Wave Packet Study Research Articles

Quantum wave-packet studies on ion–molecule reaction with and without Coriolis-coupling effect

The time-dependent quantum scattering calculation with Chebyshev wave packet propagation scheme has been carried out based on an accurate electronic potential energy surface of H2O+(X4A″). Due to the influence of the deep potential well, the reaction probability of [Formula: see text] shows resonance structures regardless of the Coriolis-coupling (CC) effect or centrifugal sudden (CS) approximation. In the range of collision energy 0.0–1.0 eV, the integral cross section obtained by the CS approximation calculation is smaller than that by the CC calculation, which indicates that the CC effect plays a significant role in the title reaction.

A phonon wave packet study of thermal energy transport across functionalized hard-soft interfaces

Thermal transport across hard-soft interfaces is critical to many applications, such as polymer composites, solar thermal evaporation, and photothermal cancer therapy. In this work, we use wave packets (WP) in molecular dynamics (MD) simulations to study the phonon energy transmission coefficients (ETCs) across different Au-self-assembled monolayer (SAM)-organic liquid interfaces. Three types of thiol SAMs with different terminal groups and chain length heterogeneities are studied, including “-CH3,” “-COOH,” and “hetero” SAMs. Two types of organic liquids, hexamine and hexane, are investigated. When the liquid changes from hexamine to hexane, the Au-CH3 SAM-liquid interfaces show similar ETCs across different phonon modes, since the interactions between nonpolar SAM and different liquids are similar, while the ETCs across the Au-CH3 SAM-liquid interfaces are much higher than those involving bare Au-liquid interfaces. Due to the -COOH functionalization, the Au-COOH SAM-hexamine interface shows the highest ETCs for all phonon modes compared to the other interfaces, which explains why its interfacial thermal conductance (ITC) is also the highest. We find that the Au-hetero-SAM-hexamine interface has higher ETCs in the longitudinal acoustic (LA) modes than the Au-CH3 SAM-hexamine interface; as a result, the ITC of the Au-hetero-SAM-hexamine interface is slightly higher. The ETCs calculated in our WP simulations can interpret the ITCs calculated from our previous MD simulations. Using the mode-resolved ETCs, the ITC contributions for each phonon mode are also calculated. We find that the LA modes play an important role in thermal transport across Au-SAM-liquid interfaces. Results from this WP study will help design interfaces with desirable thermal transport properties.

Mapping of the light-induced conical intersections in the photoelectron spectra of K2 molecules.

In the strong field regime, exploring the physical nature of molecular dynamics is still a challenge due to the dramatic change of molecular potentials. Here, we perform a quantum wave packet study on the pump-probe ionization of K2 molecules and show how the light-induced conical intersections (LICIs) are imprinted into the molecular photoelectron spectra. We demonstrate that the energy and angular distributions of photoelectron spectra provide an accurate mapping of the electronic structure under the influence of the strong laser field. The determination of correct characterization of LICIs can help us to better explore alternative ways to control dynamics.

Non-resonant vibrational excitation of HOD and selective bond breaking.

This paper reports a time-dependent quantum mechanical wave packet study for bond-selective excitation and dissociation of HOD into the H + OD and D + OH channels in the first absorption band. Prior to excitation, the HOD molecule is randomly oriented with respect to a linearly polarized laser field and accurate static dipole moment and polarizability surfaces are included in the interaction potential. Vibrational excitation is obtained with intense, non-resonant 800 nm few-cycle excitation using dynamic Stark effect/impulsive Raman scattering. Dissociation is accomplished by another ultrashort vacuum ultraviolet-laser excitation. A laser control scheme is designed with a train of simple, non-resonant laser pulses in order to enhance the selectivity between the fragmentation channels. The effect of the carrier-envelope-phase of the ultrashort laser pulses is also investigated.

Dynamical resonances of the deuterated CH2+ complex in the electronic ground state: A quantum wavepacket study.

