Polarization-independent photochromic diffraction in a dye-doped liquid crystal

We report the observation of polarization-independent photochromic diffraction in an azo-dye-doped liquid crystal. The generation of the phase grating is more than 90% independent of the polarization of the writing beams, and the diffraction by the phase grating is more than 90% independent of the polarization of the probe beam. Unpolarized lamp light was also used to generate real-time phase gratings and self-diffraction. For the first time to our knowledge, photochromic phase modulation and light diffraction that exhibit more than 90% polarization independence for both writing and probe beams were produced in an anisotropic liquid-crystalline material.     

Imaging the surface and the interface atoms of an oxide film on Ag111 by scanning tunneling microscopy: experiment and theory

High-resolution images of the vacuum oxide surface atomic structure of Ag111-p(4x4)-O reveal large terraces of a perfect (4x4) reconstruction. Under certain conditions, the surface Ag atoms at the interface between the reconstructed oxide layer and the underlying Ag111 lattice are also imaged, providing the structural registry. Scanning tunneling microscopy simulations reveal a strong sensitivity to structure and comparison with the experimental images, therefore providing an atom-by-atom model for the entire metal-oxide-vacuum structure.     

Mechanical properties of ultrananocrystalline diamond thin films relevant to MEMS/NEMS devices

The mechanical properties of ultrananocrystalline diamond (UNCD) thin films were measured using microcantilever deflection and membrane deflection techniques. Bending tests on several free-standing UNCD cantilevers, 0.5 μm thick, 20 μm wide and 80 μm long, yielded elastic modulus values of 916–959 GPa. The tests showed good reproducibility by repeated testing on the same cantilever and by testing several cantilevers of different lengths. The largest source of error in the method was accurate measurement of film thickness. Elastic modulus measurements performed with the novel membrane deflection experiment (MDE), developed by Espinosa and co-workers, gave results similar to those from the microcantilever-based tests. Tests were performed on UNCD specimens grown by both micro and nano wafer-seeding techniques. The elastic modulus was measured to be between 930–970 GPa for the microseeding and between 945–963 GPa for the nanoseeding technique. The MDE test also provided the fracture strength, which for UNCD was found to vary from 0.89 to 2.42 GPa for the microseeded samples and from 3.95 to 5.03 for the nanoseeded samples. The narrowing of the elastic modulus variation and major increase in fracture strength is believed to result from a reduction in surface roughness, less stress concentration, when employing the nanoseeding technique. Although both methods yielded reliable values of elastic modulus, the MDE was found to be more versatile since it yielded additional information about the structure and material properties, such as strength and initial stress state.     

Stereoselective α,α'-annelation reactions of 1,3-dioxan-5-ones

Pyrrolidine enamines derived from three 1,3-dioxan-5-ones undergo α,α'-annelation reactions with methyl α-(bromomethyl)acrylate to produce bridged 2,4-dioxabicyclo[3.3.1]nonane ring systems with complete stereocontrol. Stereochemical outcomes have been rationalized based on steric and stereoelectronic interactions in intermediate boat-like conformations of the 1,3-dioxane ring and subsequent kinetic protonation to set an axial ester group on the cyclohexanone ring. Base-mediated ester epimerization provides the stereochemical array found in the highly oxygenated cyclohexane ring of phyllaemblic acid and glochicoccins B and D.     

Thermodynamically controlled cyclisation reactions with double phenylsulfanyl migration

Enantiomerically enriched C2-symmetric tetrols were synthesised by a route involving a 'self-metathesis' reaction with Grubbs' second-generation ruthenium catalyst; these tetrols produced interesting bicyclic products when rearranged under acidic conditions.     

Oceanographic inverse problems

Oceanographic inverse problems are generalizations of the concept of linear regression to encompass the fitting of numerical models based on the partial-diffential equations of fluid dynamics to oceanic data by adjusting surface fluxes, initial conditions, and/or transport parameters. The limited resolution of the data requires some sort of regularizing assumptions similar to those needed for spatial interpolation. Computationally, they are nonlinear-least-squares problems involving very many variables. These problems can be ill-conditioned due to the indirect nature of the observations, to the way the dynamics enter, and to the effect of the regularizing assumptions. Solving such large optimization problems requires efficient computation of derivatives of functions defined by computational codes. The adjoint method provides derivatives with respect to all the code's many adjustable inputs for roughly the same cost as computing a single model simulation. Although computationally difficult, model fitting offers the promise of dynamically consistent multivariate analyses of oceanographic data.     

Crystal structure of methyl β-D-hamameloside

The crystal structure of methyl -D-hamameloside has been determined by direct methods. The trigonal crystals have the space groupR3. Referred to hexagonal axes, there are nine molecules in the non-primitive unit cell witha =b = 18·08(2),c = 7·22(2)Å. The molecule exists in the 1C-trans-chair conformation, with the mean C-C and C-O bond lengths being 1·531 and 1·428Å respectively. The mean value for the carbon bond angle is 110·2 °. The crystal structure is composed of columns of molecules hydrogen-bonded about 3₂ axes, and with van der Waals forces holding these columns together around the 3₁ and 3-fold axes.