Minimal fluorous tagging strategy that enables the synthesis of the complete stereoisomer library of SCH725674 macrolactones

Four mixtures of four fluorous-tagged quasiisomers have been synthesized, demixed, and detagged to make all 16 stereoisomers of the macrocyclic lactone natural product Sch725674. A new bare-minimum tagging pattern needs only two tags--one fluorous and one nonfluorous--to encode four isomers. The structure of Sch725674 is assigned as (5R,6S,8R,14R,E)-5,6,8-trihydroxy-14-pentyloxacyclotetradec-3-en-2-one. Various comparisons of spectra of 32 lactones (16 with tags, 16 without) and 16 ester precursors (8 with tags, 8 without) provide insights into when and why related compounds have the same or different spectra.     

Higgs production at the LHC: an update on cross sections and branching ratios

New theoretical and experimental information motivates a re-examination of the Standard Model Higgs production rates at the LHC pp collider. We present calculations of the relevant cross sections and branching ratios, including recently calculated QCD next-to-leading order corrections, new parton distributions fitted to recent HERA structure function data, and new values for electroweak input parameters, in particular for the top quark mass. Cross sections are calculated at two collider energies, √s = 10 TeV and 14 TeV.     

Carrier escape dynamics in a single quantum well waveguide modulator

Picosecond excite-probe studies are performed on a single quantum well waveguide modulator giving a direct measure of the escape of photogenerated carriers from a quantum well. Both the effects of exciton saturation and external field screening are observed in the transient transmission change. The results are consistent with the escape of carriers by thermionic emission.     

Photorefractive properties of ion-implanted waveguides in strontium barium niobate crystals

0.61 Ba_0.39Nb₂O₆, SBN61), either by proton or helium ion implantation. Proton-implanted samples show a large increase of dark conductivity that reduces or even prevents the recording of refractive index gratings. For waveguides formed by helium implantation this effect is absent, and they can be used for efficient holographic recording. Photorefractive properties of the waveguides are investigated by two-beam coupling. After implantation with 2.0 MeV He⁺ and doses of (0.5-5)×10¹⁵ cm^-2, the samples have to be polarized again, because heating or charge effects at the crystals surface during the implantation process decreases or even reverses the effective electrooptic coefficients in the waveguiding layer. For repoled samples, we find logarithmic gain coefficients of up to 45 cm^-1 with time constants for the build-up of the purely π/2-shifted refractive index grating of the order of 1 ms for the blue lines of an Ar⁺ laser. Photoconductivity depends nonlinearly on light intensity with an exponent x≈0.55. With increasing implanted helium dose, both electronic and nuclear damage of the waveguiding layer grows, and the photorefractive properties of the waveguides are considerably degraded. Received: 20 February 1997/Revised version: 1 May 1997     

Experimental investigation of13CD3OH infrared transitions by means of optoacoustic spectroscopy

A systematic investigation of¹³CD₃OH infrared transitions around CO₂ laser line emissions is performed by means of optoacoustic detection. Using a waveguide CO₂ laser of 290 MHz tunability and, in some cases, an acoustooptic modulator to provide an extension of 180 MHz, 108 transitions are observed. Measurements of pump off-set and relative intensity have been performed. The data may be compared with those previously reported of high resolution Fourier Transform spectroscopy.Work supported by: FAPESP, CNPq, FAEP — Brasil CNR — ItaliaPermanent address: Depto de Ciências, FEIS-UNESP (15378) I.Solteira-SP, Brasil     

e^ + e^ - \to b\bar bW^ + W^ - events at the next linear collider: colour structure of top signal and irreducible backgroundevents at the next linear collider: colour structure of top signal and irreducible background

We examine the colour structure and charged particle yield for both the $t\bar t$ signal and the irreducible background processes contributing to $e^ + e^ - \to b\bar bW^ + W^ - $ production close to the $t\bar t$ threshold. The charged particle multiplicity for the various components of the cross section is computed as a function of several kinematic variables. Our study may have important implications for recently proposed studies of interconnection phenomena in $t\bar t$ production at high-energye ⁺ e ^? colliders.     

New large offset far-infrared laser lines from CD3OH

The CD₃OH molecule has been investigated for new far-infrared laser lines by optically pumping with a cw waveguide CO₂ laser. The increased tunability (300 MHz) with respect to a conventional CO₂ laser permits to pump many new CD₃OH lines. As a consequence 108 new laser lines have been discovered, ranging from 42.9 to 1155 m in wavelength. On some lines the effect of an electric Stark field has been investigated demonstrating a laser frequency tuning. The total number of known FIR laser lines from CD₃OH is increased to about 340 making this molecule the most prolific together with CH₃OH.     

Study of the FeII ion role in the magnetic properties of the AFM ring Cr7Fe using Mössbauer spectroscopy

In recent years the synthesis of antiferromagnetic rings traced a path to the observation of interesting quantum coherence phenomena in heterometallic Single Molecule Magnets. Because of the presence of different magnetic centers, it is crucial to understand the distribution of the molecular spin on all the sites. ⁵⁷Fe Mössbauer spectroscopy is an efficient and powerful technique to contribute to this knowledge in antiferromagnetic wheels containing iron ions. We analyze the Cr₇Fe^II wheel: the Mössbauer line shape evolution of low temperature spectra (2.1 K) at different external fields is illustrated and the mean spin value of the iron ion is evaluated.