Kent R. Wilson

A computer simulation method for the calculation of equilibrium constants for the formation of physical clusters of molecules: Application to small water clusters

An improved metal stamped and formed screw is disclosed. The subject screw is stamped and formed from continuous web of metal stock to form a plurality of screws joined by a carrier strip. The thus formed strip of screws can be machine applied to prebored holes and manually withdrawn therefrom and reapplied by conventional means.

Triatomic Photofragment Spectra. I. Energy Partitioning in NO2 Photodissociation

The photofragment spectrum of NO2 has been measured in the near ultraviolet at 28 810 cm−1. A molecular beam of NO2 is crossed with brief pulses of polarized laser light and measurements are made on the distributions of speed and direction of the recoiling O and NO fragments produced by photodissociation. The average translational energy of the fragments is about 60% of the available energy. There are at least two prominent peaks in the translational energy distribution. We conclude that the two peaks most likely correspond to nearly equal probability of recoil with the NO fragment in the v=0 and v=1 vibrational states. Such vibrationally excited NO fragments produced by photodissociation in polluted atmospheres could perhaps react with different rates than ground state fragments. The positions and widths of the peaks indicate that there is a significant rotational distribution. Statistical and direct models for photodissociation energy partitioning are briefly explored, and their predictions compared wit...

Picosecond-milliangstrom lattice dynamics measured by ultrafast X-ray diffraction

Fundamental processes on the molecular level, such as vibrations and rotations in single molecules, liquids or crystal lattices and the breaking and formation of chemical bonds, occur on timescales of femtoseconds to picoseconds. The electronic changes associated with such processes can be monitored in a time-resolved manner by ultrafast optical spectroscopic techniques, but the accompanying structural rearrangements have proved more difficult to observe. Time-resolved X-ray diffraction has the potential to probe fast, atomic-scale motions. This is made possible by the generation of ultrashort X-ray pulses, and several X-ray studies of fast dynamics have been reported,. Here we report the direct observation of coherent acoustic phonon propagation in crystalline gallium arsenide using a non-thermal, ultrafast-laser-driven plasma — a high-brightness, laboratory-scale source of subpicosecond X-ray pulses. We are able to follow a 100-ps coherent acoustic pulse, generated through optical excitation of the crystal surface, as it propagates through the X-ray penetration depth. The time-resolved diffraction data are in excellent agreement with theoretical predictions for coherent phonon excitation in solids, demonstrating that it is possible to obtain quantitative information on atomic motions in bulk media during picosecond-scale lattice dynamics.

Triatomic Photofragment Spectra. II. Angular Distributions from NO2 Photodissociation

The angular distribution of recoil of O atoms from the photodissociation of NO2 at 28 810 cm−1 is presented and analyzed to obtain information about the lifetime and symmetry of the excited dissociative state. The theoretical effects on photofragment angular distributions of excited state symmetry, lifetime, angular momentum, and angular recoil distribution with respect to internal coordinates are considered. The actual angular distribution measured by photofragment spectroscopy, peaks in the direction of the electric vector of the light, indicating that the predominant upper state is of 2B2 symmetry. Some absorption leading to states of different symmetry cannot be excluded. An upper limit for the lifetime of the excited NO2 molecule before dissociation is ∼ 2 × 10−13 sec, indicating that break‐up is too rapid to be affected by collisions in the atmosphere where NO2 photodissociation is an important step in photochemical air pollution.

Nonequilibrium solvation effects on reaction rates for model SN2 reactions in water

Molecular dynamics (MD) simulations of the model SN2 reaction Cl−+CH3Cl→ClCH3+Cl− in water, and variants thereof, are presented. The resulting transmission coefficients κ, that measure the deviations of the rates from the transition state theory (TST) rate predictions due to solvent‐induced recrossings, are used to assess the validity of the generalized Langevin equation (GLE)‐based Grote–Hynes (GH) theory. The GH predictions are found to agree with the MD results to within the error bars of the calculations for each of the 12 cases examined. This agreement extends from the nonadiabatic regime, where solvent molecule motions are unimportant and κ is determined by static solvent configurations at the transition state, into the polarization caging regime, where solvent motion is critical in determining κ. In contrast, the Kramers theory predictions for κ fall well below the simulation results. The friction kernel in the GLE used to evaluate the GH κ values is determined, from MD simulation, by a fixed‐parti...

