arXiv subjects

Thomas Schnappinger

Publications and source records attributed to Thomas Schnappinger.

16 recordsLinked to original sources

arXiv2025

Selective excitation of molecular vibrations via a two-mode cavity Raman scheme

The experimental realization of strong light-matter coupling with molecules initiated the rapidly evolving field of molecular polaritonics. Most studies focus on how exciton polaritons, which combine electronic excitations with confined light modes, alter photochemistry. In this paper, we investigate their use in selectively exciting molecular vibrational states in the ground state. Selectively exciting molecules to high vibrational states with infrared lasers to catalyze ground-state chemical reactions is a challenging task. Here, we propose a two-cavity mode setup in the electronic strong coupling regime inspired by the process of Stimulated Raman Adiabatic Passage (STIRAP) to selectively populate excited vibrational states. One cavity mode actively pumps the molecular system, while the other provides a highly effective and tunable decay channel via photon leakage. We demonstrate the ability to selectively populate vibrational states for coherent and incoherent light sources using a molecular model system. Our initial findings show high efficiency and suggest a possible route to steering and controlling chemical reactions in the electronic ground state based on electronic strong coupling.

physics.chem-ph↗

arXiv2025

The role of dark polariton states for electronic strong coupling in molecules

Polaritonic chemistry investigates the possible modification of chemical and photochemical reactions by means of strong light-matter coupling in optical cavities, as demonstrated in numerous experiments over the last few years. These experiments are typically interpreted in terms of the Jaynes-Cummings or Tavis-Cummings models under the assumption that the molecular ensemble is only excited by a single photon. In such a model, two polariton states compete with an overwhelming number of dark states, inhibiting polaritonic reactions entropically. We analyze the higher excitation manifolds of the Tavis-Cummings model along with a three-level system that resembles photochemical reactions. We demonstrate that allowing for more than a single excitation makes the reaction of the involved polaritons entropically more favorable.

quant-ph↗

arXiv2025

Molecular Polarizability under Vibrational Strong Coupling

Polaritonic chemistry offers the possibility of modifying molecular properties and even influencing chemical reactivity through strong coupling between vibrational transitions and confined light modes in optical cavities. Despite considerable theoretical progress and due to the complexity of the coupled light-matter system, the fundamental mechanism how and if collective strong coupling can induce local changes of individual molecules is still unclear. We derive an analytical formulation of static polarizabilities within linear response theory for molecules under strong coupling using the cavity Born-Oppenheimer Hartree-Fock ansatz. This ab-initio method consistently describes vibrational strong coupling and electron-photon interactions even for ensembles of molecules. For different types of molecular ensemble, we observed local changes in the polarizabilities and dipole moments that are induced by collective strong coupling. Furthermore, we used the polarizabilities to calculate vibro-polaritonic Raman spectra in the harmonic approximation. This allows us to comprehensively compare the effect of vibrational strong coupling on IR and Raman spectra on an equal footing.

physics.chem-ph↗

arXiv2024

Enhanced photoisomerization with hybrid metallodielectric cavities based on mode interference

The ability to control chemical reactions by coupling organic molecules to confined light in a cavity has recently attracted much attention. While most previous studies have focused on single-mode photonic or plasmonic cavities, here we investigate the effect of hybrid metallodielectric cavities on photoisomerization reactions. Hybrid cavities, which support both photonic and plasmonic modes, offer unique opportunities that arise from the interplay between these two distinct types of modes. Specifically, we demonstrate that interference in the spectral density due to a narrow photonic mode and a broad plasmonic mode that are coupled to each other enables hybrid cavities to provide an energy-selective Purcell effect. This effect enhances electronic relaxation only to the desired molecular geometry, providing the ability to increase the yield of photoisomerization reactions. As a test case, we study the asymmetric proton transfer reaction in the electronic excited state of 3-aminoacrolein. Our results, which are robust for a range of realistic cavity parameters, highlight the advantages of hybrid cavities in cavity-induced photochemical processes.

physics.chem-ph↗

arXiv2024

Simulating Nonadiabatic Dynamics in Benzophenone: Tracing Internal Conversion Through Photoelectron Spectra

