Wave Packet Simulations Research Articles

High-Efficiency Iron Photosensitizer Explained with Quantum Wavepacket Dynamics.

Fe(II) complexes have long been assumed unsuitable as photosensitizers because of their low-lying nonemissive metal centered (MC) states, which inhibit electron transfer. Herein, we describe the excited-state relaxation of a novel Fe(II) complex that incorporates N-heterocyclic carbene ligands designed to destabilize the MC states. Using first-principles quantum nuclear wavepacket simulations we achieve a detailed understanding of the photoexcited decay mechanism, demonstrating that it is dominated by an ultrafast intersystem crossing from (1)MLCT-(3)MLCT proceeded by slower kinetics associated with the conversion into the (3)MC states. The slowest component of the (3)MLCT decay, important in the context of photosensitizers, is much longer than related Fe(II) complexes because the population transfer to the (3)MC states occurs in a region of the potential where the energy gap between the (3)MLCT and (3)MC states is large, making the population transfer inefficient.

Sound attenuation in amorphous silica at frequencies near the boson peak

We use molecular dynamics phonon wave packet (WP) simulations to study acoustic propagation and attenuation in amorphous silica $(a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2})$ at frequencies near the boson peak (BP) position and compare them with the results of equilibrium molecular dynamics (EMD) simulations. The sound attenuation coefficients obtained from WP simulations are generally consistent with those from EMD predictions and have reasonable agreement with the existing experimental data. Near the BP position, we found the frequency-dependent sound attenuation coefficients for longitudinal and transverse modes both follow the Rayleigh-scattering fourth power law. Above the BP frequency, however, the propagating phonon is essentially attenuated in $a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ within a few nanometers, and the accurate determination of the sound attenuation coefficients by the WP simulation becomes challenging. The modeling results provide a reference for future experimental investigations of sound attenuation in $a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ thin film using narrow-band coherent phonons.

Non-adiabatic dynamics of molecules in optical cavities

Strong coupling of molecules to the vacuum field of micro cavities can modify the potential energy surfaces thereby opening new photophysical and photochemical reaction pathways. While the influence of laser fields is usually described in terms of classical field, coupling to the vacuum state of a cavity has to be described in terms of dressed photon-matter states (polaritons) which require quantized fields. We present a derivation of the non-adiabatic couplings for single molecules in the strong coupling regime suitable for the calculation of the dressed state dynamics. The formalism allows to use quantities readily accessible from quantum chemistry codes like the adiabatic potential energy surfaces and dipole moments to carry out wave packet simulations in the dressed basis. The implications for photochemistry are demonstrated for a set of model systems representing typical situations found in molecules.

Photo-induced 1,3-cyclohexadiene ring opening reaction: Ab initio on-the-fly nonadiabatic molecular dynamics simulation

Abstract By a newly developed algorithm to compute global nonadiabatic switching probability only using electronic adiabatic potential energy surfaces and its gradients, we could able to perform on-the-fly trajectory surface hopping molecular dynamics at the SA-CASSCF(14, 8)/6-31G* quantum level to investigate the photo-induced 1,3-cyclohexadiene ring opening reaction from 1,3-cyclohexadiene (CHD) to 1,3,5-hexatriene (HT). We found four conical intersection zones within three singlet low-lying electronic states, in which 1B/2A and 1A/2A are well known to be the most important conical intersection zones to govern dynamics of ring opening reaction. The present simulation gained insight into nonadiabatic dynamics in which 516 sampling trajectories out of 600 can switch back and forth between two involved potential energy surfaces not only at 1B/2A, but also at 1A/2A. This insight is the key to influence quantum yields and time scales of the ring opening reaction. We estimated time constants as 46 fs, 82 fs, and 120 fs, respectively for crossing 1B/2A and 1A/2A, and excited-state lifetime in comparison with corresponding experimental values 56 fs, 80 fs and 130 fs. We estimated quantum yield 0.47/0.53 to HT/CHD in comparison to experiment values 0.30/0.70 in the gas-phase and 0.41/0.59 in the condense-phase. Moreover, the present simulation within 600 sampling trajectories indicates that there is no five membered ring forming in agreement with experiment results and with prediction from the previous wave-packet simulation as a rare case.

