Piezoelectrically stimulated dielectric dispersion in KAP single crystals

The frequency response of the dielectric constant (epsilon(r)), the loss tangent (tan delta) and impedance Z of potassium acid phthalate (KAP) single crystals, monitored along the polar axis, exhibit strong resonances in the frequency range 50-200 kHz, depending on the dimensions of the sample. The observed resonance effect, which is strongly dependent on the geometric shape and size of the sample, is attributed to its piezoelectric nature. The resonance peak positions have been monitored as a function of both temperature and uniaxial pressure. The stiffness coefficient (C), computed based on the resonance data, is found to decrease with increasing temperature and increase with increasing pressure. The electro-mechanical coupling coefficient (k), obtained by resonance-anti-resonance method, has also been found to increase with rise in temperature. The epsilon(r) behaviour along the polar axis, as a function of temperature is consistent with that of k. The preliminary results on the influence, of partial replacement of K+ ions in the KAP crystal by Cs+ and Li+ ions, on the observed piezoelectric resonance effects are also included.

Resonance de-enhancement in the 2(1)A(g) state of trans-azobenzene

We analyze the origin of de-enhancement for a number of vibrational modes in the 2(1)A(g) excited state of trans-azobenzene. We have used the time-dependent wave packet analysis of the RR intensities by including the multimode damping effects in the calculation. This avoids the use of unrealistically large values for the damping parameter. It is concluded that the de-enhancement is caused by the interference between the two uncoupled electronic states, and that the intensities observed under the so-called symmetry forbidden 2(1)A(g) <-- 1(1)A(g) transition are purely due to resonance excitation. It is also observed that the use of the time-dependent approach to study the de-enhancement effects caused by multiple electronic states on the RR intensities is not necessarily useful if one is interested in the structural dynamics.

Time-resolved resonance Raman spectroscopic studies on the triplet excited state of fluoranil

Perfluoro substituted organic compounds have attracted attention owing to their unique structure and reactivity induced by the perfluoro effect. Fluoranil, a perfluoro derivative of p-benzoquinone, is the subject of this paper. Although the perfluoro effect in the ground state seems to have been well understood there is no information available about such effects on the excited state. Here, the time-resolved resonance Raman spectra of the triplet excited state of fluoranil are reported along with the Raman excitation profiles (REPs) of the various vibrational modes. The vibrational spectral analyses have been carried out by analogy with the fluoranil ground state, triplet benzoquinone, and triplet chloranil vibrational spectral assignments. Also, the assignments are further supported by the calculated frequencies using ab initio theoretical methods. It is observed that for fluoranil in the triplet excited state, due to the perfluoro effect, the structure is considerably less distorted than benzoquinone and also the electron delocalization in the pi* antibonding orbital is less than that of triplet excited state of benzoquinone.

Mapping adaptive resonance theory onto ring and mesh architectures

In recent years, parallel computers have been attracting attention for simulating artificial neural networks (ANN). This is due to the inherent parallelism in ANN. This work is aimed at studying ways of parallelizing adaptive resonance theory (ART), a popular neural network algorithm. The core computations of ART are separated and different strategies of parallelizing ART are discussed. We present mapping strategies for ART 2-A neural network onto ring and mesh architectures. The required parallel architecture is simulated using a parallel architectural simulator, PROTEUS and parallel programs are written using a superset of C for the algorithms presented. A simulation-based scalability study of the algorithm-architecture match is carried out. The various overheads are identified in order to suggest ways of improving the performance. Our main objective is to find out the performance of the ART2-A network on different parallel architectures. (C) 1999 Elsevier Science B.V. All rights reserved.

Micromechanical modeling of granular materials: effect of confining pressure on mechanical behavior

This paper reports the effect of confining pressure on the mechanical behavior of granular materials from micromechanical considerations starting from the grain scale level, based on the results of numerically simulated tests on disc assemblages using discrete element modeling (DEM). The two macro parameters which are influenced by the increase in confining pressure are stiffness (increases) and volume change (decreases). The lateral strain coefficient (Poisson's ratio) at the beginning of the test is more or less constant. The angle of internal friction slightly decreases with increase in confining pressure. The numerical results of disc assemblages indicate very clearly a non-linear Mohr-Coulomb failure envelope with increase in confining pressure. The increase in average coordination number and accompanying decrease of fabric anisotropy reduce the shear strength at higher confining pressures. Micromechanical explanations of the macroscopic behavior are presented in terms of the force and fabric anisotropy coefficients. (C) 1999 Elsevier Science Ltd. AII rights reserved.

