化学位移各向导性对半整数自旋四极核的2D NMR章动的影响

考察了化学位移各向异性对半整数自旋四极核2D NMR章动的影响.用数值方法对2D NMR章动实验的演化期的Hamiltonian矩阵进行对角化,然后用帐篷法进行粉末平均,获得了自旋为I=3/2、5/2、7/2和9/2的四极核在不同化学位移各向异性下的2D NMR章动谱.实验结果与理论计算符合较好.     

Suppression of probe background signals via B(1) field inhomogeneity

A new approach combining a long pulse with the DEPTH sequence (Cory and Ritchey, Journal of Magnetic Resonance, 1988) greatly improves the efficiency for suppressing probe background signals arising from spinning modules. By applying a long initial excitation pulse in the DEPTH sequence, instead of a π/2 pulse, the inhomogeneous B(1) fields outside the coil can dephase the background coherence in the nutation frame. The initial long pulse and the following two consecutive EXORCYCLE π pulses function complementarily and prove most effective in removing background signals from both strong and weak B? fields. Experimentally, the length of the long pulse can be optimized around odd multiples of the π/2 pulse, depending on the individual probe design, to preserve signals inside the coil while minimizing those from probe hardware. This method extends the applicability of the DEPTH sequence to probes with small differences in B? field strength between the inside and outside of the coil, and can readily combine with well-developed double resonance experiments for quantitative measurement. In general, spin systems with weak internal interactions are required to attain efficient and uniform excitation for powder samples, and the principles to determine the applicability are discussed qualitatively in terms of the relative strength of spin interactions, r.f. power and spinning rate.     

A precessing and nutating beam with a tip mass

Stability analysis of elastic vibration of a simultaneously precessing and nutating beam with a tip mass is attempted here. A simple two-degree of freedom model is considered to have an understanding of the terms appearing in the parametric equations with precession softening and centrifugal stiffening. The stability margin is determined using a variant of Hill's method using precession and nutation speeds as parameters. It is found that the stability of an only precessing beam depends on the inclination of the beam-centerline with the axis of precession. A precessing beam with very slow nutation is unstable because of its passage through a particular range of nutation angle.     

Al nuclear magnetic resonance study of synthetic and natural corundum (α-Al2O3) Some experimental aspects of quantitative Al nuclear magnetic resonance spectroscopy

The ²⁷Al nuclear magnetic resonance (NMR) response of a series of natural and synthetic corundum (α-Al₂O₃) samples is studied quantitatively by short-pulse excitation and frequency-stepped adiabatic half-passage (FSAHP). Using on- and off-resonance nutation NMR, it was established that the quadrupole coupling parameters of visible Al is identical in all samples. Remarkably, the relaxation behavior for the aluminum is very different in the various samples and has a marked effect on the quantitative response. In natural corundum samples the ²⁷Al spin-lattice relaxation is very efficient as these samples contain paramagnetic impurities. As a result, however, the full signal could not be recovered, which is attributed to relaxation broadening of spins in the vicinity of these impurities. In synthetic samples, containing no impurities, the full signal could be recovered, although the relaxation behaviour appeared to depend strongly on the preparation method. We observed differences in the spin-lattice relaxation by a factor 20; the longest T₁ was observed in a crushed single crystal. This implies that α-Al₂O₃ can only be used as a standard in quantitative analyses if it has been characterized thoroughly. Furthermore, the effective relaxation behaviour for different types of excitation is studied. Finally, a method to measure the spin-lattice relaxation of half-integer quadrupole nuclei is introduced, using a frequency-stepped adiabatic passage (FSAP) to invert the spin system.     

Complete description of the interactions of a quadrupolar nucleus with a radiofrequency field. Implications for data fitting

We present a theory, with experimental tests, that treats exactly the effect of radiofrequency (RF) fields on quadrupolar nuclei, yet retains the symbolic expressions as much as possible. This provides a mathematical model of these interactions that can be easily connected to state-of-the-art optimization methods, so that chemically-important parameters can be extracted from fits to experimental data. Nuclei with spins >1/2 typically experience a Zeeman interaction with the (possibly anisotropic) local static field, a quadrupole interaction and are manipulated with RF fields. Since RF fields are limited by hardware, they seldom dominate the other interactions of these nuclei and so the spectra show unusual dependence on the pulse width used. The theory is tested with ²³Na NMR nutation spectra of a single crystal of sodium nitrate, in which the RF is comparable with the quadrupole coupling and is not necessarily on resonance with any of the transitions. Both the intensity and phase of all three transitions are followed as a function of flip angle. This provides a more rigorous trial than a powder sample where many of the details are averaged out. The formalism is based on a symbolic approach which encompasses all the published results, yet is easily implemented numerically, since no explicit spin operators or their commutators are needed. The classic perturbation results are also easily derived. There are no restrictions or assumptions on the spin of the nucleus or the relative sizes of the interactions, so the results are completely general, going beyond the standard first-order treatments in the literature.     

弹丸章动周期的光学立靶测量法

提出了一种新的测量弹丸初始弹道章动周期的方法,采用非接触式光学立靶测量原理测量弹丸章动周期测量弹丸通过立靶的瞬时位置,计算出弹丸章动角,根据多组章动角数据获得弹丸的章动周期并提出了实现这一目标的测量系统的组成与功能,对系统的攻角测量准确度进行了分析.     

化学位移各向导性对半整数自旋四极核的2D NMR章动的影响(英文)

考察了化学位移各向异性对半整数自旋四极核2DNMR章动的影响．用数值方法对2DNMR章动实验的演化期的Hamiltonian矩阵进行对角化，然后用帐篷法进行粉末平均，获得了自旋为I＝3／2、5／2、7／2和9／2的四极核在不同化学位移各向异性下的2DNMR章动谱．实验结果与理论计算符合较好．     

On flows having constant vorticity

Constant vorticity flows of a uniform fluid in a rigid ellipsoidal container rotating at a variable rate are considered. These include librationally driven and precessionally driven flows. The well-known Poincaré solution for precessionally driven flow in a spheroid is generalized to an ellipsoid with unequal principal axes. The dynamic stability of these flows is investigated, and of other flows in which the angular velocity of the container is constant in time. Solutions for the Chandler wobble are discussed. The role of an invariant, called here the Helmholtzian, is examined.