D. J. Diestler

Fluids in micropores. I. Structure of a simple classical fluid in a slit‐pore

Equilbrium properties of a rare‐gas fluid contained between two parallel fcc(100) planes of rigidly fixed rare‐gas atoms were computed by means of the grand‐canonical ensemble Monte Carlo method. The singlet distribution function ρ(1), and the pair‐correlation function g(2) in planes parallel to the solid layers, indicate that the structure of the pore fluid depends strongly on the distance h between the solid layers. As the separation increases from less than two atomic diameters, successive layers of fluid appear. The transitions between one and two layers and three and four layers are especially abrupt and are accompanied by changes in the character of g(2) from dense fluid‐like to solid‐like. Long‐range, in‐plane order in the fluid layers diminishes with increasing h, but is still evident in the contact layer (i.e., that nearest the solid layer) at h=16.5 atomic diameters, the largest separation considered. The structure of the contact layer reflects the solid‐layer structure and differs significantly...

Shear forces in molecularly thin films

Monte Carlo and molecular dynamics methods have been used to study the shearing behavior of an atomic fluid between two plane-parallel solid surfaces having the face-centered cubic (100) structure. A distorted, face-centered cubic solid can form epitaxially between surfaces that are separated by distances of one to five atomic diameters. Under these conditions a critical stress must be overcome to initiate sliding of the surfaces over one another at fixed separation, temperature, and chemical potential. As sliding begins, a layer of solid exits the space between the surfaces and the remaining layers become fluid.

Fluids in micropores. II. Self‐diffusion in a simple classical fluid in a slit pore

Self‐diffusion coefficients D are computed for a model slit pore consisting of a rare‐gas fluid confined between two parallel face‐centered cubic (100) planes (walls) of rigidly fixed rare‐gas atoms. By means of an optimally vectorized molecular‐dynamics program for the CYBER 205, the dependence of D on the thermodynamic state (specified by the chemical potential μ, temperature T, and the pore width h) of the pore fluid has been explored. Diffusion is governed by Fick’s law, even in pores as narrow as 2 or 3 atomic diameters. The diffusion coefficient oscillates as a function of h with fixed μ and T, vanishing at critical values of h, where fluid–solid phase transitions occur. A shift of the pore walls relative to one another in directions parallel with the walls can radically alter the structure of the pore fluid and consequently the magnitude of D. Since the pore fluid forms distinct layers parallel to the walls, a local diffusion coefficient D(i)∥ associated with a given layer i can be defined. D(i)∥ i...

Close‐Coupling Technique for Chemical Exchange Reaction of the Type A+BC→AB+C. H+H2→H2+H

A new close‐coupling technique for rearrangement collisions of the form A+BC→AB+C is presented. The general theory is developed for collinear electronically adiabatic encounters. A central feature of the technique is the expression of the scattering wavefunction as a sum of two terms expressed in two symmetric coordinate systems. The first term is expressed in coordinates RAC and RBC appropriate to the arrangement A+BC prior to reaction and the second term in coordinates RAC and RAB appropriate to the arrangement AB+C after reaction. Coupled equations in the single variable RAC are obtained and integrated numerically by an initial‐value technique to obtain accurate transition and reaction probabilities. The theory is applied to the exchange reaction H+H2→H2+H using the Porter–Karplus potential energy surface. Although the results cannot be compared directly with those of previous accurate quantal calculations, they are internally consistent and exhibit trends qualitatively similar to those of the previous...

Many‐Body Processes in Nonradiative Energy Transfer between Ions in Crystals

It is shown that many‐body interactions can play a dominant role in nonradiative energy transfer processes between ions in crystals. These interactions formally arise from the Coulomb and exchange interactions between the electrons of the donor ion and two or more acceptor ions. In particular, three‐body transfer processes arising from the dipole‐dipole perturbation Hamiltonian through virtual transitions are considered in some detail. Such processes are manifested by the quadratic concentration dependence of the per‐ion nonradiative energy transfer rate observed in concentration quenching and sensitized luminescence experiments. Many‐body interactions are important in rare‐earth ions because of the narrow widths of the 4fn crystal states and because of the large moments of the virtual transitions involving the opposite parity 4fn−1 5d states.

