Optical absorption spectrum of gold atoms deposited on SiO(2) from cavity ringdown spectroscopy

The optical properties of gold atoms supported on amorphous silica (alpha-SiO2) were studied experimentally and theoretically in the visible range. Samples were prepared in situ by depositing Au atoms at low coverages (5 x 10(12) cm(-2)) in UHV, and the optical absorption spectra were recorded by cavity ringdown spectroscopy. The atomic absorption bands can be attributed to gold atoms trapped at [triple bond] Si-O(.-) and [triple bond]Si-O(-) defect sites. The absence of optical transitions typical for Au(2) shows that the atoms are efficiently anchored at these defect sites, preventing diffusion and aggregation. Furthermore, these experimental results reveal that it is now possible to study optical properties of well-defined nanostructures at surface coverages as low as 5 x 10(11) cm(-2).     

Coverings of Groups and Types

Assume that G is a group covered by countably many sets X_n,n < . It is proved that G is generated in two or three stepsby a small number of the sets X_n. These results are generalizedfor countable coverings of types and countable colourings ofgraphs.     

Nonlinearities in the reflection and transmission spectra of the photonic bandgap heterostructures with n–i–p–i crystals

Nonlinear optical properties of photonic crystal heterostructures with embedded n–i–p–i superlattices are investigated. Self-consistent calculations of the transmission and reflection spectra near the defect mode are performed using the transfer-matrix method and taking into account the gain saturation. Analysis of features and output characteristics is carried out for one-dimensional photonic crystal heterostructure amplifiers in the GaAs–GaInP system having at the central part an active “defect” from doubled GaAs n–i–p–i crystal layers. The gain saturation in the active layers in the vicinity of the defect changes the index contrast of the photonic structure and makes worse the emission at the defect mode. Spectral bistability effect, which can be exhibited in photonic crystal heterostructure amplifiers, is predicted and the hysteresis loop and other attending phenomena are described. The bistability behavior and modulation response efficiency demonstrate the potential possibilities of the photonic crystal heterostructures with n–i–p–i layers as high-speed optical amplifiers and switches.      

Optochemical sensor for determining ozone based on novel soluble indigo dyes immobilised in a highly permeable polymeric film

An optochemical ozone sensor is described that has been manufactured by immobilisation of novel soluble indigo derivatives in permeable transparent polymeric films of polydimethylsiloxane–polycarbonate copolymer. From a number of investigated indigo derivatives, 4,4,7,7-tetraalkoxyindigo 9 has been selected for optimal sensitivity and specificity of ozone detection. A linear calibration for ozone can be obtained in the range between 0.01 and 0.5 ppm. The limit of quantitation is 0.03 ppm, and the accuracy exceeds 8%. It takes about 134 s to measure the relatively low occupational exposure concentration of 0.1 ppm. A reduction of the sensor response time could be achieved through application of double-sided coated sensors instead of single-sided variants. The stability of the sensors and the effect of external parameters like relative humidity (RH), temperature and gas flow on the sensor response have been investigated. The sensor response is affected by varying the gas flow or temperature; however, humidity in the range between 0 and 90% RH does not affect sensor response. The indigo derivative 9 remained stable inside the polymeric film and no chemical reaction, crystallisation or leaching occurred during 10 months of observation. Proper choice of indicator dye and polymeric material and successful application of kinetic evaluation method for the exposure experiments determine the desired features of the sensor.     

