An ultrasonic transducer array for velocity measurement in underwater vehicles

A correlation velocity log (CVL) is an ultrasonic navigation aid for marine applications, in which velocity is estimated using an acoustic transmitter and a receiver array. CVLs offer advantages over Doppler velocity logs (DVLs) in many autonomous underwater vehicle (AUV) applications, since they can achieve high accuracy at low velocities even during hover manoeuvres. DVLs require narrow beam widths, whilst ideal CVL transmitters have wide beam widths. This gives CVLs the potential to use lower frequencies thus permitting operation in deeper water, reducing power requirements for the same depth, or allowing the use of smaller transducers. Moving patterns in the wavefronts across a 2D receiver array are detected by calculating correlation coefficients between bottom reflections from consecutive transmitted pulses, across all combinations of receiver pairings. The position of the peak correlation value, on a surface representing receiver-pairing separations, is proportional to the vessel's displacement between pulses. A CVL aimed primarily for AUVs has been developed. Its acoustical and signal processing design has been optimised through sea trials and computer modelling of the sound field. This computer model is also used to predict how the distribution of the correlation coefficients varies with distance from the peak position. Current work seeks to increase the resolution of the peak estimate using surface fitting methods. Numerical simulations suggest that peak estimation methods significantly improve system precision when compared with simply identifying the position of the maximum correlation coefficient in the dataset. The peak position may be estimated by fitting a quadratic model to the measured data using least squares or maximum likelihood estimation. Alternatively, radial basis functions and Gaussian processes successfully predict the peak position despite variation between individual correlation datasets. This paper summarises the CVL's main acoustical features and signal processing techniques and includes results of sea trials using the device.     

Driving rate effects on crackling noise

Many systems respond to slowly changing external conditions with crackling noise, created by avalanches or pulses with a broad range of sizes. Examples range from Barkhausen noise (BN) in magnets to earthquakes. In this Letter, we discuss the effects of increasing driving rate Omega on the scaling behavior of the avalanche size and duration distributions as well as qualitative effects of Omega on the power spectra. We derive an exponent inequality as a criteria for the relevance of Omega. To illustrate these general results, we use recent experiments on BN as a successful example.     

Tunable single-frequency ultraviolet generation from a continuous-wave Ti : sapphire laser with an intracavity PPLN frequency doubler

We report a single-longitudinal-mode continuous-wave Ti:sapphire ring laser that is intracavity frequency doubled in periodically poled lithium niobate (PPLN). Using a multi-grating PPLN crystal, the second harmonic is tunable from 386 to 403 nm – close to the PPLN ultraviolet absorption edge – with a maximum generated power of 114 mW. The wavelength dependence of the critical operating temperature required to eliminate photorefractive effects in the PPLN crystal is determined. PACS 42.65.Ky; 42.70.Mp; 42.72.Bj     

Design of highly active iron-based catalysts for atom transfer radical polymerization: tridentate salicylaldiminato ligands affording near ideal Nernstian behavior

Iron(II) complexes bearing monoanionic tridentate salicyladiminato ligands are shown to be highly efficient catalysts for atom transfer radical polymerization (ATRP). Polymerization rates for styrene are among the highest reported for iron-mediated ATRP in nonpolar media, correlating well with E1/2 potentials and DeltaEp values for the complexes. The rigidity of the tridentate ligands, combined with ample space around the metal center to accommodate a halogen atom, we believe to be an important factor in the efficient ATRP behavior of these systems.     

Demonstration of a simple entangling optical gate and its use in bell-state analysis

We demonstrate a new architecture for an optical entangling gate that is significantly simpler than previous realizations, using partially polarizing beam splitters so that only a single optical mode-matching condition is required. We demonstrate operation of a controlled-z gate in both continuous-wave and pulsed regimes of operation, fully characterizing it in each case using quantum process tomography. We also demonstrate a fully resolving, nondeterministic optical Bell-state analyzer based on this controlled-z gate. This new architecture is ideally suited to guided optics implementations of optical gates.     

Search for the lepton-number-violating decay Xi(-)-->pmu(-)mu(-)

A sensitive search for the lepton-number-violating decay Xi(-)-->pmu(-)mu(-) has been performed using a sample of approximately 10(9) Xi(-) hyperons produced in 800 GeV/c p-Cu collisions. We obtain B(Xi(-)-->pmu(-)mu(-))<4.0x10(-8) at 90% confidence, improving on the best previous limit by 4 orders of magnitude.     

