Time-dependent Wave Packet Approach Research Articles

Stimulating the production of deeply bound RbCs molecules with laser pulses: the role of spin–orbit coupling in forming ultracold molecules

We investigate the possibility of forming deeply bound ultracold RbCs molecules by a two-color photoassociation experiment. We compare the results with those for Rb2 in order to understand the characteristic differences between heteronuclear and homonuclear molecules. The major differences arise from the different long-range potential for excited states. Ultracold 85Rb and 133Cs atoms colliding on the X 1Σ+ potential curve are initially photoassociated to form excited RbCs molecules in the region below the Rb(5S)+Cs(6P1/2) asymptote. We explore the nature of the Ω=0+ levels in this region, which have mixed A 1Σ+ and b 3Π character. We then study the quantum dynamics of RbCs by a time-dependent wavepacket (TDWP) approach. A wavepacket is formed by exciting a few vibronic levels and is allowed to propagate on the coupled electronic potential energy curves. We calculate the time dependence of the overlap between the wavepacket and ground-state vibrational levels. For a detuning of 7.5 cm−1 from the atomic line, the wavepacket for RbCs reaches the short-range region in about 13 ps, which is significantly faster than for the homonuclear Rb2 system; this is mostly because of the absence of an R−3 long-range tail in the excited-state potential curves for heteronuclear systems. We give a simple semiclassical formula that relates the time taken to the long-range potential parameters. For RbCs, in contrast to Rb2, the excited-state wavepacket shows a substantial peak in singlet density near the inner turning point, and this produces a significant probability of de-excitation to form ground-state molecules bound by up to 1500 cm−1. The short-range peak depends strongly on non-adiabatic coupling and is reduced if the strength of the spin–orbit coupling is increased. Our analysis of the role of spin–orbit coupling concerns the character of the mixed states in general and is important for both photoassociation and stimulated Raman de-excitation.

Quantum electron transport through carbon nanotubes with electron-phonon coupling

The channel-length dependence of the resistance of the carbon nanotubes on micron-order length is studied at finite temperature using the time-dependent wave-packet approach based on the Kubo formula within a tight-binding approximation. The authors clarify how the transport properties of the carbon nanotubes are changed from the ballistic to the diffusive regime due to electron-phonon scattering. They also obtain the reasonable mean free path, relaxation time, and temperature dependence of resistivity.

Photodissociation dynamics of lithium chloride: Contribution of interferometric predissociation

Continuum photoabsorption of lithium chloride (LiCl) was investigated using a coupled-channel time-dependent wave packet approach. Photodissociation cross sections for the production of ground-state Li and Cl atoms were computed up to temperatures of 1500 K for a thermal distribution of rotational levels. At such temperatures, LiCl is believed to be the primary Li-bearing gas in cool stellar atmospheres. Narrow Rydberg resonances in the total absorption spectrum are found to dominate the thermally averaged cross section due to the large density of Rydberg states in the predissociation gap. Comparison with measured photoabsorption cross sections, where available, is made.

Contact and phonon scattering effects on quantum transport properties of carbon-nanotube field-effect transistors

We investigated the phonon scattering effects on the transport properties of carbon nanotube devices with micron-order lengths at room temperature, using the time-dependent wave-packet approach based on the Kubo formula within a tight-binding approximation. We studied the scattering effects of both the longitudinal acoustic and the optical phonons on the transport properties. The conductance of semiconducting nanotubes is decreased by the acoustic phonon, instead of the optical phonon. Furthermore, we clarified how the electron mobilities of the devices are affected by the acoustic phonon.

Quantum dynamical study of the O(D1)+HCl reaction employing three electronic state potential energy surfaces

Quantum dynamical calculations are reported for the title reaction, for both product arrangement channels and using potential energy surfaces corresponding to the three electronic states, 1 1A', 2 1A', and 1 1A", which correlate with both reactants and products. The calculations have been performed for J=0 using the time-dependent real wavepacket approach by Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)]. Reaction probabilities for both product arrangement channels on all three potential energy surfaces are presented for total energies between 0.1 and 1.1 eV. Product vibrational state distributions at two total energies, 0.522 and 0.722 eV, are also presented for both channels and all three electronic states. Product rotational quantum state distributions are presented for both product arrangement channels and all three electronic states for the first six product vibrational states.