Potential flow simulations of Peregrine‐type deep water surface gravity wave packets

AbstractApplying a higher order spectral method for potential flow we numerically determine the time evolution of non‐linear deep water wave packets originating from initial conditions derived from the Peregrine breather solution of the non‐linear Schrödinger equation (NLS). The spatio‐temporal evolution of the wave packets qualitatively agrees well with what would be expected from lowest order weakly nonlinear estimates (NLS). Some quantitative discrepancies do, however, exist: The maximum wave envelope amplification appears retarded with respect to the NLS predictions. The amplification factor slightly exceeds the factor known from the NLS solution. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Wave packet simulations of antiproton scattering on molecular hydrogen

The problem of antiproton scattering on the molecular hydrogen is investigated by means of wave packet dynamics. The electronic potential energy surfaces of the antiproton H2 system are presented within this work. Excitation and dissociation probabilities of the molecular hydrogen for collision energies in the ultra low energy regime below 10 eV are computed

Detecting the phonon interference effect in Si/Ge nanocomposite by wave packets

As the particle size in nanocomposites approaches the order of several nanometers that is comparable with the phonon wavelength, the phonon wave interference and particle scattering effect will lead to a much more complicated energy transport. This letter reports the investigation of phonon interference and scattering in Si/Ge nanocomposites by the phonon wave packet simulation. The particle size, particle layer number, and particle shape effects on the phonon transmission are considered. For both cubic and spherical particles, the phonon transmission coefficient fluctuates obviously with the increasing particle size and there are local peaks and valleys of transmission coefficient at certain particle sizes. The transmission decreases with the increasing layers of particle. The optical multi-layer transmission model can be applied to predict the transmission coefficient of Si/Ge nanocomposites with layered particles. The enhancement of thermal conductance is up to 19.1% due to the phonon interference effect.

Links between acoustic field statistics, baroclinic wave geometry, and bathymetry computed with time-stepped three-dimensional acoustic simulations

Simulations of sound propagation in a canyon region and in an area populated with internal-wave packets have been performed. Time dependence was introduced into the three-dimensional water-column environments using dynamical ocean models. In the canyon simulation, time-dependence is limited to waves with hydrostatic pressure generated by tidal transports prescribed at the dynamical model boundaries. The wave-packet simulation includes both hydrostatic and nonhydrostatic waves. For the canyon, propagation of 300-Hz sound from shallow to deep water is simulated. For wave packets, 200-Hz sound is simulated. For each situation, statistics related to horizontal correlation are estimated at synthetic “arrays” inserted into the domain. Correlation properties are estimated from single field snapshots and from time-series analysis of the output, and the two are compared. Array performance evaluations from the two methods are also compared. Covariance matrix properties for the time-series output, including principal component behavior, are analyzed to examine the interference effects and fluctuation mechanisms that are responsible for the spatially patchy array performance results.

Separating nuclear spin isomers using a pump–dump laser scheme

The concept of nuclear spin isomers was already introduced in the early days of quantum mechanics. Despite its importance, not much work has been done to separate them experimentally by pushing the ratio away from its equilibrium value. We propose to use ultrashort laser pulses in a pump–dump-like experiment to enhance the ratio between different nuclear spin isomers. Exemplary wave packet simulations with optimized femtosecond pump and dump laser pulses are shown on a quinodimethane derivative to illustrate that the ratio between two different groups of nuclear spin isomers is enhanced.

