Nonequivalence of spatial shifts and Wigner delays at interfaces

We isolate spatial shifts and Wigner delays for reflection at the same interface and demonstrate that they can carry different information. The spatial shifts associated with Wood anomalies on gratings can be either positive or negative, while the corresponding delays are both positive. In the standard case of total reflection at a glass-air interface, a differential two-photon absorption correlation technique allows us to measure for the first time a delay of up to 35 fs in agreement with the associated 10.5 microm spatial shift. The method also allows us to isolate giant Wigner delays. The existence of similar delays in different areas of physics is discussed.     

Study of the attachment of Na+ on glucose and on some of its methylated derivatives

The mechanism of attachment of Na(+) on glucose, methyl-alpha-D-glucose, methyl-beta-D-glucose, 3-O-methylglucose, tetra-O-methylglucose, and also on galactose and methyl-beta-D-galactose, was studied. For this we measured the ion yields for the complex [sugar-Na(+)] formed by ionisation by matrix-assisted laser desorption/ionisation (MALDI) and ionspray. These data were compared with the relative volatilities and hydrophobicities of the sugars, measured by evaporative light scattering and reversed-phase liquid chromatography, respectively. Some formation enthalpies for the complexes [sugar-Na(+)], starting from the sugar and the cation, were obtained by ab initio calculations. No simple correlations could be observed between the ion yields and the parameters studied, so that the cationisation mechanism of the sugars remains unclear.     

Elliptic flow in Au+Au collisions at square root(S)NN = 130 GeV

Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at square root(S)NN = 130 GeV using the STAR Time Projection Chamber at the Relativistic Heavy Ion Collider. The elliptic flow signal, v2, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.     

Measurement of positive and negative Goos--Hänchen effects for metallic gratings near Wood anomalies

Large Goos-H?nchen effects are isolated for reflection on a metallic grating. These shifts occur in the vicinity of Wood anomalies. Depending on the nature of the anomaly, these displacements are found to be either positive or, contrary to the usual GH effect, clearly negative. Those shifts, associated with forward and backward leaky surface waves, are as large as plus or minus tens of wavelengths for a classic metallic grating.     

Quantification and improvement of the signal-to-noise ratio in a magnetic resonance image acquisition procedure

A procedure is developed to quantify and improve the signal-to-noise ratio (SNR) of magnetic resonance images. The image SNR is quantified using the correlation function of two independent acquisitions of an image. To test the performance of the quantification, SNR measurement data are fitted to theoretically expected curves. The proposed correlation technique is also used to improve the SNR by estimating the amplitude of the signal spectrum. The technique is applied to a set of MR images, and its performance in terms of gain in SNR, contrast-to-noise ratio (CNR), and resolution loss is compared to that of classical noise filters. The SNR as well as the CNR is improved significantly with minor loss of resolution. Finally, it is shown that the correlation technique can be implemented in a highly efficient way in almost any acquisition procedure of a magnetic resonance imaging system.     

Semiconducting properties and band structure of polycrystalline Bi4Mo20O62 compounds

Transport coefficient measurements (electrical conductivity, thermoelectric power and Hall effect) have been performed on compacted bars of Bi₄Mo₂₀O₆₂ in polycrystalline form over the temperature range (130–500 K) Experimental results are discussed in comparison to those obtained recently on Sb₄Mo₂₀O₆₂ They are interpreted on the basis of the same _p-type semiconductor model with two inverted deep levels near the midgap Conduction mechanisms are governed by acoustical-phonon scattering of the carrriers The top of the valence band is assumed to be formed from the d_xy orbitais of some of the Mo atoms leading to narrow bonding bands, while the donor and acceptor levels may be formed from the nonbonding d_xy orbitais of some of the Mo atoms of the distorted octahedron framework EPR measurements are discussed on the basis of this model and in comparison to the EPR results for Sb₄MO₂₀O₆₂ It is assumed that more than one narrow energy level (localized d states located below the valence band) contributes to the area of the line The XPS spectrum obtained on Bi₄Mo₂₀O₆₂ is also discussed.     

Temperature dependence of hypersonic attenuation in NaCl and KCl crystals

The usual theories of ultrasonic attenuation (Landau-Rumer and Akhiezer) are in disagreement with experiments involving hypersonic phonons (Brillouin scattering). This is shown here by measurements on NaCl and KCl. We observe a Tⁿ dependence with n approximately equal to 2.8, and an attenuation increasing with temperature in the low and high temperature range respectively. An extension of the Landau-Rumer and Akhiezer theories to hypersonic frequencies is proposed. Our theoretical expressions give a good account of the temperature dependence of the attenuation in the whole temperature range.     

An enantioselective synthesis of a 4-fluoromethyl-azetidinone

(3S,4S) 3-amino-4-fluoromethyl-azetidinone 9 is obtained conveniently from (R) N-(2-fluoroethylidene)-α-methylbenzylamine via cycloaddition.