Nuclear quadrupole resonance exchange spectroscopy with shaped radiofrequency pulses

Recent work on the nuclear quadrupole resonance (NQR) investigation of molecular dynamics in the solid state has relied on 2D methods. We report our studies of dynamic processes by 1D shaped pulse NQR spectroscopy. Significant advantages include considerably shorter experimental duration, clear definition of the exchange time window, and avoidance of off-resonance effects. The reorientation of the Cl₃C? group in polycrystalline chloral hydrate [Cl₃C–CH(OH)₂] is considered as a test case. This may be modelled as a three-site exchange process. An analysis of the generalised Bloch–McConnell equation is performed to formulate the kinetic matrix. The present approach involves simultaneous excitation of the sites that undergo chemical exchange by employing a suitably modulated shaped RF pulse, followed by a mixing time, and finally a suitable read pulse for signal detection. The experimental signal intensities are plotted against the mixing time to extract the kinetic parameters, i.e. the exchange rate and the spin-lattice relaxation rate. Variable temperature measurements are carried out to determine the activation parameters. Short experiment times are possible in our 1D mode, enabling a large number of runs to be readily performed as a function of mixing time and temperature. The kinetic and activation parameters obtained in the case of chloral hydrate are in good agreement with recent literature values.     

Rotating turbulent Rayleigh–Bénard convection subject to harmonically forced flow reversals

The characteristics of turbulent flow in a cylindrical Rayleigh–Bénard convection cell which can be modified considerably in case rotation is included in the dynamics. By incorporating the additional effects of an Euler force, i.e., effects induced by non-constant rotation rates, a remarkably strong intensification of the heat transfer efficiency can be achieved. We consider turbulent convection at Rayleigh number Ra = 10⁹ and Prandtl number σ = 6.4 under a harmonically varying rotation, allowing complete reversals of the direction of the externally imposed rotation in the course of time. The dimensionless amplitude of the oscillation is taken as 1/Ro^* = 1 while various modulation frequencies 0.1 ≤ Ro_ω ≤ 1 are applied. Both slow and fast flow-structuring and heat transfer intensification are induced due to the forced flow reversals. Depending on the magnitude of the Euler force, increases in the Nusselt number of up to 400% were observed, compared to the case of constant or no rotation. It is shown that a large thermal flow structure accumulates all along the centreline of the cylinder, which is responsible for the strongly increased heat transfer. This dynamic thermal flow structure develops quite gradually, requiring many periods of modulated flow reversals. In the course of time, the Nusselt number increases in an oscillatory fashion up to a point of global instability, after which a very rapid and striking collapse of the thermal columnar structure is seen. Following such a collapse is another, quite similar episode of gradual accumulation of the next thermal column. We perform direct numerical simulation of the incompressible Navier–Stokes equations to study this system. Both the flow structures and the corresponding heat transfer characteristics are discussed at a range of modulation frequencies. We give an overview of typical time scales of the system response.     

染料聚合物工艺母盘刻录坑形对读出信号调制幅度的影响分析

与光刻胶工艺相比,染料聚合物工艺母盘刻录的光盘坑形有很大区别。在已知文献中对坑点形貌与读出信号关系的研究结果已不再有效。使用基于角谱分解的衍射理论,利用原子力显微镜得到的坑形,对两种工艺光盘的调制幅度参数进行了仿真。仿真结果表明,对染料聚合物工艺而言,在较小的深度和宽度下能获得与光刻胶工艺光盘相当的调制幅度。对实际光盘几何尺寸和电信号参数的测试结果证明了仿真结果的正确性。     

Shear‐thinning flow in weakly modulated channels

The steady flow inside a spatially modulated channel is examined for shear‐thinning and shear‐thickening fluids. The flow is induced by the translation of the lower plate. The modulation amplitude is assumed to be small. A regular perturbation expansion of the flow field is used, coupled to a variable‐step finite‐difference scheme, to solve the problem. Convergence and accuracy assessment against finite‐volume calculations indicates that there is a significant range of validity of the perturbation approach. The influence of the wall geometry, inertia and non‐Newtonian effects are investigated systematically. In particular, the influence of the flow and fluid parameters is examined on the conditions for the onset of separation, vortex size and location. Copyright © 2005 John Wiley & Sons, Ltd.     

Theory and Applications of Modulated Temperature Programming to Thermomechanical Techniques

The application of modulated temperature programs to thermomechanical analysis can be used to separate the reversible nature of thermal expansion from irreversible deformation arising from creep under the applied load or changes in dimensions due to relaxation of orientation. The effect of experimental variables and calibration are described. Modulated temperature TMA allows the time-dependent nature of thermal expansivity to be studied. Measurements made under compression afford a means of measuring the thermal expansivity of soft specimens independently of initial load. Application of these principles to scanning thermal microscopy leads to a novel method of generating image contrast based upon local changes thermal expansivity of a specimen. This revised version was published online in July 2006 with corrections to the Cover Date.     

J‐modulated 1D directed COSY for precise measurement of proton–proton residual dipolar coupling constants of oligosaccharides

A simple selective 1D experiment for the accurate and precise determination of residual proton–proton dipolar coupling constants of oligosaccharides is proposed. The technique is based on the concept of J‐modulated spectroscopy and provides coupling constants by fitting the peak intensities from a series of spectra to known transfer functions with a precision of ±0.02 Hz for splittings >0.3 Hz. This level of precision is possible owing to limiting the number of sites for polarization transfer. Selective pulses and pulsed field gradients are used for selective excitation and coherence selection, yielding clean, artefact‐free spectra. The technique is illustrated using trisaccharide samples in Pf1 phage. Copyright © 2002 John Wiley & Sons, Ltd.     

Modulated Pulse Generations in a Laser-diode-pumped Passively Q-switched Intracavity-frequency-doubling Nd∶YVO_4 Laser

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Long-term analysis of semilinear wave equations with slowly varying wave speed

A semilinear wave equation with slowly varying wave speed is considered in one to three space dimensions on a bounded interval, a rectangle or a box, respectively. It is shown that the action, which is the harmonic energy divided by the wave speed and multiplied with the diameter of the spatial domain, is an adiabatic invariant: it remains nearly conserved over long times, longer than any fixed power of the time scale of changes in the wave speed in the case of one space dimension, and longer than can be attained by standard perturbation arguments in the two- and three-dimensional cases. The long-time near-conservation of the action yields long-time existence of the solution. The proofs use modulated Fourier expansions in time.