Benzophenone serves as a prototype chromophore for studying the photochemistry of aromatic ketones, with applications ranging from biochemistry to organic light-emitting diodes. In particular, its intersystem crossing from the first singlet excited state to triplet states has been extensively studied, but experimental or theoretical studies on the preceding internal conversion within the singlet manifold are very rare. This relaxation mechanism is particularly important because direct population transfer of the first singlet excited state from the ground state is inefficient due to its low oscillator strength. In this work, we aim to fill this gap by employing mixed quantum classical and full quantum dynamics simulations and time-resolved photoelectron spectroscopy for gas-phase benzophenone and meta-methyl benzophenone. Our results show that nonadiabatic relaxation via conical intersections leads to a linear increase in the population of the first singlet excited state. This population transfer due to conical intersections can be directly detected by a bifurcation of the photoelectron signal. In addition, we are able to clarify the role of the third singlet excited state degenerate to the second excited state - a topic that remains largely unexplored in the existing literature on benzophenone.

physics.chem-ph↗

arXiv2024

Disentangling collective coupling in vibrational polaritons with double quantum coherence spectroscopy

Vibrational polaritons are formed by strong coupling of molecular vibrations and photon modes in an optical cavity. Experiments have demonstrated that vibrational strong coupling can change molecular properties and even affect chemical reactivity. However, the interactions in a molecular ensemble are complex, and the exact mechanisms that lead to modifications are not fully understood yet. We simulate two-dimensional infrared spectra of molecular vibrational polaritons based on the double quantum coherence technique to gain further insight into the complex many-body structure of these hybrid light-matter states. Double quantum coherence uniquely resolves the excitation of hybrid light-matter polaritons and allows to directly probe the anharmonicities of the resulting states. By combining the cavity Born-Oppenheimer Hartree-Fock ansatz with a full quantum dynamics simulation of the corresponding eigenstates, we go beyond simplified model systems. This allows us to study the influence of self-polarization and the response of the electronic structure to the cavity interaction on the spectral features even beyond the single-molecule case.

quant-ph↗

arXiv2024

1.5-Femtosecond Delay in Charge Transfer

The transfer of population between two intersecting quantum states is the most fundamental dynamical event that governs a broad variety of processes in physics, chemistry, biology and material science. Whereas any two-state description implies that population leaving one state instantaneously appears in the other state, we show that coupling to additional states, present in all real-world systems, can cause a measurable delay in population transfer. Using attosecond spectroscopy supported by advanced quantum-chemical calculations, we measure a delay of 1.46$\pm$0.41 fs at a charge-transfer state crossing in CF$_3$I$^+$, where an electron hole moves from the fluorine atoms to iodine. Our measurements also fully resolve the other fundamental quantum-dynamical processes involved in the charge-transfer reaction: a vibrational rearrangement time of 9.38$\pm$0.21 fs (during which the vibrational wave packet travels to the state crossing) and a population-transfer time of 2.3-2.4 fs. Our experimental results and theoretical simulations show that delays in population transfer readily appear in otherwise-adiabatic reactions and are typically on the order of 1 fs for intersecting molecular valence states. These results have implications for many research areas, such as atomic and molecular physics, charge transfer or light harvesting.

physics.chem-ph↗

arXiv2024

Do Molecular Geometries Change Under Vibrational Strong Coupling?

As pioneering experiments have shown, strong vibrational coupling between molecular vibrations and light modes in an optical cavity can significantly alter molecular properties and even affect chemical reactivity. However, the current theoretical description is limited and far from complete. To explore the origin of this exciting observation, we investigate how the molecular structure changes under strong light-matter coupling using an ab-initio method based on the cavity Born-Oppenheimer Hartree-Fock ansatz. By optimizing H$_2$O and H$_2$O$_2$ resonantly coupled to cavity modes, we study the importance of reorientation and geometric relaxation. In addition, we show that the inclusion of one or two cavity modes can change the observed results. On the basis of our findings, we derive a simple concept to estimate the effect of the cavity interaction on the molecular geometry using the molecular polarizability and the dipole moments.

physics.chem-ph↗

arXiv2024

Extending the Tavis-Cummings model for molecular ensembles -- Exploring the effects of dipole self energies and static dipole moments

