arXiv subjects

Joseph E. Subotnik

Publications and source records attributed to Joseph E. Subotnik.

At least 73 records · Page 4Linked to original sources

arXiv2021

Collective vibrational strong coupling effects on molecular vibrational relaxation and energy transfer: Numerical insights via cavity molecular dynamics simulations

For a small fraction of hot CO2 molecules immersed in a liquid-phase CO2 thermal bath, classical cavity molecular dynamics simulations show that forming collective vibrational strong coupling (VSC) between the C=O asymmetric stretch of CO2 molecules and a cavity mode accelerates hot-molecule relaxation. The physical mechanism underlying this acceleration is the fact that polaritons, especially the lower polariton, can be transiently excited during the nonequilibrium process, which facilitates intermolecular vibrational energy transfer. The VSC effects on these rates (i) resonantly depend on the cavity mode detuning, (ii) cooperatively depend on molecular concentration or Rabi splitting, and (iii) collectively scale with the number of hot molecules, which is similar to Dicke's superradiance. For larger cavity volumes, due to a balance between this superradiant-like behavior and a smaller light-matter coupling, the total VSC effect on relaxation rates can scale slower than $1/N$, and the average VSC effect per molecule can remain meaningful for up to $N \sim10^4$ molecules forming VSC. Moreover, we find that the transiently excited lower polariton prefers to relax by transferring its energy to the tail of the molecular energy distribution rather than equally distributing it to all thermal molecules. Finally, we highlight the similarities of parameter dependence between the current finding with VSC catalysis observed in Fabry-Perot microcavities.

physics.chem-ph↗

arXiv2021

Cavity molecular dynamics simulations of vibrational polariton enhanced molecular nonlinear absorption

Recent experiments have observed that the chemical and photophysical properties of molecules can be modified inside an optical Fabry-Perot microcavity under collective vibrational strong coupling (VSC) conditions, and such modification is currently not well understood by theory. In an effort to understand the origin of such cavity induced phenomena, some recent studies have focused on the effect of the cavity environment on the nonlinear optical response of the molecular subsystem. Here, we use a recently proposed protocol for classical cavity molecular dynamics (CavMD) simulations to numerically investigate the linear and nonlinear response of liquid carbon dioxide under such VSC conditions following an optical pulse excitation. We find that applying a strong pulse of excitation to the lower hybrid light-matter state, i.e., the lower polariton (LP), can lead to an overall molecular nonlinear absorption which is enhanced by up to two orders of magnitude relative to the excitation outside the cavity. This polariton-enhanced multiphoton absorption also causes an ultrashort LP lifetime (0.2 ps) under strong illumination. Unlike usual polariton relaxation processes -- whereby polaritonic energy transfers directly to the manifold of singly excited vibrational dark states -- under the present mechanism, the LP transfers energy directly to the manifold of higher vibrationally excited dark states; these highly excited dark states subsequently relax to the manifold of singly excited states with a lifetime of tens of ps. Because the present mechanism is generic in nature, we expect these numerical predictions to be experimentally observed in different molecular systems and in cavities with different volumes.

cond-mat.mes-hall↗

arXiv2021

A Grid-free Approach for Simulating Sweep and Cyclic Voltammetry

We present a new computational approach to simulate linear sweep and cyclic voltammetry experiments that does not require a discretized grid in space to quantify diffusion. By using a Green's function solution coupled to a standard implicit ordinary differential equation solver, we are able to simulate current and redox species concentrations using only a small grid in time. As a result, where benchmarking is possible, we find that the current method is faster (and quantitatively identical) to established techniques. The present algorithm should help open the door to studying adsorption effects in inner sphere electrochemistry.

physics.chem-ph↗

arXiv2020

Nonadiabatic dynamics at metal surfaces: fewest switches surface hopping with electronic relaxation

A new scheme is proposed for modeling molecular nonadiabatic dynamics near metal surfaces. The charge-transfer character of such dynamics is exploited to construct an efficient reduced representation for the electronic structure. In this representation, the fewest switches surface hopping (FSSH) approach can be naturally modified to include electronic relaxation (ER). The resulting FSSH-ER method is valid across a wide range of coupling strength as supported by tests applied to the Anderson-Holstein model for electron transfer. Future work will combine this scheme with ab initio electronic structure calculations.

physics.chem-ph↗

arXiv2020

On the Proper Derivation of the Floquet-based Quantum Classical Liouville Equation and Surface Hopping Describing a Molecule or Material Subject to an External Field