Mechanistic investigation of vibrational fine structure in e‐H2 scattering using local complex potential‐based time dependent wave packet approach

AbstractThe structural features of vibrational excitation cross‐sections in resonant e‐H2 scattering have been investigated using a time dependent wave packet approach and a local complex potential to describe the 2Σ H anion. An analysis of the partial contributions to the vibrational excitation cross‐sections reveals that all features of the excitation profile result from simple interference between bound vibrational levels of H2 and H. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Effect of Coriolis coupling in chemical reaction dynamics

It is essential to evaluate the role of Coriolis coupling effect in molecular reaction dynamics. Here we consider Coriolis coupling effect in quantum reactive scattering calculations in the context of both adiabaticity and nonadiabaticity, with particular emphasis on examining the role of Coriolis coupling effect in reaction dynamics of triatomic molecular systems. We present the results of our own calculations by the time-dependent quantum wave packet approach for H + D2 and F(2P3/2,2P1/2) + H2 as well as for the ion-molecule collisions of He + H2 +, D(-) + H2, H(-) + D2, and D+ + H2, after reviewing in detail other related research efforts on this issue.

An eight-degree-of-freedom, time-dependent quantum dynamics study for the H2+C2H reaction on a new modified potential energy surface

An eight-dimensional time-dependent quantum dynamics wave packet approach is performed for the study of the H2+C2H-->H+C2H2 reaction system on a new modified potential energy surface (PES) [L.-P. Ju et al., Chem. Phys. Lett. 409, 249 (2005)]. This new potential energy surface is obtained by modifying Wang and Bowman's old PES [J. Chem. Phys. 101, 8646 (1994)] based on the new ab initio calculation. This new modified PES has a much lower transition state barrier height at 2.29 kcal/mol than Wang and Bowman's old PES at 4.3 kcal/mol. This study shows that the reactivity for this diatom-triatom reaction system is enhanced by vibrational excitations of H2, whereas the vibrational excitations of C2H only have a small effect on the reactivity. Furthermore, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. The comparison of the rate constant between this calculation and experimental results agrees with each other very well. This comparison indicates that the new modified PES corrects the large barrier height problem in Wang and Bowman's old PES.

Calculation of vibrational excitation cross-sections in resonant electron-molecule scattering using the time-dependent wave packet (TDWP) approach with application to the 2Π CO− shape resonance

Results from application of a new implementation of the time-dependent wave packet (TDWP) approach to the calculation of vibrational excitation cross-sections in resonant e-CO scattering are presented to examine its applicability in the treatment of e-molecule resonances. The results show that the SCF level local complex potential (LCP) in conjunction with the TDWP approach can reproduce experimental features quite satisfactorily.

Ultrafast Electronuclear Dynamics ofH2Double Ionization

The ultrafast electronic and nuclear dynamics of H2 laser-induced double ionization is studied using a time-dependent wave packet approach that goes beyond the fixed nuclei approximation. The double ionization pathways are analyzed by following the evolution of the total wave function during and after the pulse. The rescattering of the first ionized electron produces a coherent superposition of excited molecular states which presents a pronounced transient H+H- character. This attosecond excitation is followed by field-induced double ionization and by the formation of short-lived autoionizing states which decay via double ionization. These two double ionization mechanisms may be identified by their signature imprinted in the kinetic-energy distribution of the ejected protons.

Electron–phonon coupling effect on quantum transport in carbon nanotubes using time-dependent wave-packet approach

We investigate the electron–phonon coupling effects on the electronic transport properties of the metallic ( 5 , 5 ) -carbon nanotube using the time-dependent wave-packet approach based on the Kubo–Greenwood formula within a tight-binding approximation. We consider the zone-center longitudinal optical phonon among various phonon modes, and calculate the conductance for the 10 μ m carbon nanotubes at room temperature. We show that the electron–phonon coupling largely decreases the conductance around the Fermi energy, which is explained by the time-dependent behaviors of propagating length and the electronic band structure.

