Robert M. Whitnell

Vibrational relaxation of a dipolar molecule in water

The vibrational energy relaxation of a model methyl chloride molecule in water is studied through equilibrium and nonequilibrium molecular dynamics simulations. Previous work [Whitnell, Wilson, and Hynes, J. Phys. Chem. 94, 8625 (1990)] has demonstrated the validity of a Landau–Teller formula for this system in which the relaxation rate is equal to the frequency‐dependent friction that the solvent exerts on the bond. In the present work, an analysis of this friction is used to test the isolated binary interaction (IBI) approximation for vibrational energy relaxation. In this system, where long‐range electrostatic Coulomb forces dominate the interaction between the water solvent and the CH3Cl molecule, we show that the binary approximation to the friction only partially accounts for the rapid relaxation of the vibrational energy. We attribute the importance of cross correlations between different solvent molecules to the overlap of the CH3Cl vibrational frequency with the librational band of the water solv...

Optical control of molecular dynamics: Molecular cannons, reflectrons, and wave‐packet focusers

We consider the control of molecular dynamics using tailored light fields, based on a phase space theory of control [Y. J. Yan et al., J. Phys. Chem. 97, 2320 (1993)]. This theory enables us to calculate, in the weak field (one‐photon) limit, the globally optimal light field that produces the best overlap for a given phase space target. We present as an illustrative example the use of quantum control to overcome the natural tendency of quantum wave packets to delocalize on excited state potential energy curves. Three cases are studied: (i) a ‘‘molecular cannon’’ in which we focus an outgoing continuum wave packet of I2 in both position and momentum, (ii) a ‘‘reflectron’’ in which we focus an incoming bound wave packet of I2, and (iii) the focusing of a bound wave packet of Na2 at a turning point on the excited state potential using multiple light pulses to create a localized wave packet with zero momentum. For each case, we compute the globally optimal light field and also how well the wave packet produce...

Vibrational relaxation of I−2 in water and ethanol: molecular dynamics simulation

Abstract The vibrational relaxation of I − 2 in water and ethanol using molecular dynamics simulations. In both solvents, the relaxation rate is ≈0.6–0.7 ps, in qualitative agreement with the experiments of Barbara and co-workers. A Landau-Teller model for the relaxation rate is in good agreement with the full molecular dynamics calculations. Simulations of the neutral I 2 molecule vibrational relaxation in the same solvents are used to sort out the effects of solute charge and vibrational frequency. We show that the fast relaxation of the I − 2 molecule is due to both its low vibrational frequency and the long-range solvent-solute Coulombic interactions.

Femtosecond chemical dynamics in solution. Wavepacket evolution and caging of I2

Abstract Theoretical and experimental studies are discussed for the femtosecond wavepacket dynamics and reaction of iodine in compressed argon solution and in the collision-free gas phase limit. Classical and quantum calculations are compared with experiment and are used to demonstrate the influence of the solvent on wavepacket nuclear motion dephasing, on the caging and recombination of the separating iodine atoms, and on iodine vibrational relaxation. It is shown that these phenomena are observed on the time scale in which they occur.

Quantum mechanics of small Ne, Ar, Kr, and Xe clusters

We compute energy levels and wave functions of Ne, Ar, Kr, and Xe trimers, modeled by pairwise Lennard‐Jones potentials, using the discrete variable representation (DVR) and the successive diagonalization‐truncation method. For the Ne and Ar trimers, we find that almost all of the energy levels lie above the energy required classically to achieve a collinear configuration. For the Kr and Xe trimers, we are able to determine a number of energy levels both below the classical transition energy as well as above it. Energy level statistics for these heavier clusters reveal behavior that correlates well with classical chaotic behavior that has previously been observed above the transition energy. The eigenfunctions of these clusters show a wide variety of behavior ranging from very regular behavior for low lying eigenstates to a combination of regular and irregular behavior at energies above the transition energy. These results, along with quantum Monte Carlo calculations of the ground states for a variety of ...

Quantum chaos of Ar3: Statistics of eigenvalues

The successive diagonalization–truncation method is applied to the calculation of the vibrational eigenvalues of the Ar trimer bound by pairwise Lennard‐Jones potentials. The statistics of the eigenvalues reveal strongly chaotic behavior of the cluster, consistent with the classical dynamics studies. Moreover, the zero‐point energy is higher than the highest energy at which regular dynamics were found classically, indicating that for all energies physically accessible to the cluster, the dynamics are chaotic.

Classical theory of ultrafast pump-probe spectroscopy: Applications to I2 photodissociation in Ar solution

We apply a time-frequency response theory of pump-probe spectroscopy, due to Yan, to the experiments of Zewail and co-workers on I_2 photodissociation in Ar solution. This theory can be implemented with quantum, semiclassical or classical dynamics, thereby allowing the treatment of complex systems such as molecules in solution. The advantage of this theory is that it can take into account the actual light pulses that are used in the experiment, and we can therefore make a separation of those effects due to the physics of the system from those due to the finite temporal and spectral width of the pump and probe excitation pulses. We show here that the classical mechanical formulation of pump-probe dynamics is able to reproduce qualitatively many features of the experimental measurements. This agreement allows us to interpret much of the underlying physics of the pump-probe process in terms of the underlying microscopic dynamics.

‘‘Classical’’ quantum control with application to solution reaction dynamics

Quantum control theory has the potential for great success in the ability to control the dynamics of gas phase molecules. Beyond 3 or 4 atoms, however, we are at present unable to compute the quantum dynamics needed to solve the control equations. An alternative applicable to larger systems, including polyatomics, clusters, surfaces, and condensed phases, is to approximate the quantum dynamics either classically or semiclassically. In this work, we present and illustrate a classical mechanical implementation of the weak field density matrix control theory. We consider I2 wavepacket focusing in the gas phase and in Ar solutions, comparing with full quantum calculations when possible. The classical calculations give results that are qualitatively similar to the full quantum calculations, and thereby show that the basic phenomenology of the control theory is accessible for more complex systems in a straightforward manner, in cases when purely quantum effects, such as tunneling and interference, are not dominant. We present calcualtions of the focusing of the I2 phase space distribution in Ar solution and discuss how this type of control may allow us to measure the influence of the solvent on a reacting system.

Fast space-filling molecular graphics using dynamic partitioning among parallel processors

We present a novel algorithm for the efficient generation of high-quality space-filling molecular graphics that is particularly appropriate for the creation of the large number of images needed in the animation of molecular dynamics. Each atom of the molecule is represented by a sphere of an appropriate radius, and the image of the sphere is constructed pixel-by-pixel using a generalization of the lighting model proposed by Porter (Comp. Graphics 1978, 12, 282). The edges of the spheres are antialiased, and intersections between spheres are handled through a simple blending algorithm that provides very smooth edges. We have implemented this algorithm on a multiprocessor computer using a procedure that dynamically repartitions the effort among the processors based on the CPU time used by each processor to create the previous image. This dynamic reallocation among processors automatically maximizes efficiency in the face of both the changing nature of the image from frame to frame and the shifting demands of the other programs running simultaneously on the same processors. We present data showing the efficiency of this multiprocessing algorithm as the number of processors is increased. The combination of the graphics and multiprocessor algorithms allows the fast generation of many high-quality images.