Photogenerated radical intermediates of vitamin K-1: a time-resolved resonance Raman study

Quinones play a vital role in the process of electron transfer in bacterial photosynthetic reaction centers. It is of interest to investigate the photochemical reactions involving quinones with a view to elucidating the structure-function relationships in the biological processes. Resonance Raman spectra of radical anions and the time-resolved resonance Raman spectra of vitamin K-1 (model compound for Q(A) in Rhodopseudomonas viridis, a bacterial photosynthetic reception center) are presented. The photochemical intermediates of vitamin K-1, viz. radical anion, ketyl radical and o-quinone methide have been identified. The vibrational assignments of all these intermediates are made on the basis of comparison with our earlier TR3 studies on radical anions of naphthoquinone and menaquinone. (C) 1999 Elsevier Science B.V. All rights reserved.

Preparation and characterization of nanocrystalline powders of bismuth vanadate

The influence of mechanical activation on the formation of Bi2VO5.5 bismuth vanadate (BiV) phase, was investigated by ball-milling a stoichiometric mixture of bismuth oxide and vanadium pentoxide. The structural evolution of the desired BN phase, via an intermediate BiVO4,phase, was investigated using X-ray powder diffraction; (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). Milling for 54h. yielded monophasic gamma-BiV powders with an average crystallite size of 30 nm. The electron paramagnetic resonance (EPR) peaks associated with the V4+ ions are stronger and broader in nanocrystalline (n) BN than in the conventionally prepared microcrystalline (m) BN, suggesting theta significant portion of V5+ has been transformed to V4+ during milling. The optical bandgap of n-BiV was found to be higher than that of m-BiV. High density (97% of the theoretical density), fine-grained (average grain-size of 2 tun) ceramics with uniform grain-size distribution could be fabricated using n-BiV powders. These fine-grained ceramics exhibit improved dielectric, pyre and ferroelectric properties. (C) 1999 Elsevier Science S.A. All rights reserved.

Developments in quantum information processing by nuclear magnetic resonance: Use of quadrupolar and dipolar couplings

Use of dipolar and quadrupolar couplings for quantum information processing (QIP) by nuclear magnetic resonance (NMR) is described. In these cases, instead of the individual spins being qubits, the 2(n) energy levels of the spin-system can be treated as an n-qubit system. It is demonstrated that QIP in such systems can be carried out using transition-selective pulses, in (CHCN)-C-3, (CH3CN)-C-13, Li-7 (I = 3/2) and Cs-133 (I = 7/2), oriented in liquid crystals yielding 2 and 3 qubit systems. Creation of pseudopure states, implementation of logic gates and arithmetic operations (half-adder and subtractor) have been carried out in these systems using transition-selective pulses.

Excited state structure and dynamics of p-benzoquinone and bromanil from time-resolved resonance Raman spectra and simulation

p-Benzoquinone and its halogen substituted derivatives are known to have differing reactivities in the triplet excited state. While bromanil catalyzes the reduction of octaethylporphyrin most efficiently among the halogenated p-benzoquinones, the reaction does not take place in presence of the unsubstituted p-benzoquinone (T. Nakano and Y. Mori, Bull. Chem. Soc. Jpn., 67, 2627 (1994)). Understanding of such differences requires a detailed knowledge of the triplet state structures, normal mode compositions and excited state dynamics. In this paper, we apply a recently presented scheme (M. Puranik, S. Umapathy, J. G. Snijders, and J. Chandrasekhar, J. Chem, Phys., 115, 6106 (2001)) that combines parameters from experiment and computation in a wave packet dynamics simulation to the triplet states of p-benzoquinone and bromanil. The absorption and resonance Raman spectra of both the molecules have been simulated. The normal mode compositions and mode specific excited state displacements have been presented and compared. Time-dependent evolution of the absorption and Raman overlaps for all the observed modes has been discussed in detail. In p-benzoquinone, the initial dynamics is along the C=C stretching and C-H bending modes whereas in bromanil nearly equal displacements are observed along all the stretching coordinates.