We investigate two approaches to derive the proper Floquet-based quantum-classical Liouville equation (F-QCLE) for laser-driven electron-nuclear dynamics. The first approach projects the operator form of the standard QCLE onto the diabatic Floquet basis, then transforms to the adiabatic representation. The second approach directly projects the QCLE onto the Floquet adiabatic basis. Both approaches yield a form which is similar to the usual QCLE with two modifications: 1. The electronic degrees of freedom are expanded to infinite dimension. 2. The nuclear motion follows Floquet quasi-energy surfaces. However, the second approach includes an additional cross derivative force due to the dual dependence on time and nuclear motion of the Floquet adiabatic states. Our analysis and numerical tests indicate that this cross derivative force is a factitious artifact, suggesting that one cannot safely exchange the order of Floquet state projection with adiabatic transformation. Our results are in accord with similar findings by Izmaylov et al., who found that transforming to the adiabatic representation must always be the last operation applied, though now we have extended this result to a time-dependent Hamiltonian. This paper and the proper derivation of the F-QCLE should lay the basis for further improvements of Floquet surface hopping.

physics.chem-ph↗

arXiv2020

A Protocol for Spectroscopists to Isolate The Effect of Berry Geometric Magnetic Forces on Molecular Dynamics

We propose a novel means to isolate and quantify the effects of Berry force on molecular dynamics using two reasonably strong continuous wave (CW) laser fields with frequencies $ω$ and $2ω$. For molecules or materials with three frequency-matching bright transitions ($|{0}\rangle\rightarrow|{1}\rangle$, $|{1}\rangle\rightarrow|{2}\rangle$, $|{0}\rangle\rightarrow|{2}\rangle$) at frequencies ($ω$, $ω$, $2ω$) respectively, the effects of Berry curvature can be isolated by varying the phase between the two laser fields ($Δϕ$) and monitoring the dynamics. Moreover, we find that the resulting chemical dynamics can depend critically on the sign of $Δϕ$; in other words, the effects of Berry curvature can be enormous. Thus, this manuscript represents an unusual step forward towards using light-matter interactions to affect chemical dynamics, suggesting that topological concepts usually invoked in adiabatic quantum optics and condensed matter can be directly applied to non-adiabatic chemical excited state dynamics.

physics.chem-ph↗

arXiv2020

Near Total Electronic Spin Separation as Caused by Nuclear Dynamics: Perturbing a Real-Valued Conical Intersection with Complex-Valued Spin-Orbit Coupling

We investigate the nuclear dynamics near a real-valued conical intersection that is perturbed by a complex-valued spin-orbit coupling. For a model Hamiltonian with two outgoing channels, we find that even a small spin-orbit coupling can dramatically affect the pathway selection on account of Berry force, leading to extremely large spin selectivity (sometime as large as 100%). Thus, this Letter opens the door for organic chemists to start designing spintronic devices that use nuclear motion and conical intersections (combined with standard spin-orbit coupling) in order to achieve spin selection. Vice versa, for physical chemists, this Letter also emphasizes that future semiclassical simulations of intersystem crossing (which have heretofore ignored Berry force) should be corrected to account for the spin polarization that inevitably arises when dynamics pass near conical intersections.

physics.chem-ph↗

arXiv2020

Analytic Gradients and Derivative Couplings for Configuration Interaction with All Single Excitations and One Double Excitation -- En Route to Nonadiabatic Dynamics

We present analytic gradients and derivative couplings for the simplest possible multireference configuration interaction method, CIS-1D, an electronic structure ansatz that includes all single excitations and one lone double excitation on top of a Hartree-Fock reference state. We show that the resulting equations are numerically stable and require the evaluation of a similar number of integrals as compared to standard CIS theory; one can easily differentiate the required frontier orbitals ($\mathtt{h}$ and $\mathtt{l}$) with minimal cost. The resulting algorithm has been implemented within the Q-Chem electronic structure package and should be immediately useful for understanding photochemistry with $S_{0}$-$S_{1}$ crossings.

physics.chem-ph↗

arXiv2020

Cavity molecular dynamics simulations of liquid water under vibrational ultrastrong coupling

We simulate vibrational strong (VSC) and ultrastrong coupling (V-USC) for liquid water with classical molecular dynamics simulations. When the cavity modes are resonantly coupled to the O-H stretch mode of liquid water, the infrared spectrum shows asymmetric Rabi splitting. The lower polariton (LP) may be suppressed or enhanced relative to the upper polariton (UP) depending on the frequency of the cavity mode. Moreover, although the static properties and the translational diffusion of water are not changed under VSC or V-USC, we do find the modification of the orientational autocorrelation function of H$_2$O molecules especially under V-USC, which could play a role in ground-state chemistry.