Modified series model for cross-plane thermal conductivity of short-period Si/Ge superlattices

Superlattices have great application potentials in thermo-electric materials and solid laser techniques. Their complicated heat transport mechanisms due to size effects, multi-interfaces and mini-band are very important issues for the prediction of thermal conductivity. In this paper, the short-period Si/Ge superlattices are investigated by a modified series model based on the Debye-Callaway description. The Lambert Law is used to describe the phonon emission within hemisphere space. In addition, the phonon interface transmission coefficients obtained from the phonon wave packet simulation are incorporated into boundary condition of the model, which removes the fitting parameters in the model. Better agreement with experiment is obtained. The effects of temperature, wavelength-dependent phonon transmission, superlattice periods, as well as the thickness of Ge layer are considered in this paper.

Eight-dimensional quantum reaction rate calculations for the H+CH4 and H2+CH3 reactions on recent potential energy surfaces.

Eight-dimensional (8D) transition-state wave packet simulations have been performed on two latest potential energy surfaces (PES), the Zhou-Fu-Wang-Collins-Zhang (ZFWCZ) PES [Y. Zhou, B. Fu, C. Wang, M. A. Collins, and D. H. Zhang, J. Chem. Phys. 134, 064323 (2011)] and the Xu-Chen-Zhang (XCZ)-neural networks (NN) PES [X. Xu, J. Chen, and D. H. Zhang, Chin. J. Chem. Phys. 27, 373 (2014)]. Reaction rate constants for both the H+CH4 reaction and the H2+CH3 reaction are calculated. Simulations of the H+CH4 reaction based on the XCZ-NN PES show that the ZFWCZ PES predicts rate constants with reasonable high accuracy for low temperatures while leads to slightly lower results for high temperatures, in line with the distribution of interpolation error associated with the ZFWCZ PES. The 8D H+CH4 rate constants derived on the ZFWCZ PES compare well with full-dimensional 12D results based on the equivalent m-ZFWCZ PES, with a maximum relative difference of no more than 20%. Additionally, very good agreement is shown by comparing the 8D XCZ-NN rate constants with the 12D results obtained on the ZFWCZ-WM PES, after considering the difference in static barrier height between these two PESs. The reaction rate constants calculated for the H2+CH3 reaction are found to be in good consistency with experimental observations.

Charge transfer dynamics from adsorbates to surfaces with single active electron and configuration interaction based approaches

We employ wavepacket simulations based on many-body time-dependent configuration interaction (TD-CI), and single active electron theories, to predict the ultrafast molecule/metal electron transfer time scales, in cyano alkanethiolates bonded to model gold clusters. The initial states represent two excited states where a valence electron is promoted to one of the two virtual π∗ molecular orbitals localized on the cyanide fragment. The ratio of the two time scales indicate the efficiency of one charge transfer channel over the other. In both our one-and many-electron simulations, this ratio agree qualitatively with each other as well as with the previously reported experimental time scales (Blobner et al., 2012), measured for a macroscopic metal surface. We study the effect of cluster size and the description of electron correlation on the charge transfer process.

Numerical investigation of the development of three-dimensional wavepackets in a sharp cone boundary layer at Mach 6

AbstractDirect numerical simulations were performed to investigate wavepackets in a sharp cone boundary layer at Mach 6. In order to understand the natural transition process in hypersonic cone boundary layers, the flow was forced by a short-duration (localized) pulse. The pulse disturbance developed into a three-dimensional wavepacket, which consisted of a wide range of disturbance frequencies and wavenumbers. First, the linear development of the wavepacket was studied by forcing the flow with a low-amplitude pulse (0.001 % of the free-stream velocity). The dominant waves within the resulting wavepacket were identified as the second-mode axisymmetric disturbance waves. In addition, weaker first-mode oblique disturbance waves were also observed on the lateral sides of the wavepacket. In order to investigate the nonlinear transition regime, large-amplitude pulse disturbances (0.5 % of the free-stream velocity) were introduced. The response of the flow to the large-amplitude pulse disturbances indicated the presence of a fundamental resonance mechanism. Lower secondary peaks in the disturbance wave spectrum were identified at approximately half the frequency of the high-amplitude frequency band, suggesting the possibility of a subharmonic resonance mechanism. However, the spectrum also indicated that the fundamental resonance was much stronger than the subharmonic resonance. A secondary stability investigation using controlled disturbances confirmed that fundamental resonance is indeed a dominant mechanism compared to subharmonic resonance. Furthermore, strong peaks in the disturbance wave spectrum were also observed for low-azimuthal-wavenumber second-mode oblique waves, hinting at a possible oblique breakdown mechanism. Thus, the wavepacket simulations indicate that the second-mode fundamental resonance and oblique breakdown mechanisms are the strongest for the investigated flow. Hence, both mechanisms are likely to be relevant in the natural transition process for a cone boundary layer at Mach 6.