Investigation of short-time isomerization dynamics in p-nitroazobenzene from resonance Raman intensity analysis

Resonance Raman (RR) spectra are presented for p-nitroazobenzene dissolved in chloroform using 18 excitation Wavelengths, covering the region of (1)(n --> pi*) electronic transition. Raman intensities are observed for various totally symmetric fundamentals, namely, C-C, C-N, N=N, and N-O stretching vibrations, indicating that upon photoexcitation the excited-state evolution occurs along all of these vibrational coordinates. For a few fundamentals, interestingly, in p-nitroazobenzene, it is observed that the RR intensities decrease near the maxima of the resonant electronic (1)(n --> pi*) transition. This is attributed to the interference from preresonant scattering due to the strongly allowed (1)(pi --> pi*) electronic transition. The electronic absorption spectrum and the absolute Raman cross section for the nine Franck-Condon active fundamentals of p-nitroazobenzene have been successfully modeled using Heller's time-dependent formalism for Raman scattering. This employs harmonic description of the lowest energy (1)(n --> pi*) potential energy surface. The short-time isomerization dynamics is then examined from a priori knowledge of the ground-state normal mode descriptions of p-nitroazobenzene to convert the wave packet motion in dimensionless normal coordinates to internal coordinates. It is observed that within 20 fs after photoexcitation in p-nitroazobenzene, the N=N and C-N stretching vibrations undergo significant changes and the unsubstituted phenyl ring and the nitro stretching vibrations are also distorted considerably.

Mechanical properties of Nylon-6/LDPE blends containing an epoxy resin compatibilizer

Binary and ternary blends of nylon-6/low density polyethylene (nylon-6/LDPE) and Nylon-6/LDPE/poly(ethylene-co-glycidyl methacrylate) were prepared by melt mixing. The blends exhibit two phase morphology with LDPE dispersed in the form of spherical domains in the nylon-6 matrix. The mechanical properties of the blends were measured by standard methods. It is shown that the use of the epoxy copolymer as a compatibilizer improves the impact strength of the blend as compared to nylon-6, which is attributed to better stress transfer across the interface due to the compatibilizer. The data for each mechanical property were also fitted into a best fit model equation and the method of steepest ascent was applied to arrive at the optimum composition of the blend for that property.

A determination of the structure of the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) by time-resolved resonance Raman spectroscopy

Picosecond time-resolved resonance Raman spectra of the A (intramolecular charge transfer, ICT) state of DMABN, DMABN-d(6) and DMABN-N-15 have been obtained. The isotopic shifts identify the nu (s)(ph-N) mode as a band at 1281 cm(-1). The similar to 96 cm(-1) downshift of this mode from its ground state frequency rules out the electronic coupling PICT model and unequivocally supports the electronic decoupling TICT model. However, our results suggest some pyramidal character of the A state amino conformation.

The effect of mischmetal addition on the structure and mechanical properties of a cast Al-7Si-0.3Mg alloy containing excess iron (up to 0.6 Pct)

The effect of Fe content (0.2 to 0.6 pct) on the microstructure and mechanical properties of a cast Al-7Si-0.3Mg (LM 25/356) alloy has been investigated. Further, 1 pct mischmetal (MM) additions (a mixture of rare-earth (RE) elements) were made to these alloys, and their mechanical properties at room and at elevated temperatures (up to 200 degreesC) were evaluated. A structure-property correlation on this alloy was attempted using optical microstructure analysis, fractographs, X-ray diffraction, energy-dispersive analysis of X-rays (EDX), and quantitative metallography by image analysis. An increase in Fe content increased the volume percentage of Fe-bearing intermetallic compounds (beta and pi phases), contributing to the lower yield strength (YS), ultimate tensile strength (UTS), percentage elongation, and higher hardness. An addition of 1 pct MM to the alloys containing 0.2 and 0.6 pct Fe was found to refine the microstructure; modify the eutectic silicon and La, Ce, and Nd present in the MM; form different intermetallic compounds with Al, Si, Fe, and Mg; and improve the mechanical properties of the alloys both at room and elevated temperatures.