We here investigate the effects of isotopic substituents on the vibrational energy levels of the CH2+ complex, supported by the electronic ground (1 2A') potential energy surface (PES) of the H + CH+ reaction. We calculate the transition state spectrum by Fourier transforming the time-autocorrelation function of the initial wavepacket (WP) chosen in the interaction region of the PES. Using the time-dependent WP approach, the dynamical resonances are identified as bound and quasibound in nature, and they are characterized in terms of the eigenfunctions and lifetimes. The present work on the isotopic variants [CHD+(CDH+) and CD2+] is compared with our earlier work [P. Sundaram et al., Phys. Chem. Chem. Phys. 19, 20172 (2017)] on the parent CH2+ species. The isotopic variants reveal a large number of peaks in the spectra and the eigenfunctions exhibit the systematic nodal progressions and periodic orbits, the same as in CH2+. While the CD2+ complex exactly mimics the resonance behaviors (local and hyperspherical modes) of the bound and quasibound CH2+ complex, the CHD+(CDH+) complex reveals only the local mode behaviors at low energies and significantly less number of resonance structures at high energies. Lifetime analysis of the isotopic variants implies that the CD2+ complex survives much longer than the CHD+(CDH+) complex and concludes the work by noting the following order in the decay profile of the deuterated CH2+ resonances as CH2+>CHD+(CDH+) >CD2+.

Accurate Quantum Wave Packet Study of the Deep Well D+ + HD Reaction: Product Ro-vibrational State-Resolved Integral and Differential Cross Sections.

The H+ + H2 reaction and its isotopic variants as the simplest triatomic ion-molecule reactive system have been attracting much interests, however there are few studies on the titled reaction at state-to-state level until recent years. In this work, accurate state-to-state quantum dynamics studies of the titled reaction have been carried out by a reactant Jacobi coordinate-based time-dependent wave packet approach on diabatic potential energy surfaces constructed by Kamisaka et al. Product ro-vibrational state-resolved information has been calculated for collision energies up to 0.2 eV with maximal total angular momentum J = 40. The necessity of including all K-component for accounting the Coriolis coupling for the reaction has been illuminated. Competitions between the two product channels, (D+ + HD' → D'+ + HD and D+ + HD' → H+ + DD') were investigated. Total integral cross sections suggest that resonances enhance the reactivity of channel D+ + HD'→ H+ + DD', however, resonances depress the reactivity of the another channel D+ + HD' → D'+ + HD. The structures of the differential cross sections are complicated and depend strongly on collision energies of the two channels and also on the product rotational states. All of the product ro-vibrational state-resolved differential cross sections for this reaction do not exhibit rigorous backward-forward symmetry which may indicate that the lifetimes of the intermediate resonance complexes should not be that long. The dynamical observables of this deuterated isotopic reaction are quite different from the reaction of H+ + H2 → H2 + H+ reported previously.

Experimental study of turbulent-jet wave packets and their acoustic efficiency

This paper details the statistical and time-resolved analysis of the relationship between the near-field pressure fluctuations of unforced, subsonic free jets (0.4 ≤ M ≤ 0.6) and their far-field sound emissions. Near-field and far-field microphone measurements were taken on a conical array close to the jets and an azimuthal ring at 20∘ to the jet axis, respectively. Recent velocity and pressure measurements indicate the presence of linear wave packets in the near field by closely matching predictions from the linear homogenous parabolized stability equations, but the agreement breaks down both beyond the end of the potential core and when considering higher order statistical moments, such as the two-point coherence. Proper orthogonal decomposition (POD), interpreted in terms of inhomogeneous linear models using the resolvent framework allows us to understand these discrepancies. A new technique is developed for projecting time-domain pressure measurements onto a statistically obtained POD basis, yielding the time-resolved activity of each POD mode and its correlation with the far field. A single POD mode, interpreted as an optimal high-gain structure that arises due to turbulent forcing, captures the salient near-field–far-field correlation signature; further, the signatures of the next two modes, understood as suboptimally forced structures, suggest that these POD modes represent higher order, acoustically important near-field behavior. An existing Green's-function-based technique is used to make far-field predictions, and results are interpreted in terms of POD/resolvent modes, indicating the acoustic importance of this higher order behavior. The technique is extended to provide time-domain far-field predictions.

Dynamics of photoionization from molecular electronic wavepacket states in intense pulse laser fields: A nonadiabatic electron wavepacket study.

A theory for dynamics of molecular photoionization from nonadiabatic electron wavepackets driven by intense pulse lasers is proposed. Time evolution of photoelectron distribution is evaluated in terms of out-going electron flux (current of the probability density of electrons) that has kinetic energy high enough to recede from the molecular system. The relevant electron flux is in turn evaluated with the complex-valued electronic wavefunctions that are time evolved in nonadiabatic electron wavepacket dynamics in laser fields. To uniquely rebuild such wavefunctions with its electronic population being lost by ionization, we adopt the complex-valued natural orbitals emerging from the electron density as building blocks of the total wavefunction. The method has been implemented into a quantum chemistry code, which is based on configuration state mixing for polyatomic molecules. Some of the practical aspects needed for its application will be presented. As a first illustrative example, we show the results of hydrogen molecule and its isotope substitutes (HD and DD), which are photoionized by a two-cycle pulse laser. Photon emission spectrum associated with above threshold ionization is also shown. Another example is taken from photoionization dynamics from an excited state of a water molecule. Qualitatively significant effects of nonadiabatic interaction on the photoelectron spectrum are demonstrated.

