Broadband terahertz ultrasonic transducer based on a laser-driven piezoelectric semiconductor superlattice

Spectral characteristics of laser-generated acoustic waves in an InGaN/GaN superlattice structure are studied at room temperature. Acoustic vibrations in the structure are excited with a femtosecond laser pulse and detected via transmission of a delayed probe pulse. Seven acoustic modes of the superlattice are detected, with frequencies spanning a range from 0.36 to 2.5 THz. Acoustic waves up to ∼2 THz in frequency are not significantly attenuated within the transducer which indicates excellent interface quality of the superlattice. The findings hold promise for broadband THz acoustic spectroscopy.     

Hub-and-spoke network design and fleet deployment for string planning of liner shipping

All shipping liner companies divide their service regions into several rotations (strings) in order to operate their container vessels. A string is the ordered set of ports at which a container vessel will call. Each port is usually called at no more than twice along one string, although a single port may be called at several times on different strings. The size of string dictates the number of vessels required to offer a given frequency of service. In order to better use their shipping capacity, groups of Liner Service Providers sometimes make a short term agreement to merge some of their service routes (in a certain region) into one main ocean going rotation and p feeder rotations. In order to minimize the weighted sum of transit time, and fixed deployment costs, this paper proposes a mixed integer linear programming model of the network design, and an allocation of proper capacity size and frequency setting for every rotation. Given that none of the existing general-purpose MIP solvers is able to solve even very small problem instances in a reasonable time, we propose a Lagrangian decomposition approach which uses a heuristic procedure and is capable of obtaining practical and high quality solutions in reasonable times. The model will be applied on a real example, and we shall present some of the results obtained by our model which show how it facilitates a better use of assets and a significant reduction in the use of fuel, therefore allowing a more environmentally friendly service.     

State-resolved velocity map imaging of surface-scattered molecular flux

This work describes a novel surface-scattering technique which combines resonance enhanced multiphoton ionization (REMPI) with velocity-map imaging (VMI) to yield quantum-state and 2D velocity component resolved distributions in the scattered molecular flux. As an initial test system, we explore hyperthermal scattering (E(inc) = 21(5) kcal mol(-1)) of jet cooled HCl from Au(111) on atomically flat mica surfaces at 500 K. The resulting images reveal 2D (v(in-plane) and v(out-of-plane)) velocity distributions dominated by two primary features: trapping/thermal-desorption (TD) and a hyperthermal, impulsively scattering (IS) distribution. In particular, the IS component is strongly forward scattered and largely resolved in the velocity map images, which allows us to probe correlations between rotational and translational degrees of freedom in the IS flux without any model dependent deconvolution from the TD fraction. These correlations reveal that HCl molecules which have undergone a large decrease in velocity parallel to scattering plane have actually gained the most rotational energy, reminiscent of a dynamical energy constraint between these two degrees of freedom. The data are reduced to a rotational energy map that correlates with velocity along and normal to the scattering plane, revealing that exchange occurs primarily between rotation and the in-plane kinetic energy component, with v(out-of-plane) playing a relatively minor role.     

Tethered particle motion as a diagnostic of DNA tether length

The tethered particle motion (TPM) technique involves an analysis of the Brownian motion of a bead tethered to a slide by a single DNA molecule. We describe an improved experimental protocol with which to form the tethers, an algorithm for analyzing bead motion visualized using differential interference contrast microscopy, and a physical model with which we have successfully simulated such DNA tethers. Both experiment and theory show that the statistics of the bead motion are quite different from those of a free semiflexible polymer. Our experimental data for chain extension versus tether length fit our model over a range of tether lengths from 109 to 3477 base pairs, using a value for the DNA persistence length that is consistent with those obtained under similar solution conditions by other methods. Moreover, we present the first experimental determination of the full probability distribution function of bead displacements and find excellent agreement with our theoretical prediction. Our results show that TPM is a useful tool for monitoring large conformational changes such as DNA looping.     

