Slow Nuclear Motion Research Articles

Collective response in light-matter interactions: The interplay between strong coupling and local dynamics.

A model designed to mimic the implications of the collective optical response of molecular ensembles in optical cavities on molecular vibronic dynamics is investigated. Strong molecule-radiation field coupling is often reached when a large number N of molecules respond collectively to the radiation field. In electronic strong coupling, molecular nuclear dynamics following polariton excitation reflects (a) the timescale separation between the fast electronic and photonic dynamics and the slow nuclear motion on one hand and (b) the interplay between the collective nature of the molecule-field coupling and the local nature of the molecules nuclear response on the other. The first implies that the electronic excitation takes place, in the spirit of the Born approximation, at an approximately fixed nuclear configuration. The second can be rephrased as the intriguing question of whether the collective nature of optical excitation leads to collective nuclear motion following polariton formation resulting in so-called polaron decoupled dynamics. We address this issue by studying the dynamical properties of a simplified Holstein-Tavis-Cummings-type model, in which boson modes representing molecular vibrations are replaced by two-level systems, while the boson frequency and the vibronic coupling are represented by the coupling between these levels (that induces Rabi oscillations between them) and electronic state dependence of this coupling. We investigate the short-time behavior of this model following polariton excitation as well as its response to CW driving and its density of states spectrum. We find that, while some aspects of the dynamical behavior appear to adhere to the polaron decoupling picture, the observed dynamics mostly reflect the local nature of the nuclear configuration of the electronic polariton rather than this picture.

Charge localization induced by reorientation of FA cations greatly suppresses nonradiative electron-hole recombination in FAPbI3 perovskites: A time-domain Ab Initio study

Recent experiments report the rotation of FA (FA=HC[NH2]2+) cations significantly influence the excited-state lifetime of FAPbI3. However, the underlying mechanism remains unclear. Using ab initio nonadiabatic (NA) molecular dynamics combined with time-domain density functional simulations, we have demonstrated that reorientation of partial FA cations significantly inhibits nonradiative electron-hole recombination with respect to the pristine FAPbI3 due to the decreased NA coupling by localizing electron and hole in different positions and the suppressed atomic motions. Slow nuclear motions simultaneously increase the decoherence time, which is overcome by the reduced NA coupling, extending electron-hole recombination time scales to several nanoseconds and being about 3.9 times longer than that in pristine FAPbI3, which occurs within sub-nanosecond and agrees with experiment. Our study established the mechanism for the experimentally reported prolonged excited-state lifetime, providing a rational strategy for design of high performance of perovskite solar cells and optoelectronic devices.

Current-induced atomic motion, structural instabilities, and negative temperatures on molecule-electrode interfaces in electronic junctions

Molecule-electrode interfaces in molecular electronic junctions are prone to chemical reactions, structural changes, and localized heating effects caused by electric current. These can be exploited for device functionality or may be degrading processes that limit performance and device lifetime. We develop a nonequilibrium Green's function based transport theory in which the central region atoms and, more importantly, atoms on molecule-electrode interfaces are allowed to move. The separation of time-scales of slow nuclear motion and fast electronic dynamics enables the algebraic solution of the Kadanoff-Baym equations in the Wigner space. As a result, analytical expressions for dynamical corrections to the adiabatically computed Green's functions are produced. These dynamical corrections depend not only on the instantaneous molecular geometry but also on the nuclear velocities. To make the theoretical approach fully self-consistent, the same time-separation approach is used to develop expressions for the adiabatic, dissipative, and stochastic components of current-induced forces in terms of adiabatic Green's functions. Using these current induced forces, the equation of motion for the nuclear degrees of freedom is cast in the form of a Langevin equation. The theory is applied to model molecular electronic junctions. We observe that the interplay between the value of the spring constant for the molecule-electrode chemical bond and electronic coupling strength to the corresponding electrode is critical for the appearance of structural instabilities and, consequently, telegraphic switching in the electric current. The range of model parameters is identified to observe structurally stable molecular junctions as well as various different kinds of current-induced telegraphic switching. The interfacial structural instabilities are also quantified based on current noise calculations.

Subfemtosecond Tracing of Molecular Dynamics during Strong-Field Interaction.