Strong coupling of organic molecules to the vacuum field of a nanoscale cavity can be used to modify their chemical and physical properties. We extend the Tavis-Cummings model for molecular ensembles and show that the often neglected interaction terms arising from the static dipole moment and the dipole self-energy are essential for a correct description of the light-matter interaction in polaritonic chemistry. On the basis of a full quantum description, we simulate the excited-state dynamics and spectroscopy of MgH$^+$ molecules resonantly coupled to an optical cavity. We show that the inclusion of static dipole moments and the dipole self-energy is necessary to obtain a consistent model. We construct an efficient two-level system approach that reproduces the main features of the real molecular system and may be used to simulate larger molecular ensembles.

physics.chem-ph↗

arXiv2024

Analytic Model Reveals Local Molecular Polarizability Changes Induced by Collective Strong Coupling in Optical Cavities

Despite recent numerical evidence, one of the fundamental theoretical mysteries of polaritonic chemistry is how and if collective strong coupling can induce local changes of the electronic structure to modify chemical properties. Here we present non-perturbative analytic results for a model system consisting of an ensemble of $N$ harmonic molecules under vibrational strong coupling (VSC) that alters our present understanding of this fundamental question. By applying the cavity Born-Oppenheimer partitioning on the Pauli-Fierz Hamiltonian in dipole approximation, the dressed many-molecule problem can be solved self-consistently and analytically in the dilute limit. We discover that the electronic molecular polarizabilities are modified even in the case of vanishingly small single-molecule couplings. Consequently, this non-perturbative local polarization mechanism persists even in the large-$N$ limit. In contrast, a perturbative calculation of the polarizabilities leads to a qualitatively erroneous scaling behavior with vanishing effects in the large-$N$ limit. Nevertheless, the exact (self-consistent) polarizabilities can be determined from single-molecule strong coupling simulations instead. Our fundamental theoretical observations demonstrate that hitherto existing collective-scaling arguments are insufficient for polaritonic chemistry and they pave the way for refined single- (or few-) molecule strong-coupling ab-initio simulations of chemical systems under collective strong coupling.

quant-ph↗

arXiv2023

Ab-Initio Vibro-Polaritonic Spectra in Strongly Coupled Cavity-Molecule Systems

Recent experiments have revealed the profound effect of strong light-matter interactions in optical cavities on the electronic ground state of molecular systems. This phenomenon, known as vibrational strong coupling (VSC), can modify reaction rates and induce the formation of molecular vibrational polaritons, hybrid states involving both photon modes and vibrational modes of molecules. We present an ab-initio methodology, based on the cavity Born-Oppenheimer Hartree-Fock ansatz, which is specifically powerful for ensembles of molecules, to calculate vibro-polaritonic IR spectra. This method allows a comprehensive analysis of these hybrid states. Our semi-classical approach, validated against full quantum simulations, reproduces key features of the vibro-polaritonic spectra. The underlying analytic gradients also pave the way for optimizing cavity-coupled molecular systems and performing semi-classical dynamics simulations

quant-ph↗

arXiv2023

Cavity-Born-Oppenheimer Hartree-Fock Ansatz: Light-matter Properties of Strongly Coupled Molecular Ensembles

Experimental studies indicate that optical cavities can affect chemical reactions, through either vibrational or electronic strong coupling and the quantized cavity modes. However, the current understanding of the interplay between molecules and confined light modes is incomplete. Accurate theoretical models, that take into account inter-molecular interactions to describe ensembles, are therefore essential to understand the mechanisms governing polaritonic chemistry. We present an ab-initio Hartree-Fock ansatz in the framework of the cavity Born-Oppenheimer approximation and study molecules strongly interacting with an optical cavity. This ansatz provides a non-perturbative, self-consistent description of strongly coupled molecular ensembles taking into account the cavity-mediated dipole self-energy contributions. To demonstrate the capability of the cavity Born-Oppenheimer Hartree-Fock ansatz, we study the collective effects in ensembles of strongly coupled diatomic hydrogen fluoride molecules. Our results highlight the importance of the cavity-mediated inter-molecular dipole-dipole interactions, which lead to energetic changes of individual molecules in the coupled ensemble.

quant-ph↗

arXiv2023

Unraveling a cavity induced molecular polarization mechanism from collective vibrational strong coupling