Time-dependent wave-packet approach to the quantum transport of carbon nanotubes with vacancies

We investigate the vacancy effect on the electronic transport properties of the (5,5)-metallic and (5,0)-semiconducting carbon nanotubes using the time-dependent wave-packet approach based on the Kubo–Greenwood formula within the tight-binding approximation. We found that the metallic and semiconducting carbon nanotubes show different electronic transport properties for the states created by vacancies.

Calculation of the cross section of epithermal neutron scattering from water by a time-dependent wavepacket approach

An expression for an epithermal neutron scattering cross section from a water cluster is derived by using a time-dependent wavepacket approach. Both the translational motion of the center of mass of the cluster and the vibrational modes of a proton in the cluster are taken into account. Incident energy dependence on the neutron–proton scattering cross sections calculated for a simplified cluster model agrees with experimental results of neutron scattering from water. Time correlation functions of momentum transfer clearly show that both the translational and vibrational motions significantly contribute to the scattering cross section in the low energy regime.

RESONANCES IN H+HLi SCATTERING FOR NONZERO TOTAL ANGULAR MOMENTUM (J > 0): A TIME-DEPENDENT WAVE PACKET APPROACH

The resonances in H + HLi scattering for nonzero total angular momentum (J > 0) are examined here by a time-dependent wave packet approach employing an ab initio potential energy surface (PES) [Dunne LJ, Murrell JN, Jemmer P, Chem Phys Lett336: 1, 2001] of the system. The resonances are identified by calculating a set of pseudospectra corresponding to the Franck–Condon transition of a hypothetical initial state to the interaction region of the H + HLi PES. The resonances are characterized by calculating their eigenfunctions and linewidth lifetimes. The resonances for J ≠ 0 are discussed in relation to their counterpart for J = 0. The effect of Coriolis coupling on the resonances obtained from the centrifugal sudden approximation for J = 2 and for K = 0,1,2 is examined.

Effect of Reagent Rotation on Isotopic Branching in (He, HD+) Collisions

A three-dimensional time-dependent quantum mechanical wave packet approach is used to calculate reaction probability (P(R)) and integral reaction cross section (sigma(R)) values for both the channels of the reaction He + HD(+) (v = 1; j = 0, 1, 2, 3) --> HeH(D)(+) + D(H), over a range of translational energy (E(trans)) on the McLaughlin-Thompson-Joseph-Sathyamurthy (MTJS) potential energy surface using centrifugal sudden approximation for nonzero total angular momentum (J) values. The reaction probability plots as a function of translational energy for different J values exhibit several oscillations, which are characteristic of the system. It is shown that HeH(+) is preferred over HeD(+) for large J values and that HeD(+) is preferred over HeH(+) for small J values for all the rotational (j) states studied. The integral reaction cross section for both the channels and therefore the isotopic branching ratio for the reaction depend strongly on j in contrast to the marginal dependence shown by earlier QCT calculations. The computed results are in overall agreement with the available experimental results.

Time-dependent wave packet approach to the pulse delay effect upon RbI photoelectron spectrum

The time-resolved photoelectron spectrum (TRPES) of RbI molecule is simulated using the time-dependent wave-packet method. Both the normal three-photon ionization process and auto-ionization process are involved in the simulation. The calculated results show that the change of delay time will influence the shape of the photoelectron spectrum (PES), and the influence is sub- stantially due to the existence of the crossing between excited states and the strong laser field which will change the position of relevant curves.

A full dimensional, nine-degree-of-freedom, time-dependent quantum dynamics study for the H2+C2H reaction

A full dimensional, nine-degree-of-freedom (9DOF), time-dependent quantum dynamics wave packet approach is presented for the study of the H2+C2H-->H+C2H2 reaction system. This is the first full dimensional quantum dynamics study for a diatom-triatom reaction system. The effects of the initial vibrational and rotational excitations of the reactants on the reactivity of this reaction are investigated. This study shows that vibrational excitations of H2 enhance the reactivity; whereas, the vibrational excitations of C2H only have a small effect on the reaction probability. In addition, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. Comparison of the ground state reaction probabilities of the 9DOF and 8DOF shows the reaction probability from the full dimensional calculation is larger, with more prominent resonance features.

