The effect of an electric field on the local stoichiometry of front waves in an ionic chemical system

A model based on an autocatalytic, two-step reaction mechanismincluding two ionic components (of the same charge) and twonon-ionic components, where both reactions are of second orderoverall, is considered when an electric field is applied tothe system. The model is motivated by experimental observationson the iodate-arsenous acid system. The travelling wave equationsare examined first and conditions obtained for the existenceand form of these waves. These conditions are then used to interpretthe results obtained from numerical simulations of the fullsystem. These results display all the main features observedexperimentally, the change in the local stoichiometry and thepossible wave annihilation for sufficiently strong fields. Themodel provides a clear explanation for these features as wellas predicting new features not reported from the experiments.The main one of which is the occurrence of an internal wavepropagating in the reacted part of the system in the directioninduced by the applied field.     

A quantum-mechanical explanation of vibronic phenomena in atom-molecule collisions

The quantum-mechanical equivalent of a classically vibrating molecular ion during an ion-pair formation collision is presented. The classical vibration in the quantal representation is explained as an interference between partial waves which evolve along neighbouring vibronic states during the collision.     

The absolute determination of the rotational coupling range from diffraction fringes

We present strong evidence of the experimental observation of Fraunhofer diffraction effects in differential cross sections of Li⁺ ions scattered by K atoms. The fringes can be explained to be due to the sharp edges of the rotational coupling mechanism and can be compared to the Fraunhofer diffraction from an annular diaphragm.     

The trapping of potassium atoms by a polycrystalline tungsten surface as a function of energy and angle of incidence

The trapping probability of potassium atoms on a polycrystalline tungsten surface has been measured as a function of the angle of incidence and as a function of the energy of the incoming atoms. Below an energy of 1 eV the trapping was complete; above 20 eV only reflection occurred. The trapping probability increased with increasing angle of incidence. The measurements are compared with a simple model of the fraction of atoms initially trapped. The model, a one-dimensional cube model including a Boltzmann distribution of the velocities of oscillating surface atoms, partially explains the data. The trapping probability as a function of incoming energy is well described for normal incidence, justifying the inclusion of thermal motion of the surface atoms in the model. The angular dependence can be explained in a qualitative way, although there is a substantial discrepancy for large angles of incidence, showing the presence of surface structure.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Extracellular β-class carbonic anhydrase of the alkaliphilic cyanobacterium Microcoleus chthonoplastes

The gene for β-class carbonic anhydrase (CA), which was designated as cahB1, was cloned from the genomic library of the alkaliphilic cyanobacterium Microcoleus chthonoplastes. The product of the cahB1 gene was expressed in Escherichia coli. The protein revealed high specific activity of CA, which was inhibited with ethoxyzolamide. The maximum activity of the recombinant CA was detected at alkaline pH (～9.0) and its minimum - at neutral pH (～7.0). Western blotting analysis with the antibodies raised against the recombinant CahB1 protein revealed its localization in cell envelopes of M. chthonoplastes. Immunocytochemical localization of the CahB1 in cells confirmed its extracellular location. The newly characterized CahB1 of Microcoleus was similar in amino acid and nucleotide sequences to well known β-CAs of Synechococcus sp. PCC 7942 (IcfA) and Synechocystis sp. PCC 6803 (CcaA), although those CAs were attributed to the carboxysomal shells of cyanobacteria. Previously we have reported β-class CA which was associated with PS II of alkaliphilic cyanobacteria. Here we first report extracellular localization of β-class CA and provide a scheme for its possible involvement in the maintenance of a balance between external sources of inorganic carbon and photosynthesis in extreme environments of soda lakes.     

Synthesis and characterization of uncoated and gold-coated magnetite nanoparticles

We report on the synthesis and characterization of uncoated and gold coated magnetite nanoparticles. Structural characterizations, carried out using X-ray diffraction, confirm the formation of magnetite phase with a mean size of ~7 and ~8 nm for the uncoated and gold covered magnetite nanoparticles, respectively. The value of the gold coated Fe₃O₄ nanoparticles is consistent with the mean physical size determined from transmission electron microscopy images. Mössbauer spectra at room temperature are consistent with the thermal relaxation of magnetic moments mediated by particle-particle interactions. The 77 K Mössbauer spectra are modeled with four sextets. Those sextets are assigned to the signal of iron ions occupying the tetrahedral and octahedral sites in the core and shell parts of the particle. The room-temperature saturation magnetization value determined for the uncoated Fe₃O₄ nanoparticles is roughly ~60 emu/g and suggests the occurrence of surface effects such as magnetic disorder or the partial surface oxidation. These surface effects are reduced in the gold-coated Fe₃O₄ nanoparticles. Zero-field–cooled and field-cooled curves of both samples show irreversibilities which are consistent with a superparamagnetic behavior of interacting nanoparticles.     

Polarization dependence on the holographic recording in spiropyran-doped polymers

We report on the photoinduced anisotropy in the holographic recording in spiropyran doped polymers as a result of the photoizomerization of spiropyran into merocyanine. Photoizomerization is achieved with UV polarized light ( = 355 nm) from the third harmonic of Q-switched Nd:YAG laser. After UV illumination a strong and broad polarization-dependent absorption peak centered 600 nm appears. Absorption holograms were recorded with low power He–Ne lasers with 2.2% output diffraction efficiency for polarization parallel to the polarization of the UV laser. The extremely long lifetime of the merocyanine states (12 days in the dark) and their high resolution makes this material promising as an optical memory element.