Terahertz wide aperture reflection tomography

We describe a powerful imaging modality for terahertz (THz) radiation, THz wide aperture reflection tomography (WART). Edge maps of an object's cross section are reconstructed from a series of time-domain reflection measurements at different viewing angles. Each measurement corresponds to a parallel line projection of the object's cross section. The filtered backprojection algorithm is applied to recover the image from the projection data. To our knowledge, this is the first demonstration of a reflection computed tomography technique using electromagnetic waves. We demonstrate the capabilities of THz WART by imaging the cross sections of two test objects.     

Ultrasound-assisted hydrogenation of cinnamaldehyde

The hydrogenation, employing hydrogen gas, of cinnamaldehyde was performed using Pd-black and Raney Ni catalysts at 298+/-3 K in a water-cooled (jacketed) reaction vessel. Sampling at pre-determined time intervals and GC/MS analysis yielded time-dependent product state distribution information. A kinetic modeling of the data revealed that cinnamaldehyde was both hydrogenated directly to the final product benzenepropanol, as well as a fraction being converted to the intermediate benzenepropanal, where the latter was subsequently hydrogenated to benzenepropanol. Comparing the ultrasound-assisted and blank (stirred) experiments revealed that a higher maximum relative concentration of the intermediate benzenepropanal was formed in the ultrasound experiments compared to the stirred experiment. The activity of the ultrasound experiments compared to blank were 9-fold and 20-fold greater for the Pd-black and Raney Ni catalysts, respectively. Finally, an application of the Bell-Evans-Polanyi principle to yield an estimate of the ratio of rate coefficients for benzenepropanal and benzenepropanol formation was performed by considering chemical group energy differences and surface adsorption energy differences in the first mechanistic step of hydrogenation.     

Characterisation of derivatised monomeric and prepolymeric isocyanates by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry and structural elucidation by tandem mass spectrometry

Isocyanates are an important class of compounds in occupational hygiene monitoring due mainly to their behaviour as respiratory sensitisers. Here, we demonstrate the application of matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS) and MALDI tandem mass spectrometry (MS-MS) to the analysis of derivatised isocyanate monomers and prepolymers. The aim of the work has been to gauge the selectivity obtainable from the direct analysis of isocyanate mixtures without prior separation. Monomeric and prepolymeric isocyanate mixtures were analysed as their 1-(2-methoxyphenyl) piperazine derivatives and the potential of MALDI time-of-flight (ToF)-MS for an NCO monitoring program was assessed. The results obtained demonstrated the possibility of direct mixture analysis by this method. MALDI-MS-MS was used for the elucidation of fragment structures in the prepolymer samples. The developed methodology was then applied to the analysis of swabs from an occupational hygiene monitoring scheme and enabled the identification of the isocyanate species detected.     

A study of N-nitrosoamide-mediated friedel-crafts type benzylation of benzene-toluene and benzene-Anisole(1)(a)

Nitrogen-separated carbocation-carboxylate ion pairs were employed as sources of carbocations in the alkylation of aromatic compounds. The N-nitrosoamide approach to these nitrogenous-molecule-separated ion pairs is an excellent alternative to the standard acid-catalyzed Friedel-Crafts approach for studies of the alkylation because of the following variables: high reactivity of the electrophile, stability of the products, strict kinetic control, homogeneity, lack of overalkylation, straightforward chemistry, and good product balance. In deaminative benzylations of benzene-toluene and anisole-benzene mixtures, the values of k(X)/k(B) and % meta isomer are significantly different from those observed in the standard benzylations in a manner that indicates the deaminative electrophiles are more reactive than those generated by the standard Friedel-Crafts approach. The reactions show a direct proportionality between inter- and intramolecular selectivities and thus follow the Brown selectivity relationship (BSR). The benzylation of 2,4, 6-trideuteriotoluene provided no evidence for deuterium rearrangements (or coupled benzyl rearrangements) in the arenium ion intermediate. Kinetic isotope effects were not detected. The methyl substituent on toluene appears to affect intermolecular selectivity (k(T)/k(B)) and intramolecular selectivity (o, m, p distribution) to similar degrees. A mechanism is proposed in which both selectivities are determined by activation energy differences in the transition states leading to the sigma-complex intermediates. The observation that most standard Friedel-Crafts benzylations do not follow the BSR is discussed in terms of possible systematic errors in those cases. Silver ion-assisted Friedel-Crafts benzylations were performed under dry, basic conditions to investigate whether the standard approach could lead to data that obey the BSR.