Coupled 3D time-dependent quantum wave-packet study of the O + OH reaction in hyperspherical coordinates on the CHIPR potential energy surface

We report quantum dynamics calculations of the O+OH→H+O2 reaction on the CHIPR (Varandas, 2013) and DMBE IV (Pastrana et al., 1990) potential energy surfaces (PESs) for ground-state HO2 using the 3D time-dependent wavepacket formalism based on hyperspherical coordinates. Reaction probabilities for J=0 are calculated for several initial rovibrational states of the OH radical (v=0; j=0–5). The J-shifting approximation is used to obtain initial state selected rate-coefficients in the range 0–400K. Total and state-to-state rate-coefficients are predicted in good agreement with recent experimental studies and theoretical calculations.

A full-dimensional time-dependent wave packet study of the H + CO2 → OH + CO reaction

A full dimensional quantum dynamics calculation has been carried out to study the prototypical complex forming H+CO2→OH+CO reaction. The total reaction probability is converged with a propagation time much shorter than the reverse reaction, and only exhibits very small oscillatory structures, indicating the reaction proceeds mainly through a direct mechanism. The strong Fermi resonance between the (020) and (100) vibrationally excited states makes it hard to define their efficacy for reactivity, although it is conceivable that the bending excitation is much more effective than the symmetric excitation according to the geometry of the transition state in the entrance channel.

Time-Dependent Quantum Wave Packet Study of the Si + OH → SiO + H Reaction: Cross Sections and Rate Constants.

The dynamics of the Si(3P) + OH(X2Π) → SiO(X1Σ+) + H(2S) reaction is investigated by means of the time-dependent wave packet (TDWP) approach using an ab initio potential energy surface recently developed by Dayou et al. ( J. Chem. Phys. 2013 , 139 , 204305 ) for the ground X2A' electronic state. Total reaction probabilities have been calculated for the first 15 rotational states j = 0-14 of OH(v=0,j) at a total angular momentum J = 0 up to a collision energy of 1 eV. Integral cross sections and state-selected rate constants for the temperature range 10-500 K were obtained within the J-shifting approximation. The reaction probabilities display highly oscillatory structures indicating the contribution of long-lived quasibound states supported by the deep SiOH/HSiO wells. The cross sections behave with collision energies as expected for a barrierless reaction and are slightly sensitive to the initial rotational excitation of OH. The thermal rate constants show a marked temperature dependence below 200 K with a maximum value around 15 K. The TDWP results globally agree with the results of earlier quasi-classical trajectory (QCT) calculations carried out by Rivero-Santamaria et al. ( Chem. Phys. Lett. 2014 , 610-611 , 335 - 340 ) with the same potential energy surface. In particular, the thermal rate constants display a similar temperature dependence, with TDWP values smaller than the QCT ones over the whole temperature range.

Photoinduced Charge Separation Catalyzed by Manganese Oxides onto a Y-Shaped Branching Acceptor Efficiently Preventing Charge Recombination.

A full-dimensional nonadiabatic electron wavepacket study is performed on Mn oxide catalytic charge separation to be created on an accepting molecular system, which is of Y-shaped structure and has a track-branching function for protons and electrons. This branching is necessary in cases in which the transferred electrons and protons are to be eventually carried to mutually different destinations without quick annihilation of the created pair (charge separation). However, as a result of the larger size of such a branched acceptor, the distance between the Mn oxide and the acceptor is so great that it is far from obvious whether an electron is successfully delivered through conical intersections. It is shown here that this can actually occur.

Dynamics and resonances of the H(2S) + CH+(X1Σ+) reaction in the electronic ground state: a detailed quantum wavepacket study.