physics.chem-ph↗

arXiv2020

On the Origin of Ground-State Vacuum-Field Catalysis: Equilibrium Consideration

Recent experiments suggest that vibrational strong coupling (VSC) may significantly modify ground-state chemical reactions and their rates even without external pumping. The intrinsic mechanism of this "vacuum-field catalysis" remains largely unclear. Generally, modifications of thermal reactions in the ground electronic states can be caused by equilibrium or non-equilibrium effects. The former are associated with modifications of the reactant equilibrium distribution as expressed by the transition state theory of chemical reaction rates, while the latter stem from the dynamics of reaching and leaving transition configurations. Here, we examine the VSC effect in a cavity environment on chemical rates as calculated by transition state theory. Our approach is to examine the effect of coupling to cavity mode(s) on the potential of mean force (PMF) associated with the reaction coordinate. Within the context of classical nuclei and classical photons, we find that while the PMF can be affected by the cavity environment, this effect is negligible for the usual cavities used to examine VSC situations.

physics.chem-ph↗

arXiv2019

Quasiclassical Modeling of Cavity Quantum Electrodynamics

We model a collection of $N$ two-level systems (TLSs) coupled to a multimode cavity via Meyer-Miller-Stock-Thoss (MMST) dynamics, sampling both electronic and photonic zero-point energies (ZPEs) and propagating independent trajectories in Wigner phase space. By investigating the ground state stability of a single TLS, we use MMST dynamics to separately study both electronic ZPE effects (which would naively lead to the breakdown of the electronic ground state) as well as photonic ZPE effects (which would naively lead to spontaneous absorption). By contrast, including both effects (i.e., sampling both electronic and photonic ZPEs) leads to the dynamical stability of the electronic ground state. Therefore, MMST dynamics provide a practical way to identify the contributions of self-interaction and vacuum fluctuations. More importantly, we find that MMST dynamics can predict accurate quantum dynamics for both electronic populations and electromagnetic field intensity in the high saturation limit. For a single TLS in a cavity, MMST dynamics correctly predict the initial exponential decay of spontaneous emission, Poincaré recurrences, and the positional dependence of a spontaneous emission rate. For an array of $N$ equally spaced TLSs with only one TLS excited initially, MMST dynamics correctly predict the modification of spontaneous emission rate as a function of the spacing between TLSs. Finally, MMST dynamics also correctly model Dicke's superradiance and subradiance (i.e., the dynamics when all TLSs are excited initially) including the correct quantum statistics for the delay time (as found by counting trajectories, for which a full quantum simulation is hard to achieve). Therefore, this work raises the possibility of simulating large-scale collective light-matter interactions with methods beyond mean-field theory.

quant-ph↗

arXiv2019

Configuration interaction approaches for solving quantum impurity models

We develop several configuration interaction approaches for characterizing the electronic structure of an adsorbate on a metal surface (at least in model form). When one can separate adsorbate from substrate, these methods can achieve a reasonable description of adsorbate on-site electron-electron correlation in the presence of a continuum of states. While the present paper is restricted to the Anderson impurity model, there is hope that these methods can be extended to ab initio Hamiltonians, and provide insight into the structure and dynamics of molecule-metal surface interactions.

physics.chem-ph↗

arXiv2019

Understanding the Nature of Mean-Field Semiclassical Light-Matter Dynamics: An Investigation of Energy Transfer, Electron-Electron Correlations, External Driving and Long-Time Detailed Balance

Semiclassical electrodynamics is an appealing approach for studying light-matter interactions, especially for realistic molecular systems. However, there is no unique semiclassical scheme. On the one hand, intermolecular interactions can be described instantaneously by static two-body interactions connecting different molecules plus a classical transverse E-field; we will call this Hamiltonian #I. On the other hand, intermolecular interactions can also be described as effects that are mediated exclusively through a classical one-body E-field without any quantum effects at all (assuming we ignore electronic exchange); we will call this Hamiltonian #II. Moreover, one can also mix these two Hamiltonians into a third, hybrid Hamiltonian, which preserves quantum electron-electron correlations for lower excitations but describes higher excitations in a mean-field way. To investigate which semiclassical scheme is most reliable for practical use, here we study the real-time dynamics of a pair of identical two-level systems (TLSs) undergoing either resonance energy transfer (RET) or collectively driven dynamics. While all approaches perform reasonably well when there is no strong external excitation, we find that no single approach is perfect for all conditions. Each method has its own distinct problems: Hamiltonian #I performs best for RET but behaves in a complicated manner for driven dynamics. Hamiltonian #II is always stable, but obviously fails for RET at short distances. One key finding is that, under externally driving, a full configuration interaction description of Hamiltonian #I strongly overestimates the long-time electronic energy, highlighting the not obvious fact that, if one plans to merge quantum molecules with classical light, a full, exact treatment of electron-electron correlations can actually lead to worse results than a simple mean-field treatment.

physics.chem-ph↗

arXiv2019

Predictive Semiclassical Model for Coherent and Incoherent Emission in the Strong Field Regime: The Mollow Triplet Revisited