Third harmonic generation microscopy.

Third harmonic generation microscopy is used to make dynamical images of living systems for the first time. A 100 fs excitation pulse at 1.2 aem results in a 400 nm signal which is generated directly within the specimen. Chara plant rhizoids have been imaged, showing dynamic plant activity, and non-fading image characteristics even with continuous viewing, indicating prolonged viability under these THG-imaging conditions.

Molecular dynamics of a modelSN2 reaction in water

Molecular dynamics are computed for a model SN2 reaction Cl−+CH3Cl→ClCH3+Cl− in water and are found to be strongly dependent on the instantaneous local configuration of the solvent at the transition state barrier. There are significant deviations from the simple picture of passage over a free energy barrier in the reaction coordinate, and thus, a marked departure from transition state theory occurs in the form of barrier recrossings. Factors controlling the dynamics are discussed, and, in particular, the rate of change of atomic charge distribution along the reaction coordinate is found to have a major effect on the dynamics. A simple frozen solvent theory involving nonadiabatic solvation is presented which can predict the outcome of a particular reaction trajectory by considering only the interaction with the solvent of the reaction system at the gas‐phase transition barrier. The frozen solvent theory also gives the transmission coefficient κ needed to make the transition state theory rate agree with the...

MOLECULAR DYNAMICS OF PHOTODISSOCIATION. QUASIDIATOMIC MODEL FOR ICN.

A model for the photodissociation of ICN in its lowest continuum is developed and is used to predict the partitioning of available energy between translational, rotational, and vibrational energies of the recoiling fragments. The model is based on three major assumptions: (i) that only one upper electronic state is involved, (ii) that light absorption affects only the breaking C–I bond, thus allowing the upper‐state potential surface to be calculated quasidiatomically from spectral and thermodynamic data, and (iii) that the mechanics of the “half‐collision” of the recoiling fragments on this potential surface may be treated as classical to predict the average partitioning between translational, rotational, and vibrational energies, and by a classical‐energy forced quantum oscillator approximation to predict the partitioning between vibrational states. The model predicts that most of the available energy will go into translation, which is consistent with flash photolysis studies and with crude measurements...

Molecular dynamics and spectra. I. Diatomic rotation and vibration

The pure rotational and vibrational–rotational absorption bands for a diatomic are calculated directly from classical molecular dynamics, classical linear response theory, and classical statistical mechanical ensemble averaging, with the use of simple quantum corrections. The experimental spectral band intensities and contours are well reproduced for CO from dilute gas phase through solution in compressed Ar to solution in liquid Ar by these ’’Newtonian’’ classical spectral calculations. The typical evolution seen in vibrational spectra from multiple‐peaked gas phase bands to single‐peaked solution bands is observed. The Newtonian gas phase calculations also match quantum and correspondence principle classical spectral calculations. This molecular dynamic approach may be applied to compute the spectra of complex molecules or of liquids for which a normal model analysis may be impractical, and may also be extended to nonequilibrium systems, for example, to compute transient vibrational spectra during chemi...

Thermodynamics and quantum corrections from molecular dynamics for liquid water

In principle, given the potential energy function, the values of thermodynamic variables can be computed from statistical mechanics for a system of molecules. In practice for the liquid state, however, two barriers must be overcome. This paper treats the first problem, how to quantum correct the classical mechanical thermodynamic values available from molecular dynamics, Monte Carlo, perturbation, or integral methods in order to compare with experimental quantum reality. A subsequent paper will focus on the second difficulty, the effective computation of free energy and entropy. A simple technique, derived from spectral analysis of the atomic velocity time histories, is presented here for the frequency domain quantum correction of classical thermodynamic values. This technique is based on the approximation that potential anharmonicities mainly affect the lower frequencies in the velocity spectrum where the system behaves essentially classically, while the higher spectral frequencies, where the deviation f...