Phonon thermal transport through tilt grain boundaries in strontium titanate

In this work, we perform nonequilibrium molecular dynamics simulations to study phonon scattering at two tilt grain boundaries (GBs) in SrTiO3. Mode-wise energy transmission coefficients are obtained based on phonon wave-packet dynamics simulations. The Kapitza conductance is then quantified using a lattice dynamics approach. The obtained results of the Kapitza conductance of both GBs compare well with those obtained by the direct method, except for the temperature dependence. Contrary to common belief, the results of this work show that the optical modes in SrTiO3 contribute significantly to phonon thermal transport, accounting for over 50% of the Kapitza conductance. To understand the effect of the GB structural disorder on phonon transport, we compare the local phonon density of states of the atoms in the GB region with that in the single crystalline grain region. Our results show that the excess vibrational modes introduced by the structural disorder do not have a significant effect on phonon scattering at the GBs, but the absence of certain modes in the GB region appears to be responsible for phonon reflections at GBs. This work has also demonstrated phonon mode conversion and simultaneous generation of new modes. Some of the new modes have the same frequency as the initial wave packet, while some have the same wave vector but lower frequencies.

Finite-size effects on molecular dynamics interfacial thermal-resistance predictions

Using molecular dynamics simulations, we study the role of finite size effects on the determination of interfacial thermal resistance between two solids characterized by high phonon mean free paths. In particular, we will show that a direct, heat source-sink method leads to strong size effect, associated with ballistic phonon transport to and from, and specular reflections at the simulation domain boundary. Lack of proper account for these effects can lead to incorrect predictions about the role of interfacial bonding and structure on interfacial thermal resistance. We also show that the finite size effect can be dramatically reduced by introduction of rough external boundaries leading to diffuse phonon scattering, as explicitly demonstrated by phonon wave-packet simulations. Finally, we demonstrate that when careful considerations are given to the effects associated with the finite heat capacity of the simulation domains and phonon scattering from the external surfaces, a size-independent interfacial resistance can be properly extracted from the time integral of the correlation function of heat power across the interface. Our work demonstrates that reliable and consistent values of the interfacial thermal resistance can be obtained by equilibrium and nonequilibrium methods with a relatively small computational cost.

Effect of the Intermolecular Excitation in the Vibrational Predissociation Dynamics of van der Waals Complexes and the Implications for Control

The effect of intermolecular excitation on the vibrational predissociation lifetime is investigated systematically for different vdW complexes Rg-X2(B, v') (Rg = rare gas atom, X = halogen atom) by means of wave packet simulations. The lifetime as a function of intermolecular excitation displays a pattern of maxima and minima, with a similar shape for the different Rg-X2(B, v') complexes. The pattern is consistent with previous experimental findings involving lifetimes of intermolecular excitations in similar systems. The structure of the lifetime pattern is found to be determined by the shape of the resonance wave functions in the two van der Waals degrees of freedom, and more specifically by the magnitude of the overlap between the wave function and the coupling responsible for predissociation. Lifetime maxima and minima are associated with minima and maxima of this overlap, respectively. Implications for control of the complex lifetime are discussed.