Comparative Infrared,Raman, and Natural‐Bond‐Orbital Analyses of King's Sultam

By means of ¹H‐NOESY‐ and Raman‐spectroscopic analyses, we experimentally demonstrated the presence of the equatorial N? Me conformer of King's sultam 4b in solution, resulting from a rapid equilibrium. As a consequence, the value of the N lone‐pair anomeric stabilization should be revised to 1.5–1.6 kcal/mol. Independently from the N tilting, natural bond orbital (NBO)‐comparative analyses suggest that the S d* orbitals do not appear as primordial and stereospecific acceptors for the N lone pair. Second, the five‐membered‐ring sultams do not seem to be particularly well‐stabilized by the S? C σ* orbital in the N‐substituted pseudo‐axial conformation, as opposed to an idealized anti‐periplanar situation for the six‐membered‐ring analogues. In this latter case, the other anti‐periplanar C? C σ* and C(1′)? H/C(2′) σ*orbitals are as important, if not more, when compared to the S? C σ* participation. In the pseudo‐equatorial conformation, γ‐sultams particularly benefit from the N lone‐pair hyperconjugation with the anti‐periplanar S? O₁ σ* and C(2)? H/C or C(1′)? H/C σ* orbitals. This is also the case for δ‐sultams when the steric requirement of the N‐substituent exceeds 1.6 kcal/mol. When both axial and equatorial conformations are sterically too exacting, the N‐atom is prone to sp² hybridization or/and conformational changes (i.e., 12c ). In that case also, the mode of stereoelectronic stabilization differs from γ‐ to δ‐sultams.     

A monomeric thallium(I) amide in the solid state: synthesis and structure of TlN(Me)ArMes2 (ArMes2 = C6H3-2,6-Mes2)

Reaction of TlCl and [LiN(Me)Ar(Mes)2](2) [Ar(Mes)2 = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me(3))(2)] in Et(2)O generated the thallium amide, TlN(Me)Ar(Mes)2 (1). X-ray data showed that it has a monomeric structure with an average Tl-N distance of 2.364(3) Angstroms. There was also a Tl-arene approach [Tl-centroid = 3.026(2) Angstroms (avg)] to a flanking mesityl ring from the terphenyl substituent. DFT calculations showed that this interaction is weak and supported essentially one coordination for thallium. The electronic spectrum of 1 is hypsochromically shifted in comparison to the monomeric TlAr(Trip)2 (Trip = C(6)H(2)-2,4,6-Pr(i)(3)).     

Actions of compact quantum groups on -algebras

In this paper we investigate a structure of the fixed point algebra under an action of compact matrix quantum group on a -algebra . We also show that the categories of -comodules in and inner endomorphisms restricted to the fixed point algebra coincide when the relative commutant of the fixed point algebra is trivial. Next we show a version of the Tannaka duality theorem for twisted unitary groups.

     

Resolutions of the Presidium of the Russian Academy of Sciences

Information

Resolutions of the Presidium of the Russian Academy of Sciences     

PEDOT films: multifunctional membranes for electrochemical ion sensing

Properties of electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) films were studied from the point of view of direct use as ion-sensing membranes in potentiometric or amperometric sensors. Stable and reproducible potentiometric characteristics were obtained for PEDOT doped by poly(4-styrenesulfonate) ions, PEDOT(PSS) (cationic characteristics), and PEDOT doped by hexacyanoferrate(II) anions, PEDOT(HCF) (anionic characteristics). As shown by voltammetric and EDAX results, the anion exchange properties of the latter polymer result from gradual replacement of HCF ions by Cl^– anions from solution. The zero-current potentiometric detection limit of PEDOT(PSS), equal to 3×10^–6 M, can be shifted to 7×10^–7 M by polarization using a cathodic current density of 3×10^–7 A cm^–2. PEDOT films doped by Cl^– or PSS^– ions can be used as membranes for sensing anions or cations, respectively, under pulse amperometric conditions, within the range from 10^–4 to 1 M, comparable with that accessible by zero-current potentiometry. Dissolved oxygen (redox interferent of low charge transfer rate) exerts a minor influence on the slope of the potentiometric and amperometric characteristics of PEDOT films. Although the presence of redox reactants characterized by a high rate of charge transfer [Fe(CN)₆ ^3–/4–] results in the disappearance of the potential dependence on KCl concentration, this disadvantageous effect is much less significant under pulse amperometric conditions.Contribution to the 3rd Baltic Conference on Electrochemistry, GDASK-SOBIESZEWO, 23–26 April 2003. Dedicated to the memory of Harry B. Mark, Jr. (February 28, 1934–March 3rd, 2003)