We introduce and experimentally demonstrate a method where the two intrinsic timescales of a molecule, the slow nuclear motion and the fast electronic motion, are simultaneously measured in a photoelectron photoion coincidence experiment. In our experiment, elliptically polarized, 750nm, 4.5fs laser pulses were focused to an intensity of 9×10^{14} W/cm^{2} onto H_{2}. Using coincidence imaging, we directly observe the nuclear wave packet evolving on the 1sσ_{g} state of H_{2}^{+} during its first round-trip with attosecond temporal and picometer spatial resolution. The demonstrated method should enable insight into the first few femtoseconds of the vibronic dynamics of ionization-induced unimolecular reactions of larger molecules.

Time-Dependent Many-Electron Treatment of Electronic Energy and Charge Transfer in Atomic Collisions

Electronic energy and charge transfer in atomic collisions are described within a first principles molecular dynamics including an explicit treatment of electronic motions, in terms of time-dependent many-electron wavefunctions. Following an overview of treatments in the literature based on expansions in sets of adiabatic and diabatic electronic states, this article emphazises the use of time-dependent molecular orbitals and time-dependent Hartree−Fock states. Three fundamental problems are identified in a first principles dynamics, relating to the calculation of state-to-state transition probabilities and expectation values, to the translational motion of electrons moving with nuclei, and to the coupling of fast electronic transitions and slow nuclear motions. Solutions to these problems are described on the basis of an eikonal representation of wavefunctions and sums over initial conditions, of the use of traveling atomic functions to expand molecular orbitals, and of a relax-and-drive propagation procedure for electrons and nuclei. Examples are presented of applications in ion−atom and ion−surface collisions, relating to electronic excitation and charge transfer, orbital polarization, and light emission during collisions.

Multiple Time Scale Quantum Wavepacket Propagation: Electron−Nuclear Dynamics

We have developed a multiple time scale (MTS) quantum wavepacket propagation algorithm for the electron−nuclear dynamics in which the electron and nuclear motions are followed simultaneously. The large mass disparity between the electron and the nuclear degrees of freedom poses a problem when one propagates the total wavefunction using a single time step. The standard split operator propagation method requires a small time step to be used for stable propagation of the fast electronic degrees of freedom, which leads to an unnecessarily large number of propagations for slow nuclear motions. A new factorization of the time evolution operator based on the decomposition of the total Hamiltonian into the nuclear part and the electronic Hamiltonian is proposed. The MTS factorization allows one to choose the spatial grid size and the time step appropriate for each subsystem separately. The MTS algorithm is applied to a charge-transfer model for the evaluation of optical spectra and thermal rate constants. It is s...

All-particle Hamiltonians for polyatomic molecules. I. Body-fixed frames and coordinates. Classical treatment

With a view to quantization, exact classical expressions of Hamiltonians are derived for a molecular system made up of ν electrons and N nuclei, by applying a body-fixed (BF) matrix procedure recently introduced. Two cases are considered, depending on whether the BF frame origin is at the centre of mass of the nuclei or at the total centre of mass. The arrangement of the nuclei is described by 3 N — 6 internal coordinates, and the 3ν electron BF Cartesian coordinates are used. All terms which contribute to the total energy are identified. The similarities and the differences between the two cases are emphasized, particularly with regard to the consequences of (i) neglecting the electron/total system mass ratio, (ii) the assumption of infinitely slow nuclear motion and the fact that (iii) there can be either no distinction between the body-fixed and the space-fixed frames, i.e. no overall rotation, or not, i.e. no total angular momentum is considered. The separability of the Hamiltonians is discussed.

All-particle Hamiltonians for polyatomic molecules. II. Born—Oppenheimer and other adiabatic approximations

A non-relativistic exact quantum-mechanical expression of the Hamiltonian operator is derived for a molecular system made up of ν electrons and N nuclei, by following a body-fixed quantization procedure recently introduced. The BF frame origin is at the total centre of mass. The arrangement of the nuclei is described by 3 N — 6 internal coordinates, and the 3ν electron body-fixed Cartesian coordinates are used. All terms that contribute, beyond the Born—Oppenheimer electronic Hamiltonian, to the non-adiabatic couplings are identified. The difference between the Born—Oppenheimer approximation (based on the infinitely slow nuclear motion hypothesis and no distinction between the body-fixed and space-fixed frames, i.e. no overall rotation), and the adiabatic approximations based on the infinitely slow nuclear motion hypothesis and no total or nuclear angular momentum, is emphasized. The separability of the total Hamiltonian operator is discussed.