Nonadiabatically Coupled π-Electron Rotation and Molecular Vibration in Aromatic Molecules Excited by Polarized UV/Vis Laser Pulses

We review our theoretical studies on laser-driven ultrafast π-electron rotation (ring current) and vibronically coupled molecular vibrations in aromatic molecules with quasi-degenerate excited states. The main focus of discussion is on the role of laser polarization in controlling the coherent vibronic dynamics. We first present general formulations of the coherent electronic wave packet and expectation value of electronic angular momentum for arbitrary laser polarization within a frozen-nuclei model. We show that the relative quantum phase of the superposed quasi-degenerate states, which determines the oscillating behavior of angular momentum, can be manipulated by the ellipticity and orientation of the incident laser. The controllability of π-electron rotation is demonstrated by electronic wave packet simulations for a model molecule. The results of nuclear wave packet simulations on effective potential energy surfaces of the molecule have revealed that the angular momentum is gradually reduced by nonadiabatic transitions. The amplitude of induced molecular vibration depends significantly on the direction of linear polarization vectors but is insensitive to the helicity of circular polarization. The characteristic feature in vibrational amplitudes is attributed to the interference between nuclear wave packets that acquire different quantum phases in nonadiabatic transition. This feature offers a new strategy for laser control of molecular vibrations through the wave packet interference in nonadiabatic transition.

Mode-Specific Tunneling in the Unimolecular Dissociation of cis-HOCO to H + CO2

We report mode-specific tunneling in the unimolecular dissociation of cis-HOCO to H + CO2 using a recent projection theory that makes use of a tunneling path along the imaginary-frequency normal mode, Qim, of a relevant saddle point. The tunneling probabilities and lifetimes are calculated for the ground vibrational state of cis-HOCO and highly excited overtones and combination bands of the modes that have large projections onto the Qim path. To go beyond the harmonic approximation, which is important for the OH stretch, energies and classical turning points are calculated using the anharmonic one-dimensional (1D) potential in that normal mode and used in the projection theory. The tunneling lifetimes calculated for a number of combination states of the OCO bend and CO stretch are in good accord with those estimated in a previous five degrees-of-freedom quantum wavepacket simulation of the dissociative photodetachment of HOCO(-). The present results are also consistent with the interpretation of the tunneling of cis-HOCO to H + CO2 seen in recent experiments.

Resonant detection of the signature of control of a resonance state lifetime using a pump–probe scheme

Realistic wave packet simulations demonstrate that the signature of control of the survival probability and lifetime of a specific resonance state can be observed and probed in typical time-resolved pump–probe experiments.

A dispersion and norm preserving finite difference scheme with transparent boundary conditions for the Dirac equation in [formula omitted

A finite difference scheme is presented for the Dirac equation in (1+1)D. It can handle space- and time-dependent mass and potential terms and utilizes exact discrete transparent boundary conditions (DTBCs). Based on a space- and time-staggered leap-frog scheme it avoids fermion doubling and preserves the dispersion relation of the continuum problem for mass zero (Weyl equation) exactly.Considering boundary regions, each with a constant mass and potential term, the associated DTBCs are derived by first applying this finite difference scheme and then using the Z-transform in the discrete time variable. The resulting constant coefficient difference equation in space can be solved exactly on each of the two semi-infinite exterior domains. Admitting only solutions in l2 which vanish at infinity is equivalent to imposing outgoing boundary conditions. An inverse Z-transformation leads to exact DTBCs in form of a convolution in discrete time which suppress spurious reflections at the boundaries and enforce stability of the whole space–time scheme.An exactly preserved functional for the norm of the Dirac spinor on the staggered grid is presented. Simulations of Gaussian wave packets, leaving the computational domain without reflection, demonstrate the quality of the DTBCs numerically, as well as the importance of a faithful representation of the energy–momentum dispersion relation on a grid.