Water-induced morphology changes in BaO/gamma-Al2O3 NOx storage materials

Exposure of NO(2)-saturated BaO/gamma-Al(2)O(3) NO(x) storage materials to H(2)O vapour results in the conversion of surface nitrates to Ba(NO(3))(2) crystallites, causing dramatic morphological changes in the Ba-containing phase, demonstrating a role for water in affecting the NO(x) storage/reduction properties of these materials.     

Probing the contribution of electronic coupling to the directionality of electron transfer in photosynthetic reaction centers

Subpicosecond transient absorption studies are reported for a set of Rhodobacter (R.) capsulatus bacterial photosynthetic reaction centers (RCs) designed to probe the origins of the unidirectionality of charge separation via one of two electron transport chains in the native pigment-protein complex. All of the RCs have been engineered to contain a heterodimeric primary electron donor (D) consisting of a bacteriochlorophyll (BChl) and a bacteriopheophytin (BPh). The BPh component of the M heterodimer (Mhd) or L heterodimer (Lhd) is introduced by substituting a Leu for His M200 or His L173, respectively. Previous work on primary charge separation in heterodimer mutants has not included the Lhd RC from R. capsulatus, which we report for the first time. The Lhd and Mhd RCs are used as controls against which we assess RCs that combine the heterodimer mutations with a second mutation (His substituted for Leu at M212) that results in replacement of the native L-side BPh acceptor with a BChl (beta). The transient absorption spectra reveal clear evidence for charge separation to the normally inactive M-side BPh acceptor (H(M)) in Lhd-beta RCs to form D+H(M)- with a yield of approximately 6%. This state also forms in Mhd-beta RCs but with about one-quarter the yield. In both RCs, deactivation to the ground state is the predominant pathway of D decay, as it is in the Mhd and Lhd single mutants. Analysis of the results indicates an upper limit ofV2L/V2m < or = 4 for the contribution of the electronic coupling elements to the relative rates of electron transfer to the L versus M sides of the wild-type RC. In comparison to the L/M rate ratio (kL/kM) approximately 30 for wild-type RCs, our findings indicate that electronic factors contribute approximately 35% at most to directionality with the other 65% deriving from energetic considerations, which includes differences in free energies, reorganization energies, and contributions of one- and two-step mechanisms on the two sides of the RC.     

Determination of colorimetric uncertainties in the spectrophotometric measurement of colour

A method for the determination of colorimetric uncertainties has been developed in order to meet the requirements for accreditation by the UK accreditation service (UKAS), which include a statement of uncertainty for all certified quantities. The values of the principal sources of spectrophotometric uncertainty are first determined and are used to calculate corresponding components of colorimetric uncertainty using a simple model. The components of uncertainty analysed are 100% level (diffuse reflectance), photometric non-linearity, dark level and wavelength error. Bandwidth error is not significant for NPL surface colour standards because a small bandwidth is always used.

The gloss trap error and specular beam error are determined and corrected so that only the uncertainties after correction need be considered. These can be treated as components of dark level uncertainty. The uncertainties are determined for the following colour data: x, y, Y, u′, v′, L^*, a^* and b^* for the CIE 10° Standard Observer and the CIE Standard Illuminant D₆₅ for three geometries: specular included, specular excluded and 0°/45°. These are now quoted routinely on NPL certificates for ceramic colour standards, white and black ceramic tile standards and Russian opal standards.     

Detection and imaging of He2 molecules in superfluid helium

We present data that show a cycling transition can be used to detect and image metastable He2 triplet molecules in superfluid helium. We demonstrate that limitations on the cycling efficiency due to the vibrational structure of the molecule can be mitigated by the use of repumping lasers. Images of the molecules obtained using the method are also shown. This technique gives rise to a new kind of ionizing radiation detector. The use of He2 triplet molecules as tracer particles in the superfluid promises to be a powerful tool for visualization of both quantum and classical turbulence in liquid helium.     

High-Resolution Measurements of HBr Transitions in the First Overtone Band Using Tunable Diode Lasers

High-resolution absorption lineshapes of the R(7) and P(2) transitions in the first overtone (v = 0-2) band of H(79)Br have been recorded at room temperature using a pair of distributed feedback diode lasers operating near 1.95 and 2.00 μm, respectively. Spectral line intensities and self-broadening coefficients were determined by fitting the measured spectra (for various pressures P = 10-100 Torr) with Voigt profiles and compared with values in the literature. Measured line intensities for the P(2) and R(7) transitions are approximately 11 and 16% higher than those listed in the HITRAN database, respectively. The measured self-broadening coefficient of the P(2) transition is approximately 14% lower than the value listed in HITRAN. Measurements of the P(2) lineshapes at low pressure (100 mTorr) were modeled with eight-line Gaussian profiles based on ground state (v = 0) hyperfine constants to include the effects of nuclear electric quadrupole interactions. Copyright 2000 Academic Press.     

Sum-free sets and Ramsey numbers

In this note we obtain new lower bounds for the Ramsey numbers R(5, 5) and R(5, 6). The method is based on computational results of partitioning the integers into sum-free sets. We obtain R(5, 5)?42 and R(5, 6)?53.     

Cavity-enhanced absorption spectroscopy for shocktubes: Design and optimization

Cavity-enhanced absorption spectroscopy (CEAS) has generated much interest in shocktube kinetics studies because of its recent success in achieving improved sensitivity and high time resolution with robust optical alignment. While recent progress demonstrated experimental schemes including off-axis scanned-wavelength approach and on-axis ps-pulsed laser approach, that both successfully suppressed the laser-cavity coupling noise, this paper develops a theoretical model to predict the CEAS sensor performance that can be used as a design tool applicable to more generalized cases. The method models the optical field in the cavity based on the decentered Gaussian beam model, from which the cavity transmission spectrum and the laser-cavity coupling noise can be numerically calculated. The simulation results predict sensor performance for different cavity configurations and laser characteristics, including various degrees of laser-cavity mode-matching, laser linewidths, scanning rates, and cavity filling conditions. Simulation with example wavelengths in the ultraviolet, near-infrared, and mid-infrared showed increasing mode-matched beam waist size for increasing wavelengths. An off-axis alignment scheme was found to be capable of suppressing the coupling noise by two orders-of-magnitude at a moderate laser linewidth of 1?GHz. Coupling noise level on the order of 1e-5 for scanned-wavelength off-axis alignment case with a narrowband mid-infrared laser was obtained by model calculation and agreed with experimental results within acceptable uncertainty range. The developed method can serve to guide future design and optimization of CEAS system in shocktube studies.     

Using external coils to correct field errors in tokamaks

Non axisymmetric correction coils can be highly effective at canceling the low order magnetic islands generated by magnetic field errors. Correction coil currents which, by themselves, mould produce an island of the same size and opposite phase as the error field, will result in cancelled magnetic islands. In computing the magnetic island sizes and toroidal phases of perturbations, the use of a straight-cylinder approximation can cause substantial error. An n=1 error with arbitrary phase can be corrected at a single resonant surface with one set of four correction coils. Correcting higher n harmonics requires more coils toroidally. Correcting same-n errors at more than one rational surface requires more correction coil sets