Initial state-selected and energy resolved channel-specific reaction probabilities, integral cross sections and thermal rate constants of the H(2S) + CH+(X1Σ+) reaction are calculated within the coupled states approximation by a time-dependent wave packet propagation method. The new ab initio global potential energy surface (PES) of the electronic ground state (1 2A') of the system, recently reported by Li et al. [J. Chem. Phys., 2015, 142, 124302], is employed for this purpose. All partial wave contributions up to the total angular momentum J = 60 are considered to obtain the converged integral reaction cross section up to a collision energy of 1.0 eV. Thermal rate constants are calculated by averaging the reaction cross sections over the Boltzmann distribution of energies and compared with the available theoretical and experimental results for the temperature range 10-1000 K. Investigation of the channel-specific reaction attributes shows that the H abstraction (CH+ destruction) channel is highly favored over the H exchange channel. The effect of rotational and vibrational excitations of the CH+ reagent on the dynamics is also studied. The resonances formed during the course of the reaction are also identified by calculating the transition state spectrum and characterized in terms of the eigenfunctions and lifetimes. More than 260 vibrational levels are obtained and their eigenfunctions are calculated, which are represented in terms of the nodal assignments and the eigenenergies. They reveal both the local and hyperspherical behavior for the bound and quasibound states of the CH2+ complex in the ground 1 2A' surface. The lifetime analysis of the quasibound states indicates that the CH2+ resonances survive for as long as ∼400 fs at high energies (E ∼ 2.0 eV) and are expected to decay faster with further increasing energy. Finally, the type of mechanism for the formation of the product (C+ + H2) is elucidated.

Full-dimensional quantum dynamics study of the H2 + C2H → H + C2H2 reaction on an ab initio potential energy surface

This work performs a time-dependent wavepacket study of the H2 + C2H → H + C2H2 reaction on a new ab initio potential energy surface (PES). The PES is constructed using neural network method based on 68 478 geometries with energies calculated at UCCSD(T)-F12a/aug-cc-pVTZ level and covers H2 + C2H↔H + C2H2, H + C2H2 → HCCH2, and HCCH2 radial isomerization reaction regions. The reaction dynamics of H2 + C2H → H + C2H2 are investigated using full-dimensional quantum dynamics method. The initial-state selected reaction probabilities are calculated for reactants in eight vibrational states. The calculated results showed that the H2 vibrational excitation predominantly enhances the reactivity while the excitation of bending mode of C2H slightly inhibits the reaction. The excitations of two stretching modes of C2H molecule have negligible effect on the reactivity. The integral cross section is calculated with J-shift approximation and the mode selectivity in this reaction is discussed. The rate constants over 200-2000 K are calculated and agree well with the experimental measured values.

Effect of Vibrational Pre-Excitation on the Dissociation Dynamics of HOD(2+).

Preferential breaking of chemical bonds using few-cycle, intense laser pulse to obtain desired products offer a formidable challenge in understanding ultrafast chemical reactivity. In a recent study [J. Chem. Phys. 2015, 143, 244310], it was found that carrier-envelope phase influences the bond-selective fragmentation in HOD with up to 3-fold enhancement. We present a detailed theoretical study to understand the influence of initial vibrational states governing the dissociation dynamics. We have carried out a time-dependent quantum mechanical wave packet study on the ground electronic state (X̃ (3)B1) of HOD(2+). Analytical potential energy surface for the ground electronic states of both the neutral molecule and dication has been developed at multireference configuration interaction level of theory with aug-cc-pVQZ basis set. Branching ratio is computed from the accumulated flux in H(+) + OD(+) and D(+) + OH(+) dissociation channels. Our investigation demonstrate a strong dependency on the initial conditions, and thereby preferential cleavage of bonds can be achieved. We have also compared our results with experimental and other theoretical studies.

Probing the Dynamics of Rydberg and Valence States of Molecular Nitrogen with Attosecond Transient Absorption Spectroscopy.

An attosecond pulse is used to create a wavepacket in molecular nitrogen composed of multiple bound and autoionizing electronic states of Rydberg and valence character between 12 and 16.7 eV. A time-delayed, few-femtosecond, near-infrared (NIR) laser pulse is used to couple individual states in the wavepacket to multiple neighboring states, resulting in time-dependent modification of the absorption spectrum and revealing both individual quantum beats of the wavepacket and the energy shifts of the excited states in the presence of the strong NIR field. The broad bandwidth of the attosecond pulse and high energy resolution of the extreme ultraviolet spectrometer allow the simultaneous observation of time-dependent dynamics for many individual vibrational levels in each electronic state. Quantum beating with periods from 1.3 to 12 fs and transient line shape changes are observed among vibrational levels of a progression of electronically autoionizing Rydberg states leading to the excited A (2)Πu N2(+) ion core. Vibrational levels in the valence b (1)Πu state exhibit 50 fs oscillation periods, revealing superpositions between individual vibrational levels within this state. Comparisons are made to previous studies of electronic wavepackets in atoms that highlight similarities to atomic behavior yet illustrate unique contributions of the diatomic molecular structure to the wavepacket, including the influence of different electronic potentials and vibrational-level-specific electronic dynamics.