We re-investigate the famous Mollow triplet and show that most of the well-known quantum characteristics of the Mollow triplet--including incoherent emission and a non-standard dependence of the sidebands on detuning--can be recovered quantitatively using semiclassical dynamics with a classical light field. In fact, by not relying on the rotating wave approximation, a semiclassical model predicts some quantum effects beyond the quantum optical Bloch equation, including higher order scattering and asymmetric sideband features. This letter highlights the fact that, with strong intensities, many putatively quantum features of light-matter interactions arise from a simple balance of mean-field electrodynamics and elementary spontaneous emission which requires minimal computational cost. Our results suggest that the application of semiclassical electrodynamics to problems with strong light-matter coupling in the fields of nanophotonics and superradiance are likely to yield a plethora of new information.

physics.chem-ph↗

arXiv2019

Understanding Detailed Balance for an Electron-Radiation System Through Mixed Quantum-Classical Electrodynamics

We investigate detailed balance for a quantum system interacting with thermal radiation within mixed quantum-classical theory. For a two-level system coupled to classical radiation fields, three semiclassical methods are benchmarked: (1) Ehrenfest dynamics over-estimate the excited state population at equilibrium due to the failure of capturing vacuum fluctuations. (2) The coupled Maxwell-Bloch equations, which supplement Ehrenfest dynamics by damping at the full golden rule rate, under-estimate the excited state population due to double-counting of the self-interaction effect. (3) Ehrenfest+R dynamics recover detailed balance and the correct thermal equilibrium by enforcing the correct balance between the optical excitation and spontaneous emission of the quantum system. These results highlight the fact that, when properly designed, mixed quantum-classical electrodynamics can simulate thermal equilibrium in the field of nanoplasmonics.

physics.chem-ph↗

arXiv2019

A Comparison of Different Classical, Semiclassical and Quantum Treatments of Light-Matter Interactions: Understanding Energy Conservation

The optical response of an electronic two-level system (TLS) coupled to an incident continuous wave (cw) electromagnetic (EM) field is simulated explicitly in one dimension by the following five approaches: (i) the coupled Maxwell-Bloch equations, (ii) the optical Bloch equation (OBE), (iii) Ehrenfest dynamics, (iv) the Ehrenfest+R approach and (v) classical dielectric theory (CDT). Our findings are as follows: (i) standard Ehrenfest dynamics predict the correct optical signals only in the linear response regime where vacuum fluctuations are not important; (ii) both the coupled Maxwell-Bloch equations and CDT predict incorrect features for the optical signals in the linear response regime due to a double-counting of self-interaction; (iii) by exactly balancing the effects of self-interaction versus the effects of quantum fluctuations (and insisting on energy conservation), the Ehrenfest+R approach generates the correct optical signals in the linear regime and slightly beyond, yielding, e.g., the correct ratio between the coherent and incoherent scattering EM fields. As such, Ehrenfest+R dynamics agree with dynamics from the quantum OBE, but whereas the latter is easily applicable only for a single TLS in vacuum, the former should be applicable to large systems in environments with arbitrary dielectrics. Thus, this benchmark study suggests that the Ehrenfest+R approach may be very advantageous for simulating light-matter interactions semiclassically.

physics.optics↗

arXiv2018

A Necessary Trade-off for Semiclassical Electrodynamics: Accurate Short-Range Coulomb Interactions versus the Enforcement of Causality?

We investigate two key representative semiclassical approaches for propagating resonant energy transfer between a pair of electronic two-level systems (donor and acceptor) with coupled Maxwell-Liouville equations. On the one hand, when the electromagnetic (EM) field is treated classically and Coulomb interactions are treated quantum-mechanically, we find that a quantum-classical mismatch leads to a violation of causality, i.e., the acceptor can be excited before the retarded EM field arrives. On the other hand, if we invoke a classical intermolecular Coulomb operator, we find that the energy transfer in the near field loses quantitative accuracy compared with Förster theory, even though causality is strictly obeyed. Thus, our work raises a fundamental paradox when choosing a semiclassical electrodynamics algorithm. Namely, which is more important: Accurate short range interactions or long range causality? Apparently, one cannot have one's cake and eat it too.

physics.chem-ph↗

arXiv2018

Ehrenfest+R Dynamics I: A Mixed Quantum-Classical Electrodynamics Simulation of Spontaneous Emission

The dynamics of an electronic system interacting with an electromagnetic field is investigated within mixed quantum-classical theory. Beyond the classical path approximation (where we ignore all feedback from the electronic system on the photon field), we consider all electron-photon interactions explicitly according to Ehrenfest (i.e. mean-field) dynamics and a set of coupled Maxwell-Liouville equations. Because Ehrenfest dynamics cannot capture certain quantum features of the photon field correctly, we propose a new Ehrenfest+R method that can recover (by construction) spontaneous emission while also distinguishing between electromagnetic fluctuations and coherent emission.

physics.chem-ph↗