Potassium hydride in paraffin: a useful base for organic synthesis

The preparation of potassium hydride as a 1:1 homogenate with paraffin, termed KH(P), is reported. KH(P), a solid at room temperature, is stable without special handling. On suspension in THF with a phosphonium salt, KH(P) rapidly generates the ylide. Wittig condensation with aromatic, aliphatic, and alpha,beta-unsaturated aldehydes proceeds with high Z selectivity. KH(P) should be a generally useful base for organic synthesis.     

Cross-reactive conjugated polymers: analyte-specific aggregative response for structurally similar diamines

A single cross-reactive conjugated polymer (poly(thiophene) 1) generates unique spectral patterns in response to structurally similar diamines. Multivariate statistics are used to deconvolute subtle variations in these spectral responses, allowing for identification and quantification of the analytes with >99% accuracy.     

Catalytic asymmetric assembly of stereodefined propionate units: an enantioselective total synthesis of (-)-pironetin

Double diastereoselection in alkaloid-catalyzed acyl halide-aldehyde cyclocondensation (AAC) reactions provides a strategy for realizing syn- or anti-selective propionate aldol additions from a common reaction manifold. Matched AAC homologation of enantioenriched aldehydes afford cis-disubstituted beta-lactones as surrogates for syn aldols; the mismatched AAC reactions provide anti-selective aldols in the form of trans-disubstituted 2-oxetanones. The utility of this reaction technology in synthesis activities is exemplified in a catalytic asymmetric total synthesis of (-)-pironetin.     

Extraction and determination of elemental selenium in sediments--a comparative study

This paper proposes a new technique to extract elemental Se from soil and sediment samples. In this study, we have identified that the purchased red elemental selenium standard (PF-Se) was impure and rather consisted of a mixture of CS₂ soluble amorphous elemental Se (ca. 10%, w/w), water soluble oxidized Se (ca. 15-17%, w/w) and, CS₂ insoluble red monoclinic elemental Se. In more recent studies, a slow oxidation and a mineral phase transition of this sample was also observed. The solubility of the amorphous elemental Se in CS₂ was at least 0.64 mg L⁻¹. The black elemental Se purchased from Sigma-Aldrich had a much lower solubility in CS₂ (7.2 μg mL⁻¹) compared to that given in the literature. Any selenium compounds with electrical charge and polar nature is insoluble in CS₂. In a sodium sulphite solution, PF-Se was completely dissolved thus giving a clear indication of the lack of selectivity in that extraction system. Other comparative studies also demonstrated that over extraction did occur with the Na₂SO₃ method. Compared to Na₂SO₃, CS₂ extraction of elemental Se is not only much simpler, straightforward and with higher analytical precision, but also much more selective and accurate. With HG-AFS, the detection limit can reach as low as 1.0 ng g⁻¹ in sediment sample owing to a low reagent blank of CS₂ solvent.     

Characterization of amino acids in silk sericin protein from Gonometa rufobrunnae by MEKC with phenyl isothiocyanate derivatization

An MEKC method was developed for the separation and characterization of phenyl-isothiocyanate (PITC)-labeled amino acids derived from Gonometa rufobrunnae silkworm after microdialysis sample cleanup. The influence of the buffer and SDS concentration on the resolution of the amino acids was investigated. A buffer system consisting of 25 mM phosphate, 10 mM borate buffer at pH 9.00, and 70 mM SDS showed the best results, with 13 PITC-amino acid derivatives being resolved out of 15 possible amino acids that were under study. Microdialysis sampling demonstrated its efficiency as a sample cleanup technique. Sericin protein from G. rufobrunnae was found to be characterized by at least 11 positively identified amino acids. These included His, Tyr, Ser, Ala, Phe, Lys, Gly, Arg, Cys, Glu, and Asp. Leu/Met and Val/Thr were coeluting pairs and hence could not be positively confirmed.