An extension of the frank and Wolfe method of feasible directions

The Frank and Wolfe method of feasible directions is shown to be a case of the more general computational approach of inner linearization followed by restriction. An extension is proposed based on this observation. The extended procedure converges, and under certain conditions the asymptotic convergence rate is geometric. Limited computational experience comparing the two procedures is reported.The author is grateful to Mr. Peter Friesen for help with the computational work and to the Graduate School of Business, Stanford University for their support of this research.     

Intrinsic step-related surface states and 4f-core levels on Ir(332)

Using angle-resolved photoemission, we have identified a new surface state on Ir(332), which can be unambigously associated with step sites. This state is located near the Fermi level and can be distinguished from a terrace-related state by preferential adsorption of hydrogen on the rows of step atoms. Terrace and step atoms are shown to exhibit different 4f-core level binding energies, a fact which can be used to independently monitor preferential adsorption.     

Dissociative adsorption of H2: Time-dependent quantum studies

In this short review we consider some selected results for the dissociation of hydrogen on metal surfaces. In particular we focus our attention on the effects that surface corrugation have on the dissociation and also how rotations and vibrations couple in the interaction region. Results from time dependent quantum wavepacket calculations have been particularly helpful in forming our ideas of how diatomics dissociate. We describe this method along with two examples of propagation schemes. The power of limited dimensional models to rationalize experimental observations is clearly demonstrated.     

The dynamics of coupled nonlinear model Boltzmann equations

A multispecies gas described by coupled nonlinear Boltzmann equations is studied as a dynamical system. Properties are determined of theN coupled nonlinear ODEs for the number densities obtained from the Boltzmann equations for the spatially uniform system ofN species undergoing binary scattering, removal, and regeneration in the presence of an external force field and a reservoir of background gas. The physically realizable setQ, the nonnegative cone in theN-dimensional phase space of species number densities, is established as invariant under the flow. The fixed-point equations for the ODEs are shown to be equivalent to 2 ^N linear systems, and conditions for the stability and instability of the fixed points are then established. Stable fixed points are demonstrated to exist inQ by showing that they enter via a sequence of transcritical bifurcations as physical parameters are varied. For the two-species case the typical global structure of the solutions is established. Various particular cases are described including one which possesses an infinite family of periodic solutions and one that depends delicately upon initial conditions due to a separatrix that separatesQ into two invariant sets.     

O implantation in ZnSe: lattice distortion by Raman measurement

We probe the “bulk” and near-surface regions of O-implanted heteroepitaxial ZnSe films, using the full-width at half-maximum (FWHM) of the Raman line. The results are compared to those after N implantation. A tensile stress attributed to a deformation in the lattice around O is found. Under rapid thermal annealing, the optimum temperature for the recovery of the implantation damage without suffering from thermally generated point defects is T_a = 500°C. It corresponds here to the maximum FWHM from the uncompensated tensile stress, while identical FWHM's are found for the top and the “bulk” part of the film just after implantation, and after implantation and T_a = 600°C.     

Theoretical analysis of desorption from surfaces

The general theory of the desorption of atoms from surfaces is discussed and a method given for the calculation of the surface residence time. Potential parameters are deduced for a number of experimental systems using a variety of adsorption sites to model the interactions. By using this technique valuable information about the surface-atom potential may be obtained.     

Treatment of municipal solid wastes by steam classification for recycling and biomass utilization

A patented method of steam classification (the Holloway Process) and a prototype process unit have been investigated. The process treats municipal solid wastes (MSW) with steam and pressure to yield sterilized recyclable metals, glass, and plastics and a cellulosic pulp. After processing, the components of the mixture are separated into three sizes by vibratory screening. The recyclable materials must be subsequently separated from the screened fractions. Process conditions have been optimized to maximize the yield of a sterile cellulosic fraction that has a variety of possible end uses, such as compost, combustion fuel, and feedstock for conversion to fuels and chemicals.     

Electronic nonadiabatic effects in the adsorption of hydrogen atoms on metals

The time-dependent, mean-field Newns-Anderson model for a spin-polarized adsorbate approaching a metallic surface is solved in the wide-band limit. Equations for the time evolution of the occupation of the spin dependent adsorbate states and for the nonadiabatic and nearly adiabatic adsorbate-surface energy transfer rates are derived. Numerical solutions are obtained using characteristic parameters derived from density functional theory calculations for the H/Cu(111) system. The time evolution of the model system is shown to be strongly nonadiabatic in the vicinity of the transition point between spin-polarized and nonpolarized ground states. Away from the spin transition the nonadiabatic energy transfer is in close agreement with the nearly adiabatic limit. Near the transition, nonadiabatic effects are large and the nearly adiabatic approximation fails.     

Photoinduced intramolecular charge separation in donor/acceptor-substituted bicyclohexylidene and bicyclohexyl

The photophysical properties of a bicyclohexylidene (1DA) and a bicyclohexyl (2DA) substituted with an anilino electron donor and a dicyanoethylene electron acceptor have been studied. Quenching of local donor emission is observed for these compounds as well as quenching of the "pseudo-local" acceptor emission. Transient absorption spectra show dialkylanilino-type radical-cation and dicyanoethylene-type radical-anion absorptions. These results show that intramolecular charge separation takes place in 1DA and 2DA. This was corroborated by time-resolved microwave conductivity measurements from which large excited-state dipole moments were found for both 1DA and 2DA. Time-resolved fluorescence spectroscopy revealed that in the charge-separated state in cyclohexane for 2DA, molecular folding takes place on a nanosecond timescale. For 1DA in cyclohexane, either charge separation takes place in a (fully) folded conformation or very rapid (subnanosecond timescale) folding takes place subsequent to charge separation. In addition to this difference in conformational behavior, the presence of the exocyclic double bond between the cyclohexyl-type rings results in efficient quenching of the anilino donor triplet state and acceleration of the charge recombination rate by a factor of 20.