Optical second harmonic generation studies of the nature of the GaAs(100) surface in air

We present in this paper a study of the non-linear optical response of the GaAs/air interface as measured by second harmonic generation. By careful choice of incidence and analysis electric vectors for the radiation the second harmonic response as a function of crystallographic orientation has been characterized for a GaAs(100) surface. The exact origin of the response remains unclear but comments on the surface specificity of the technique are made and the observed anisotropy in the data is explained in terms of the step density of the crystal surface.     

Application of computer simulation free-energy methods to compute the free energy of micellization as a function of micelle composition. 1. Theory

The widespread use of surfactant mixtures and surfactant/solubilizate mixtures in practical applications motivates the development of predictive theoretical approaches to improve fundamental understanding of the behavior of these complex self-assembling systems and to facilitate the design and optimization of new surfactant and surfactant/solubilizate mixtures. This paper is the first of two articles introducing a new computer simulation-free-energy/molecular thermodynamic (CS-FE/MT) model. The two articles explore the application of computer simulation free-energy methods to quantify the thermodynamics associated with mixed surfactant/cosurfactant and surfactant/solubilizate micelle formation in aqueous solution. In this paper (article 1 of the series), a theoretical approach is introduced to use computer simulation free-energy methods to compute the free-energy change associated with changing micelle composition (referred to as DeltaDeltaGi). In this approach, experimental critical micelle concentration (CMC) data, or a molecular thermodynamic model of micelle formation, is first used to evaluate the free energy associated with single (pure) surfactant micelle formation, g(form,single), in which the single surfactant micelle contains only surfactant A molecules. An iterative approach is proposed to combine the estimated value of gform,single with free-energy estimates of DeltaDeltaGi based on computer simulation to determine the optimal free energy of mixed micelle formation, the optimal micelle aggregation number and composition, and the optimal bulk solution composition. After introducing the CS-FE/MT modeling framework, a variety of free-energy methods are briefly reviewed, and the selection of the thermodynamic integration free-energy method is justified and selected to implement the CS-FE/MT model. An alchemical free-energy pathway is proposed to allow evaluation of the free-energy change associated with exchanging a surfactant A molecule with a surfactant/solubilizate B molecule through thermodynamic integration. In article 2 of this series, the implementation of the CS-FE/MT model to make DeltaDeltaGi free-energy predictions for several surfactant/solubilizate systems is discussed, and the predictions of the CS-FE/MT model are compared with the DeltaDeltaGi predictions of a molecular thermodynamic model fitted to relevant experimental data.     

A mechanistic probe for asymmetric reactions: deuterium isotope effects at enantiotopic groups

The development of a mechanistic probe that is especially suitable for the study of asymmetric reactions is presented. Chemically innocuous enantiotopic methyl groups are utilized as probes for the distinct environments that develop at the transition state for the (-)-B-chlorodiisopinocampheylborane reduction of 4'-methylisobutyrophenone. 2H kinetic isotope effects (KIEs) are determined for both enantiotopic methyl groups using two types of competition reactions. One competition is that between the d3-methyl enantiomeric isotopomers. The other competition reaction is that between the d6-dimethyl and perprotiated isotopologues. The rate constant ratios can be converted into kinetic isotope effects upon each of the individual enantiotopic methyl groups by invoking the rule of the geometric mean. The resulting isotope effect measurements yield highly precise values and contribute further understanding to the transition structure for this stereoselective reduction. The results are discussed in the context of steric isotope effects and the origins of these effects, which arise from the impact of steric crowding upon the anharmonicity of C-H bonds in the transition structure relative to the reactant state.     

MOVPE of II–VI materials

The development of II–VI MOVPE is reviewed, contrasting the narrow bandgap materials with the wide bandgap. Common issues are the need to grow the layers at lower temperatures than their III–V cousins in order to avoid point defects. This means that II–VI MOVPE occurs in a surface kinetic regime for precursor decomposition and has stimulated a lot of research on alternative precursors. The narrow bandgap II–VI growers have settled on dimethyl cadmium (DMCd) combined with diisopropyl telluride (DIPTe) and a liquid Hg source but wide bandgap growers are split between pyrolytic and photo-assisted growth. Recent progress in p-type doping has enabled the demonstration of some new devices, including two colour infrared detectors and the first MOVPE grown green emitting laser structure. The common theme appears to be hydrogen passivation of the Group V dopant and some novel precursor solutions to this problem are discussed.     

KINETIC FORMALDEHYDE ANALYSIS OF DNA ULTRAVIOLET IRRADIATED IN THE PRESENCE OF SILVER IONS*

Abstract— DNA from Escherichia coli was irradiated at 254 nm in the presence of silver in order to preferentially enhance the rate of formation of pyrimidine-dimer damage over nondimer damage. The irradiated DNA was treated with formaldehyde in order to measure the unwinding velocity of the defects associated with the pyrimidine dimers. This velocity was found to be 0.18 base pairs/min per pyrimidine dimer, which is nearly 8 times less than that found for a double-strand break (1.37 base pairs/min) obtained by use of sheared DNA whose size was determined by electron microscopy. The rate of reaction of the DNA with formaldehyde varied linearly with the pyrimidine dimer concentration and showed no inflection due to clustering. Treatment of irradiated DNA with UV endonuclease enhanced the formaldehyde reaction by ? 7-fold, consistent with the conversion of a dimer into the faster-reacting defect associated with a single-strand break. These results indicate that the distribution of dimers in DNA is random and not clustered, and that previous interpretations of clustering were based on the false assumption that dimer and chain break defects unwind with similar velocities when treated with formaldehyde.     

Photocatalytic degradation of 4-chlorophenol: A mechanistically-based model

The photocatalytic degradation of 4-CP was mathematically modelled using the mechanistic insights and data presented in an earlier study [1]. The solution and surface concentrations of reacting species were calculated by solving a system of differential equations that account for oxidation reactions of dissolved and adsorbed species, adsorption and desorption, reduction of oxygen, and hole-electron recombination. The differential equations were integrated over discrete time-periods and annular regions of the photoreactor. The resulting model predicts the trends observed in studies in other laboratories using different experimental apparati. Using the model it is possible to predict effects of reactor geometry, TiO₂ loading, light intensity, and mixing on the course of TiO₂ photocatalytic oxidation. The model verifies the importance of surface reactions, and reveals the need to better understand the fate and role of oxygen in TiO₂ photocatalytic systems.