Calculation and Measurement of Terahertz Active Normal Modes in Crystalline PETN

The terahertz frequency spectrum of pentaerythritol tetranitrate (PETN) is calculated using Discover 1 with the COMPASS 2 force field, CASTEP^[3] and PWscf. 4 The calculations are compared to each other and to terahertz spectra (0.3–3 THz) of crystalline PETN recorded at 4 K. A number of analysis methods are used to characterise the calculated normal modes.     

Photobleaching on Photonic Crystal Enhanced Fluorescence Surfaces

The effect of resonant fluorescent enhancement from a photonic crystal surface upon the fluorescent photobleaching rate of Cyanine-5 labeled protein has been investigated. We show that the enhanced excitation mechanism for photonic crystal enhanced fluorescence, in which the device surface resonantly couples light from an excitation laser, accelerates photobleaching in proportion to the coupling efficiency of the laser to the photonic crystal. We also show that the enhanced extraction mechanism, in which the photonic crystal directs emitted photons approximately normal to the surface, does not play a role in the rate of photobleaching. We show that the photobleaching rate of dye molecules on the photonic crystal surface is accelerated by 30x compared to an ordinary glass surface, but substantial signal gain is still evident, even after extended periods of continuous illumination at the resonant condition.     

DTI at 7 and 3 T: systematic comparison of SNR and its influence on quantitative metrics

Diffusion tensor imaging (DTI) and advanced related methods such as diffusion spectrum and kurtosis imaging are limited by low signal-to-noise ratio (SNR) at conventional field strengths. DTI at 7 T can provide increased SNR; however, B0 and B1 inhomogeneity and shorter T2? still pose formidable challenges. The purpose of this study was to quantify and compare SNR at 7 and 3 T for different parallel imaging reduction factors, R, and TE, and to evaluate SNRs influences on fractional anisotropy (FA) and apparent diffusion coefficient (ADC). We found that R>4 at 7 T and R≥2 at 3 T were needed to reduce geometric distortions due to B0 inhomogeneity. For these R at 7 T, SNR was 70-90 for b=0 s/mm² and 22-28 for b=1000s/mm² in central brain regions. SNR was lower at 3 T (40 for b=0 s/mm² and 15 for b=1000 s/mm²) and in lateral brain regions at 7 T due to B1 inhomogeneity. FA and ADC did not change with MRI field strength, SENSE factor or TE in the tested range. However, the coefficient of variation for FA increased for SNR <15 and for SNR <10 in ADC, consistent with published theoretical studies. Our study demonstrates that 7 T is advantageous for DTI and lays the groundwork for further development. Foremost, future work should further address challenges with B0 and B1 inhomogeneity to take full advantage for the increased SNR at 7 T.     

A covering lemma for L(ℝ)

Jensen's celebrated Covering Lemma states that if 0^# does not exist, then for any uncountable set of ordinals X, there is a Y∈L such that X⊆Y and |X| = |Y|. Working in ZF + AD alone, we establish the following analog: If ℝ^# does not exist, then L(ℝ) and V have exactly the same sets of reals and for any set of ordinals X with |X| ≥Θ ^{L (ℝ)}, there is a Y∈L(ℝ) such that X⊆Y and |X| = |Y|. Here ℝ is the set of reals and Θ is the supremum of the ordinals which are the surjective image of ℝ. Received: 29 October 1999 / Published online: 12 December 2001     

Internal-strain measurements of longitudinal pulses in conical bars

Longitudinal stress waves in a truncated 20-deg solid cone were investigated using embedded semiconductor strain gages. The cone, composed of an aluminum-filled epoxy, was struck normally at its small end with a 1/2-in.-diam steel ball traveling at a velocity of 170 jps. The results show the magnitude of the resulting stress wave to be nonuniform over a plane cross section perpendicular to the cone axis, the strain being greater at the center of the cone than near the surface, and the nonuniformity to increase with distance of travel from the impact end. The surface-strain measurements were compared with the one-dimensional theory of longitudinal waves in cones developed by Landon and Quinney as solved by Kenner and Goldsmith for a onehalf cycle sine-squared input pulse, and found to be in qualitative agreement with this theory, but to vary significantly in strain magnitude due to the strain nonuniformity over plane cross sections. The nonuniformity was compared with the Pochhammer-Chree theory for stress waves in cylindrical bars when that theory was evaluated for a cross section equivalent to the cone cross section. The trends of the deviations were similar, but the variations measured in the cone were consistently greater than that predicted by the theory.     

Synthesis of polymeric microspheres from a Merrifield resin by surface‐initiated nitroxide‐mediated radical polymerization

Polymeric microspheres were prepared from a Merrifield resin via nitroxide‐mediated radical polymerization. Polystyrene, poly(acetoxystyrene), and poly[styrene‐b‐(methyl methacrylate‐co‐styrene)], poly(acetoxystyrene‐b‐styrene), and poly(styrene‐co‐2‐hydroxyethyl methacrylate) copolymers were demonstrated to graft onto 2,2,6,6‐tetramethyl‐1‐piperidinyloxy nitroxide bound Merrifield resins. The polymerization control was enhanced both on the surface and in solution by the addition of sacrificial nitroxide. The significant increase in the particle diameter (more than a fivefold volume increase for polystyrene brushes) showed that polymer growth was not only on the surface but also within the particles, and this diameter increase could be adjusted through changes in the molecular weight of the polymers. The microspheres were characterized by elemental analysis, IR spectroscopy, particle size analysis, and optical microscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2145–2154, 2005     

Transference numbers for anhydrous methanol solutions at 10 and 25°C

Transference numbers are reported for LiCl and NaCl in methanol at 25°C and for NaCl, KCl, and Bu₄NBr in methanol at 10°C. The potentiometric moving-boundary method as developed by Kay and Fratiello was employed to give a precision of about 0.05% and an accuracy of at least 0.1% as indicated by two independent determinations of the conductances of the Cl^– and Br^– ions. The data are extrapolated by the Fuoss-Onsager theory, and the magnitude of the electrophoretic effect is calculated as described by Kay and Dye. The agreement with this theory is quite good at both temperatures, although the å value required in the case of Bu₄NBr is considerably larger than that obtained from conductance data. This agreement contrasts with that obtained for ethanol and acetone solutions where the measured electrophoretic effect is considerably larger than the corresponding calculated values. The importance of this fact in the determination of ion-pair association constants is discussed.     

Vanadium- or vanadylacetylacetonate as a cocatalyst for the terpolymerization of ethylene,propylene, and dicyclopentadiene

Vanadium trisacetylacetonate [V(C₅H₇O₂)₃] and vanadyl bisacetylacetonate [VO-(C₅H₇O₂)₂] were found to be satisfactory catalysts (with Al₂Et₃Cl₃ cocatalyst) for the terpolymerization of ethylene, propylene, and dicyclopentadiene to unsaturated, sulfurcurable elastomers. Polymerization solvents of heptane or benzene were used. Best yields of terpolymers were obtained in benzene. Terpolymers with unsaturations of greater than ?0.20 mole C?C/kg. can be cured with a sulfur-based vulcanzing recipe. Both acetylacetonates produced terpolymers, in benzene, with practically equivalent properties. They also appeared to be nearly equal to corresponding terpolymers made with catalysts of VOCl₃ or VCl₄.