Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

On-line LC-GC-AED and LC-GC-MS – multidimensional methods for the analysis of PAC in fuels,combustion emissions and atmospheric samples

The composition and concentration of polycyclic aromatic compounds (PAC) in fuels. Theier combustion products and in the atmosphere remains a topic of considerable interest. Despite the wealth of literature on the identification of PAC, speciation at low concentrations remains difficult due to instrument limitation and the complexity of fuel and environmental samples. Consequently on line sample preparation procedures (SPE, SFE, LC, etc.) are becomeing an increasingly important step in the analysis procedure particularly where sample clean-up and fractionation are essential for improving analytical resolution. In this study a normal phase high pressure analytical resolution. In this study a normal phase high pressure liquid chromatography-gas chromatography (LC-GC) system has been developed to provide quantitative analysis of samples, as diverse as coal liquids, petroleum fuels, diesel exhaust particulates, and urban air particulates. Separation and identification of parent and alkylated PAH, hetercycline nitro-and oxy-PAC can be achieved by direct coupling to an atomic emission detector and a bech top mass spectrometer. For both systems the primary LC separation combined with the large sample volume transferred to GC vastly improves detection limits. Furthermore the complimentary nature of the two detectors used enables the positive indentification of many unknowns.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Reply to comment on an unexpected symmetry of the Coulomb potential in the Debye-Smoluchowski equation

A properly modified inner boundary condition is used in an analysis of the conditional reaction probability, φ^*_R (r₀, t), using the Debye-Smoluchowski equation. It is shown that the Coulomb potential modifies the reaction velocity at the boundary by the addition of a drift term. φ^*_R (r₀, t) is identifical for attractive and repulsive cases if a constant term is added to the boundary velocity for the attractive potential.     

An unexpected symmetry of the coulomb potential in the debye-smoluchowski equation

Generalised diffusion processes are discussed using the theory of stochastic processes and several elementary results are proved for the survival probability of a pair of particles with an arbitrary potential. The reaction probability conditioned on reaction ultimately occurring is considered and its backward equation is determined. In the case of the Coulomb potential the probability is shown analytically to be identical in both the attractive and repulsive cases for a given absolute value of r_c. The utility of this result in numerical solutions of the Debye—Smoluchowski equation is discussed.     

Kinetics and mechanisms of the reactions of OH with some oxygenated compounds of importance in tropospheric chemistry

The kinetics of the reactions have been studied in a discharge flow system under pseudo-first-order conditions. The OH concentration was monitored by laser induced fluorescence and helium was used as the carrier gas. Values of k₁ = (8.1 ± 1.7) × 10^?13, k₂ = (1.31 ± 0.26) × 10^?11, k₃ = (2.6 ± 0.5) × 10^?11, and k₄ = (2.5 ± 0.4) × 10^?11 cm³ molecule^?1 s^?1, at 298 K and 1 torr total pressure, were obtained. To validate the newly constructed system the rate constant for the reaction was determined in a similar manner. The value of k₅ = (6.7 ± 0.9) × 10^?12 cm³ molecule^?1 s^?1 at 298 K and 1 torr total pressure is in very good agreement with other literature values. The mechanisms for the atmospheric degradation of these compounds have been constructed to allow their incorporation in a photochemical trajectory computer model, to assess their impact on photochemical ozone creation in the troposphere. © 1994 John Wiley & Sons, Inc.     

Experimental and modeling studies of the pressure and temperature dependences of the kinetics and the OH yields in the acetyl + O2 reaction

The acetyl + O(2) reaction has been studied by observing the time dependence of OH by laser-induced fluorescence (LIF) and by electronic structure/master equation analysis. The experimental OH time profiles were analyzed to obtain the kinetics of the acetyl + O(2) reaction and the relative OH yields over the temperature range of 213-500 K in helium at pressures in the range of 5-600 Torr. More limited measurements were made in N(2) and for CD(3)CO + O(2). The relative OH yields were converted into absolute yields by assuming that the OH yield at zero pressure is unity. Electronic structure calculations of the stationary points of the potential energy surface were used with a master equation analysis to fit the experimental data in He using the high-pressure limiting rate coefficient for the reaction, k(∞)(T), and the energy transfer parameter, (ΔE(d)), as variable parameters. The best-fit parameters obtained are k(∞) = 6.2 × 10(-12) cm(-3) molecule(-1) s(-1), independent of temperature over the experimental range, and (ΔE(d))(He) = 160(T/298?K) cm(-1). The fits in N(2), using the same k(∞)(T), gave (ΔE(d))(N(2)) = 270(T/298?K) cm(-1). The rate coefficients for formation of OH and CH(3)C(O)O(2) are provided in parametrized form, based on modified Troe expressions, from the best-fit master equation calculations, over the pressure and temperature ranges of 1 ≤ p/Torr ≤ 1.5 × 10(5) and 200 ≤ T/K ≤ 1000 for He and N(2) as the bath gas. The minor channels, leading to HO(2) + CH(2)CO and CH(2)C(O)OOH, generally have yields <1% over this range.     

The effect of temperature on collision induced intersystem crossing in the reaction of CH2 with H2

Laser flash photolysis of ketene at 308 nm, coupled with H atom vacuum ultraviolet laser induced fluorescence, was used to determine the branching ratio for the CH₃ + H channel (1a) in the reaction of CH₂¹A₁ (¹CH₂) with H₂, over the temperature range 300–500 K. This reaction channel competes with collision induced intersystem crossing (CIISC) to form triplet methylene, CH₂³B₁ (³CH₂) (channel 1b). The branching ratio for H formation, k_1a/k₁, was determined by measuring the relative H atom yield in three time resolved measurements of H: (i) in ketene, H₂ mixtures, where H is exclusively formed by reaction 1a, (ii) in ketene, H₂, NO mixtures ([NO] [H₂]), where H is formed at short times by 1a and at longer times by ³CH₂ + NO, following 1b, and (iii) in ketene, He, NO mixtures ([NO] [He]), where H is exclusively formed from ³CH₂ + NO, following deactivation of singlet to triplet methylene by He. k_1a/k₁ was found to increase from 0.85 at 300 K to unity at 500 K, with the yield of CIISC decreasing from 0.15 to zero. This is the first measurement of the temperature dependence of the rate coefficient for CIISC in a reactive system. The rate coefficient for CIISC with an inert gas increases with T. It has been suggested that the fractional yield of CIISC will increase with temperature in reactive systems, thus reducing the rate coefficient for reaction at high temperature, with significant consequences for combustion systems. The present experiments demonstrate that this is not the case for reaction with H₂ and implies a different CIISC mechanism for reactive vs inert collision partners.