavity is reported on.
Abstract: We report on the observation of the strong-coupling regime between the excitonic transition of a single GaAs quantum dot and a discrete optical mode of a microdisk microcavity. Photoluminescence is performed at various temperatures to tune the quantum dot exciton with respect to the optical mode. At resonance, we observe a clear anticrossing behavior, signature of the strong-coupling regime. The vacuum Rabi splitting amounts to 400 microeV and is twice as large as the individual linewidths.
722 citations
TL;DR: It is demonstrated here that one can indeed influence a chemical reaction by strongly coupling the energy landscape governing the reaction pathway to vacuum fields.
Abstract: is typically achieved by placing the material in an optical cavity, such as that formed by two parallel mirrors, which is tuned to be resonant with a transition to an excited state. Theory, discussed below, shows that even in the absence of light, a residual splitting always exists due to coupling to vacuum (electromagnetic) fields in the cavity. While cavity strong coupling and the associated hybrid states have been extensively studied due to the potential they offer in physics such as room temperature Bose–Einstein condensates and thresholdless lasers, the implication for chemistry remains totally unexplored. This is despite the fact that strong coupling with organic molecules lead to exceptionally large vacuum Rabi splittings (hundreds of meV) due to their large transition dipole moments. The molecules plus the cavity must thus be thought of as a single entity with new energy levels and therefore should have its own distinct chemistry. We demonstrate here that one can indeed influence a chemical reaction by strongly coupling the energy landscape governing the reaction pathway to vacuum fields. In the absence of dissipation, the Rabi splitting energy h WR (Figure 1) between the two new hybrid light–matter states is given, for a two-level system at resonance with a cavity mode, by the product of the electric field amplitude E in the cavity and the transition dipole moment d :
640 citations
TL;DR: The reactivity of a compound bearing two possible silyl bond cleavage sites is studied as a function of VSC of three distinct vibrational modes in the dark, showing that VSC can indeed tilt the reactivity landscape to favor one product over the other.
Abstract: Many chemical methods have been developed to favor a particular product in transformations of compounds that have two or more reactive sites. We explored a different approach to site selectivity using vibrational strong coupling (VSC) between a reactant and the vacuum field of a microfluidic optical cavity. Specifically, we studied the reactivity of a compound bearing two possible silyl bond cleavage sites—Si–C and Si–O, respectively—as a function of VSC of three distinct vibrational modes in the dark. The results show that VSC can indeed tilt the reactivity landscape to favor one product over the other. Thermodynamic parameters reveal the presence of a large activation barrier and substantial changes to the activation entropy, confirming the modified chemical landscape under strong coupling.
490 citations
TL;DR: It is shown that strong resonant coupling of a cavity field with an electronic transition can effectively decouple collective electronic and nuclear degrees of freedom in a disordered molecular ensemble, even for molecules with high-frequency quantum vibrational modes having strong electron-vibration interactions.
Abstract: The demonstration of strong and ultrastrong coupling regimes of cavity QED with polyatomic molecules has opened new routes to control chemical dynamics at the nanoscale. We show that strong resonant coupling of a cavity field with an electronic transition can effectively decouple collective electronic and nuclear degrees of freedom in a disordered molecular ensemble, even for molecules with high-frequency quantum vibrational modes having strong electron-vibration interactions. This type of polaron decoupling can be used to control chemical reactions. We show that the rate of electron transfer reactions in a cavity can be orders of magnitude larger than in free space for a wide class of organic molecular species.
464 citations