Selective deoxygenation of leurosine: concise access to anhydrovinblastine

Straightforward access to anhydrovinblastine starting from the parent alkaloid leurosine is reported. The key deoxygenation step was first optimized on a model substrate. However, applied to leurosine, only the low-valent Cp2TiCl gave satisfactory results.     

Demonstration of a Ni-like Kr optical-field-ionization collisional soft x-ray laser at 32.8 nm

We report the first experimental demonstration of a Ni-like optical-field ionization collisional soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 760 mJ, 30 fs, 10-Hz Ti:sapphire laser beam in a few mm cell filled with krypton. We have measured a gain coefficient of 78 cm(-1) on the 3d(9)4d 1S0-3d(9)4p(1)P1 transition at 32.8 nm, which is here amplified for the first time. This radiation source represents the shortest wavelength optical-field ionization collisional soft x-ray laser ever produced. The influence of the gas pressure and the pumping energy on the lasing output are also presented.     

Description of pulse propagation in a dispersive medium by use of a pulse quality factor

We investigated how the duration of short laser pulses evolves in a dispersive material, using rms widths and a propagation law based on a pulse quality factor. Experiments were carried out with femtosecond pulses (10 to 25fs at the temporal waist) propagating in bulk fused silica. Excellent agreement was found between theory and experiment. This approach does not require complete characterization of laser pulses and eliminates the need for any assumption regarding the interpretation of autocorrelation traces. The method is of general validity, and it can be applied to pulses of arbitrary shape.     

Consequences of affinity in heterogeneous catalytic reactions: highly chemoselective hydrogenolysis of iodoarenes

The catalytic hydrodeiodination reaction using molecular hydrogen and Pd/C has been revisited. It is shown, for the first time, that the chemoselectivity of this reaction is controlled by the high affinity of the iodinated compound for the catalyst. This reaction is compatible with most easily reducible functional groups (nitro, aldehyde, olefin, etc.). Using this reaction, the first general method for tritium labeling of 3-(trifluoromethyl)-3-phenyldiazirine is described.     

Clustering for routing in densely populated areas

This paper introduces a new approach for generating school bus routes in a dense urban area. First, a districting algorithm is used to determine clusters including appropriate numbers of students. Then, for each cluster, a route and the stops along this route are determined. Numerical results are reported and compared with those obtained previously. Although the algorithm has been developped and tested in a specific context, it could easily be extended to more general vehicle routing problems.     

Co stretching vibration of carbon monoxide adsorbed on nickel(111) studied by high resolution electron loss spectroscopy

The frequency of the v-CO stretching vibration measured by HRELS has been followed as a function of the CO coverage and in the presence of coadsorbed hydrocarbons on the Ni(111) face. The v-CO frequency shifts continuously from 225 meV (1814 cm^?1) to 237 meV (1911 cm^?1) when the CO coverage increases from 0 to 0.41. Coadsorption of electron donor molecules, such as ethylene and benzene, generates a significant lowering of the v-CO frequency. Results are discussed in terms of the back donation of metallic electrons into the 2π ¹ antibonding orbitals of CO, the dipole-dipole coupling and the coordination number of the CO adsorbed molecules. The back donation is found to play the major role in the range of coverage explored but we cannot exclude some contribution of a dipole-dipole coupling effect.     

On Sobolev infinitesimal rigidity of linear hyperbolic actions on the 2-torus*

Let A be a symmetric hyperbolic matrix in SL(2, ℤ) and Γ the subgroup of SL(2, ℤ) generated by A. We aim to study the infinitesimal rigidity of the standard action of Γ on the torus . More precisely, we will consider the Sobolev W^s–infinitesimal rigidity of this action (that is to determine if the cohomology space H¹(Γ,W^s (T M)) is trivial or not), and show that it is W^s–infinitesimally rigid only if 0 ≤ s < 1. A consequence will be that this action is not C^∞–infinitesimally rigid. *I would like to thank A. El Kacimi for introducing me this problem about which we had many fruitful discussions.     

High-resolution gamma-ray radiography produced by a laser-plasma driven electron source

An electron beam from a laser-plasma accelerator is converted into a gamma-ray source using bremsstrahlung radiation in a dense material. The gamma-ray beam has a pointlike source size because it is generated by a high quality electron beam with a small source size and a low divergence. Using this gamma-ray source, the radiography of complex and dense objects with submillimeter resolution is performed. It is the first evidence of a gamma-ray source size of a few hundreds micrometers produced with laser-driven accelerators. This size is consistent with results from Monte Carlo simulations.     

Phase separation in the two-dimensional bosonic Hubbard model with ring exchange

We show that soft-core bosons in two dimensions with a ring exchange term exhibit a tendency for phase separation. This observation suggests that the thermodynamic stability of normal Bose liquid phases driven by ring exchange should be carefully examined.