We demonstrate that collective vibrational strong coupling of molecules in thermal equilibrium can give rise to significant local electronic polarizations in the thermodynamic limit. We do so by first showing that the full non-relativistic Pauli-Fierz problem of an ensemble of strongly-coupled molecules in the dilute-gas limit reduces in the cavity Born-Oppenheimer approximation to a cavity-Hartree equation for the electronic structure. Consequently, each individual molecule experiences a self-consistent coupling to the dipoles of all other molecules, which amount to non-negligible values in the thermodynamic limit (large ensembles). Thus collective vibrational strong coupling can alter individual molecules strongly for localized "hotspots" within the ensemble. Moreover, the discovered cavity-induced polarization pattern possesses a zero net polarization, which resembles a continuous form of a spin glass (or better polarization glass). Our findings suggest that the thorough understanding of polaritonic chemistry, requires a self-consistent treatment of dressed electronic structure, which can give rise to numerous, so far overlooked, physical mechanisms.

quant-ph↗

arXiv2021

Coupled nuclear and electron dynamics in the vicinity of a conical intersection

Ultrafast optical techniques allow to study ultrafast molecular dynamics involving both nuclear and electronic motion.To support interpretation, theoretical approaches are needed that can describe both the nuclear and electron dynamics.Hence, we revisit and expand our ansatz for the coupled description of the nuclear and electron dynamics in molecular systems (NEMol). In this purely quantum mechanical ansatz the quantum-dynamical description of the nuclear motion is combined with the calculation of the electron dynamics in the eigenfunction basis. The NEMol ansatz is applied to simulate the coupled dynamics of the molecule NO2 in the vicinity of a conical intersection (CoIn) with a special focus on the coherent electron dynamics induced by the non-adiabatic coupling. Furthermore, we aim to control the dynamics of the system when passing the CoIn. The control scheme relies on the carrier envelope phase (CEP) of a few-cycle IR pulse. The laser pulse influences both the movement of the nuclei and the electrons during the population transfer through the CoIn.

physics.comp-ph↗

arXiv2020

Visualizing Conical Intersection Passages via Vibronic Coherence Maps Generated by Stimulated Ultrafast X--Ray Raman Signals

The rates and outcomes of virtually all photophysical and photochemical processes are determined by Conical Intersections. These are regions of degeneracy between electronic states on the nuclear landscape of molecules where electrons and nuclei evolve on comparable timescales and become strongly coupled, enabling radiationless relaxation channels upon optical excitation. Due to their ultrafast nature and vast complexity, monitoring Conical Intersections experimentally is an open challenge. We present a simulation study on the ultrafast photorelaxation of uracil, which demonstrates a new window into Conical Intersections obtained by recording the transient wavepacket coherence during this passage with an x-ray free electron laser pulse. We report two major findings. First, we find that the vibronic coherence at the conical intersection lives for several hundred femtoseconds and can be measured during this entire time. Second, the time-dependent energy splitting landscape of the participating vibrational and electronic states is directly extracted from Wigner spectrograms of the signal. These offer a novel physical picture of the quantum Conical Intersection pathways through visualizing their transient vibronic coherence distributions. The path of a nuclear wavepacket around the Conical Intersection is directly mapped by the proposed experiment.

physics.chem-ph↗

arXiv2020

Photoprotecting uracil by coupling with lossy nanocavities

We analyze how the photorelaxation dynamics of a molecule can be controlled by modifying its electromagnetic environment using a nanocavity mode. In particular, we consider the photorelaxation of the RNA nucleobase uracil, which is the natural mechanism to prevent photodamage. In our theoretical work, we identify the operative conditions in which strong coupling with the cavity mode can open an efficient photoprotective channel, resulting in a relaxation dynamics twice as fast than the natural one. We rely on a state-of-the-art chemically-detailed molecular model and a non-Hermitian Hamiltonian propagation approach to perform full-quantum simulations of the system dissipative dynamics. By focusing on the photon decay, our analysis unveils the active role played by cavity-induced dissipative processes in modifying chemical reaction rates, in the context of molecular polaritonics. Remarkably, we find that the photorelaxation efficiency is maximized when an optimal trade-off between light-matter coupling strength and photon decay rate is satisfied. This result is in contrast with the common intuition that increasing the quality factor of nanocavities and plasmonic devices improves their performance. Finally, we use a detailed model of a metal nanoparticle to show that the speedup of the uracil relaxation could be observed via coupling with a nanosphere pseudomode, without requiring the implementation of complex nanophotonic structures.

cond-mat.mes-hall↗