Analysis of infrared absorption line shapes in condensed media: Application of a classical limit of Heisenberg’s equations of motion

A microscopic dynamical treatment of chemical systems comprising both light particles that require a quantal description and heavy ones that may be described adequately by classical mechanics is presented. A (partial) classical limit of Heisenberg’s equations yields a self‐consistent set of ‘‘hemiquantal’’ equations (HQE) of motion. The adiabatic limit of the HQE, in which energy is not shared between high‐frequency modes of the quantal subsystem and relatively low‐frequency modes of the classical subsystem, is applied to a one‐dimensional model for infrared absorption by matrix‐isolated impurites. The predictions of the model are consistent with recent experimental measurements on the ‘‘Q’’ feature of HCl isolated in Ar.

A velocity reset method of simulating thermal motion and damping in gas–solid collisions

We present a velocity reset procedure for the approximate description of the molecular dynamics of a tractable subset of the atoms composing a macroscopic solid which is subjected to collisions. The coupling of the subset to the remainder (the reservoir) is taken into account in a stochastic manner by periodically resetting the velocities of subset particles which interact with the reservoir. The Cartesian velocity components are reset to vnew =(1−θ)1/2vold +θ1/2vT, where vold is the previous velocity, vT is a random velocity chosen from a Maxwellian distribution at temperature T, and θ is a parameter which controls the strength of the reset. In the limit θ=1 and all subset particles are reset, the method is similar to Andersen’s thermostat procedure [J. Chem. Phys. 72, 2384 (1980)]. In the double limit that θ→0 and the interval between resets Δtrs →0 such that β=θ/2Δtrs is fixed, the equations of motion for the subset reduce to Langevin form, where β is the frictional damping rate. This partial velocity ...

Quantum dynamics of vibrational relaxation in condensed media

The Zwanzig−Mori projection−operator formalism is employed to describe the dynamics of relaxation in a prototypic model well known for its application to a variety of physical problems: a harmonic oscillator coupled to a heat bath. First an exact equation of motion in generalized Langevin form is obtained for G (t), the expectation value of the occupation number of the oscillator, known to be in a definite initial nonequilibrium state. This equation is then solved in the appropriate van Hove weak−coupling, long−time limit for the usual case of physical interest in which the oscillator−bath interaction is linear in the oscillator coordinate. In this case G (t) is found to decay exponentially with a time constant τl = τd/2, where τd is the so called ’’dephasing’’ time, i.e., the time constant associated with the exponentially decaying time correlation function of the oscillator coordinate, 〈Q (0) Q (t) 〉. The approach taken here, which may be easily generalized, leads in a rather natural and straightforward...

Quantum‐mechanical treatment of collision‐induced dissociation

A close‐coupling technique for calculating quantum‐mechanical probabilities of collision‐induced dissociation (CID) of a diatomic molecule by an atom is presented. The internal Hamiltonian (of the diatomic) is first diagonalized in a discrete, square‐integrable basis. The lowest several of the resulting discrete eigenstates approximate the true bound states and the remaining (pseudocontinuum) states represent the true continuum. Next, the stationary collision wavefunction is expanded in the diagonal basis to obtain a discrete set of close‐coupled equations, which are integrated numerically by standard procedures. The method is applied to a collinear model in which the diatomic is bound by a Morse potential and the interaction is a repulsive exponential. The total CID probabilities appear to be converged to 1% or 2% in most cases. Vibrational ’’enhancement’’ of CID is observed in this model. A very general problem associated with the use of the exponential interaction in conjunction with a binding potentia...

A model study of collision induced dissociation of a diatomic molecule by an atom

The time‐dependent Schrodinger equation for the collinear collision of an atom with a diatomic molecule is solved numerically after the manner of McCullough and Wyatt. The binding potential is taken to be a truncated square well and the interaction is impulsive (hard sphere). For the case in which all three masses are equal final relative momentum distributions and dissociation probabilities are obtained as a function of both the initial relative kinetic energy and the initial vibrational level. For purposes of comparison quasiclassical trajectory analyses of the same cases were performed. Quantum effects on collision‐induced dissociation (CID) are seen to be important for this model. A very notable characteristic of the model, observed in both the quantum and classical results, yet not in most experimental results, is that CID is severely vibrationally inhibited, i.e., the probability of CID decreases as the initial vibrational quantum number increases at a fixed total collision energy. Probable causes o...