Time-dependent Quantum Dynamics Research Articles

Reaction of [formula omitted]. A three-dimensional quantum dynamics study

A time-dependent quantum dynamics calculation is reported for the O( 1D) + H 2 reaction in three dimensions. Total (final state summed) reaction probabilities, cross sections and rate constants for the title reaction are presented in this study using the potential energy surface of Schinke and Lester (SL1). Despite the presence of the deep well corresponding to the stable species of OH 2, no long-lived resonances are found in quantum dynamics calculations. The present quantum study also shows that the insertion mechanism plays a major role in this reaction. The calculated cross sections are in generally good agreement with an earlier classical trajectory calculation of Schinke and Lester.

Accurate quantum calculations for H2+OH→H2O+H: Reaction probabilities, cross sections, and rate constants

Following a previous Communication [J. Chem. Phys. 99, 5615 (1993)], which reported several initial state-selected total reaction probabilities for the title reaction for J=0, we present in this paper the methodologies of the previous calculation and show results of new calculations. In particular, the present calculations are extended to all angular momentum J≳0 and obtained reaction cross sections for a range of energies using the centrifugal sudden (CS) approximation. The computed cross sections are used to obtain the state-specific thermal rate constants for both the ground and the excited vibrations of H2. The dynamics calculation, in which the nonreactive OH bond is frozen, includes explicitly five degrees of freedom in the time-dependent quantum dynamics treatment. The comparison of the present accurate cross sections with other approximate theoretical calculations shows discrepancies. The computed rate constants (from the ground rotation state) are larger than experimental measurements at low temperatures, the v=0 rate is larger than the corresponding experimental rate by a factor of 1.8, and the ratio of v=1 to v=0 rate is a factor of 4.8 greater than the experimental ratio at 300 K. The calculated reaction rates are also compared to those of other theoretical calculations and the differences are discussed in the text.

Femtosecond spectroscopy of ultrafast nonadiabatic excited-state dynamics on the basis of ab initio potential-energy surfaces: the S2 state of pyrazine

A theoretical description of femtosecond time-resolved optical spectroscopy of isolated polyatomic molecules is outlined. Our ultimate goal is the characterization of the system response on a microscopic level, that is, time-dependent quantum dynamics. Employing a simplified model Hamiltonian which includes the most relevant electronic states and vibrational modes, the intramolecular quantum dynamics is treated numerically exactly. The model Hamiltonian is constructed, via a Taylor expansion in terms of ground-state normal coordinates, from ab initio potential-energy surfaces. Additional phase relaxation effects (optical pure dephasing) due to weakly coupled modes have been included in a phenomenological manner using the density-matrix formalism

Self-consistent Monte Carlo particle transport with model quantum tunneling dynamics: Application to the intrinsic bistability of a symmetric double-barrier structure

The intrinsic bistability in a symmetric resonant tunneling device (RTD) is simulated by the ensemble particle Monte Carlo technique, coupled with a simple model of the space- and time-dependent particle quantum dynamics inside the double-barrier region of the RTD. This model particle quantum dynamics is based upon the phase-time delay, which is obtained from a piecewise-linear-potential Airy function approach to the calculation of the transmission amplitude. An unambiguous hysteresis in the negative differential resistance (NDR) region of the current-voltage (I-V) characteristic is observed for a symmetric AlGaAs/GaAs double-barrier structure. The dynamical accumulation of carriers in the well is seen to be the cause of this marked bistability/hysteresis. However, the plateau-like features of the I-V curve are not resolved, although oscillations in the quantum well carrier density in the NDR are prominent. This article strongly suggests that a more accurate treatment of the space- and time-dependent particle quantum dynamics across the RTD is of paramount importance.

Grid methods for solving the Schrödinger equation and time dependent quantum dynamics of molecular photofragmentation and reactive scattering processes

Abstract This review will concentrate on new theoretical methods for solving the quantum dynamics of molecular systems. The approach will be that of solving the time dependent Schrodinger equation, and extracting from it the measurable quantities of experimental interest. Applications to the modelling of molecular photodissociation processes and to the theory of reactive molecular collisions will be discussed.

Dissociative chemisorption of H2 on Ni surface: Time-dependent quantum dynamics calculation and comparison with experiment

A time-dependent quantum wave packet method has been applied to studying the process of activated dissociative chemisorption of H2 on Ni(100) surface. The Ni surface is treated as static and the effect of weak surface correlation is neglected in our dynamics calculation. The three-dimensional calculation is fully quantum mechanical without any reduced dimension approximation. An empirical London–Erying–Polanyi–Sato (LEPS) potential surface has been used and modified in our dynamics calculation to produce a reasonable barrier height compatible with experiment. Sticking probabilities have been computed as a function of initial normal incident kinetic energy and are compared to experimental results as well as another 3D quantum dynamics calculation. Good agreement has been found between our theoretical calculation and molecular beam experiment in the energy dependence of the dissociation probability. It is also found that vibrational excitation of H2 enhances dissociation probability, in agreement with the prediction of an earlier theoretical calculation. Snapshots of the wave function are plotted that provide intimate details of the dissociation dynamics in time and space.

Probing the transition state by time-dependent quantum dynamics

Probing the transition state by time-dependent quantum dynamics

Probing the transition state by time-dependent quantum dynamics

In most reactive scattering experiments a measurement of the sticking fraction, S0, is employed to deduce aspects of the potential energy surface topology. Almost no attention is paid to that fraction which (for S0 < 1) reflects from the surface and, potentially, contains important information regarding the dissociation dynamics. We have studied some theoretical aspects of the dynamics of dissociative adsorption by calculating the orientation and alignment moments of this scattered fraction. Time-dependent quantum-mechanical scattering of H2 has been studied using a range of model potential energy surfaces corresponding to the low-index faces of an fcc crystal. It has shown that valuable information on the mechanism of dissociation and the geometry of the transition state is contained in the alignment moments and the scattered rotational distributions. In terms of reactive processes there has not been much active experimentation regarding rotational state selectivity and it is the purpose of this work to suggest possible scenarios for obtaining detailed geometrical information of the transition state in dissociative adsorption.

The calculation of product quantum state distributions and partial cross-sections in time-dependent molecular collision and photodissociation theory

A new method of analyzing the results of multidimensional time-dependent quantum dynamical wavepacket calculations in terms of the product quantum state distributions is presented. The method requires knowledge only of the time-dependent projection coefficients of the wavepacket onto individual product quantum states along a cut in the exit valley of the photodissociation or reaction process. The squares of the Fourier transforms of these coefficients then directly yield the cross-sections of interest. The great advantage of time-dependent quantum dynamics, namely that a single wavepacket calculation yields the cross-sections at all energies of interest, is fully exploited.

Time-dependent quantum dynamics of reactive scattering and the calculation of product quantum state distributions ? A study of the collinear F+H2(v=0) ? HF(v?)+H reaction

A new method for the calculation of partial cross sections in the time-dependent quantum theory of molecular reactive scattering processes is discussed. Preliminary calculations are presented which clearly illustrate the power of the method. They show how all the partial cross sections associated with a single initial quantum state may be computed over a very wide energy range from a single propagation of a prepared wavepacket. The resonance behaviour is obtained without difficulty and the energies of the reactive scattering resonances are exactly reproduced.