Coriolis-Coupled Wave Packet Dynamics of H + HLi Reaction

We investigated the effect of Coriolis coupling (CC) on the initial state-selected dynamics of H+HLi reaction by a time-dependent wave packet (WP) approach. Exact quantum scattering calculations were obtained by a WP propagation method based on the Chebyshev polynomial scheme and ab initio potential energy surface of the reacting system. Partial wave contributions up to the total angular momentum J=30 were found to be necessary for the scattering of HLi in its vibrational and rotational ground state up to a collision energy approximately 0.75 eV. For each J value, the projection quantum number K was varied from 0 to min (J, K(max)), with K(max)=8 until J=20 and K(max)=4 for further higher J values. This is because further higher values of K do not have much effect on the dynamics and also because one wishes to maintain the large computational overhead for each calculation within the affordable limit. The initial state-selected integral reaction cross sections and thermal rate constants were calculated by summing up the contributions from all partial waves. These were compared with our previous results on the title system, obtained within the centrifugal sudden and J-shifting approximations, to demonstrate the impact of CC on the dynamics of this system.

Semirigid vibrating rotor target model for atom-polyatom reactions: Application to F+CH4→CH3+HF

The semirigid vibrating rotor target (SVRT) model for the polyatomic reaction has been applied to the reaction of F+CH4→HF+CH3. The time-dependent wave packet approach has also been used in the calculation. In the current study, reaction probability, cross-section, and rate constant are calculated for the title reaction on the modified J1 (MJ1) potential energy surface (PES). Numerical calculation shows oscillatory structures in the energy dependence of the calculated reaction probability. Those structures are generally associated with broad dynamical resonance. They are almost washed-out in the energy dependence of integral cross-sections due to summation over partial waves. The calculated rate constant is in good agreement with experimental measurement.

The time-dependent quantum wave packet approach to the electronically nonadiabatic processes in chemical reactions

The time-dependent quantum wave packet approach has been improved and formulated to treat the multiple surface problems and thus provided a new simple, yet a clear quantum picture for interpreting the reaction mechanism underlying the nonadiabatic dynamical processes. The method keeps the salient feature of the original quantum wave packet theory developed for single surface problems, i.e. the introduction of the absorbing potential and the grid basis including the discrete variable representation and the fast Fourier transformation, which makes the present methodology a very efficient implement for the nonadiabatic quantum scattering calculations. Here, we review the theoretical basis of this approach and its applications to the fundamental triatomic chemical reactions, the latter include the nonadiabatic dynamics calculations on the F + H2, F + HD, F + D2, O(1D) + N2, O(3P, 1D) + H2, D+ + H2, and H+ + D2 reactions. We also present a thorough historical overview of the theoretically nonadiabatic dynamical investigations particularly on the triatomic systems, and show how the time-dependent wave packet approach complements the time-independent quantum scattering theory. Contents PAGE 1. Introduction 202 2. Historical overview 203 3. Time-dependent quantum wave packet approach for A + BC reaction 206 3.1. Propagation of the wave function 206 3.2. Preparation of the initial wave function 208 3.3. Analysis of the final wave function 209 4. Examples 210 4.1. Nonadiabatic effects on the reaction mechanism of F(2P3/2,2P1/2) + H297 210 4.2. The reactivity of the ground and the excited spin state F(2P3/2,2P1/2) atoms with D298 212 4.3. Nonadiabatic investigation on the F(2P3/2,2P1/2) + HD reaction 99,100 213 4.4. Electronic quenching process in the O(1D) + N2 → O(3P) + N2 reaction 101 218 4.5. The intersystem crossing effects in the O(3P,1D) + H2 reaction 102 221 4.6. Nonadiabatic quantum calculations on the D+ + H2 reaction 103 225 4.7. Nonadiabatic investigation on the H+ + D2 reaction 104 229 5. Conclusions 231 Acknowledgments 233 References 233