Nonadiabatic electron wavepacket study on symmetry breaking dynamics of the low-lying excited states of cyclic-B4

Symmetry allowed conical intersection plays a central role in excited state symmetry-forbidden reactions. As an illustrative example as such, we track the dynamical sequence of spatial-symmetry breaking of B4 cluster, which has a rich electronic structure in the low-lying excited states, to see how the relevant reaction proceeds. We use the semiclassical Ehrenfest method to detect the nonadiabatic electronic state mixing along the reactions. The essential feature of the nonadiabatic electron dynamics is clarified in terms of electron flux and unpaired-electron distribution induced by the nonadiabatic transitions. To facilitate understanding electron dynamics of symmetry breaking, we begin with symmetry consideration in terms of the Hückel orbitals, which are shown to be qualitatively useful enough to foresee the possible existence of symmetry allowed conical intersections.

A new potential energy surface for the H2S system and dynamics study on the S((1)D) + H2(X(1)Σg(+)) reaction.

We constructed a new global potential energy surface (PES) for the electronic ground state (1A′) of H2S based on 21,300 accurate ab initio energy points over a large configuration space. The ab initio energies are obtained from multireference configuration interaction calculations with a Davidson correction using basis sets of quadruple zeta quality. The neural network method is applied to fit the PES, and the root mean square error of fitting is small (1.68 meV). Time-dependent wave packet studies for the S(1D) + H2(X1Σg+) → H(2S) + SH(X2Π) reaction on the new PES are conducted to study the reaction dynamics. The calculated integral cross sections decrease with increasing collision energy and remain fairly constant within the high collision energy range. Both forward and backward scatterings can be observed as expected for a barrierless reaction with a deep well on the PES. The calculated integral cross sections and differential cross sections are in good agreement with the experimental results.

Kinetic isotope effect of the (16)O + (36)O2 and (18)O + (32)O2 isotope exchange reactions: Dominant role of reactive resonances revealed by an accurate time-dependent quantum wavepacket study.

The O + O2 isotope exchange reactions play an important role in determining the oxygen isotopic composition of a number of trace gases in the atmosphere, and their temperature dependence and kinetic isotope effects (KIEs) provide important constraints on our understanding of the origin and mechanism of these and other unusual oxygen KIEs important in the atmosphere. This work reports a quantum dynamics study of the title reactions on the newly constructed Dawes-Lolur-Li-Jiang-Guo (DLLJG) potential energy surface (PES). The thermal reaction rate coefficients of both the (18)O + (32)O2 and (16)O + (36)O2 reactions obtained using the DLLJG PES exhibit a clear negative temperature dependence, in sharp contrast with the positive temperature dependence obtained using the earlier modified Siebert-Schinke-Bittererova (mSSB) PES. In addition, the calculated KIE shows an improved agreement with the experiment. These results strongly support the absence of the "reef" structure in the entrance/exit channels of the DLLJG PES, which is present in the mSSB PES. The quantum dynamics results on both PESs attribute the marked KIE to strong near-threshold reactive resonances, presumably stemming from the mass differences and/or zero point energy difference between the diatomic reactant and product. The accurate characterization of the reactivity for these near-thermoneutral reactions immediately above the reaction threshold is important for correct characterization of the thermal reaction rate coefficients.

Photodesorption of water from rutile(110): ab initio calculation of five-dimensional potential energy surfaces of ground and excited electronic states and wave packet studies.

In this paper, we report on our results concerning the interaction of water with titanium dioxide in its rutile modification. The (110) surface is modelled by an embedded Ti9O18Mg7(14+) cluster. We present up to five-dimensional potential energy surfaces for the water molecule on this surface and include the dissociation of one hydrogen atom. The electronic ground state as well as one electronically excited state is included. To deal with the multi-configurational character of the wave function, we use the complete active space self-consistent field (CASSCF) approach. The resulting potential energy surfaces are fitted by means of an artificial neural network. As a first example of quantum dynamical studies based on our potential surfaces, we present results on the photodesorption of water from rutile(110).