Product operator analysis of the influence of chemical exchange on relaxation rates

Measurements of chemical-exchange processes by NMR are widely used to obtain valuable information about molecular dynamics and structure. Here, a computational method is introduced to assess the influence of chemical exchange on spin relaxation rates. The method is based on the inclusion of a random exchange process in product operator calculations on a microscopic level. This product operator approach can be applied to estimate exchange contributions when using sophisticated pulse sequences that cannot be easily described analytically. The method applies to the full range of exchange times measurable by NMR and can incorporate interference effects between exchange and other processes such as scalar coupling. To demonstrate its utility, simulated relaxation data were compared with theoretical predictions of spin-locking and Carr-Purcell spin-echo sequences with hard and adiabatic pulses, using different time scales for a two-site chemical-exchange process. Finally, simulations were used to examine a system in which a second random process is superimposed on a simple two-site exchange process. The method was found to provide a simple and robust tool to analyze pulse sequences and equations commonly used to study exchange-induced relaxation.     

混沌伪随机序列的谱熵复杂性分析

为了准确分析混沌伪随机序列的结构复杂性,采用谱熵算法对Logistic映射、Gaussian映射和TD-ERCS系统产生的混沌伪随机序列复杂度进行了分析.谱熵算法具有参数少、对序列长度N(惟一参数)和伪随机进制数K鲁棒性好的特点.采用窗口滑动法分析了混沌伪随机序列的复杂度演变特性,计算了离散混沌系统不同初值和不同系统参数条件下的复杂度.研究表明,谱熵算法能有效地分析混沌伪随机序列的结构复杂度;在这三个混沌系统中,TD-ERCS系统为广域高复杂度混沌系统,复杂度性能最好;不同窗口和不同初值条件下的混沌系统复杂度在较小范围内波动.为混沌序列在信息安全中的应用提供了理论和实验依据.     

Self-calibration of multi-camera networks without feature correspondence between different cameras

An automatic calibration approach for multi-camera networks is proposed to calibrate the intrinsic parameters of each camera and the extrinsic parameters between different cameras. Firstly, the moving objects are tracking, and the feature points are detected and calculated by a matching method from image sequences. And then we estimate the intrinsic parameters of each camera respectively by a self-calibration method from the motion of feature points, while estimating the rotation and translation of each camera with respect to the object. Thirdly, we estimate the extrinsic parameters between different cameras from the rotation and the motion of each camera with respect to the object. Our method only needs to track the motion of objects in each camera without the correspondence between different cameras. It avoids the difficulty of the correspondence between different cameras in real networks. Experiments with simulated data and real images are carried out to verify the theoretical correctness and numerical robustness.     

Gradient-enhanced heteronuclear correlation spectroscopy: theory and experimental aspects. 1992

A detailed theoretical and experimental treatment is given for gradient-enhanced heteronuclear correlation spectroscopy. Both multiple-quantum and single-quantum sequences are described. In addition to a comparison with conventional experiments using phase cycling, the effects of different gradient combinations are examined with respect to artifacts occurring in the heteronuclear dimension. The influence of gradient performance and diffusion on sensitivity is discussed. Approaches to attain phase-sensitive spectra are also analyzed.     

Recent progress on spectral data for X-ray lasers at the NationalBureau of Standards

The Atomic Spectroscopy Group at the National Bureau of Standards is using low-inductance sparks, laser-produced plasmas, and tokamaks to observe spectra that are important for the development of X-ray lasers. Attention is given to recent results on Ne-like ions and ions in neighboring isoelectronic sequences, Ni-like ions and ions in neighboring isoelectronic sequences, and the spectra of rare-gas ions. In addition to contributing to a knowledge of the energy levels and transitions of possible lasing media, the data provide reference lines for wavelength calibration of X-ray laser experiments and reference data for testing theoretical methods used for predicting the properties of lasing ions     

Systematical Study on Ground-State Ionization Potentials for Boron andCarbon Isoelectronic Sequences with Z=6-42

The multi-configuration Dirac-Fock method is used in this work to calculateground-state ionization potentials of Boron and Carbon isoelectronicsequences from Z=6 to Z=42. And the contribution of relativisticcorrections, nuclear volume effect, Breit and QED effects to the results isdiscussed in the calculation. The results are compared with the scantyexisting theoretical and experimental data in the literature and goodagreements are achieved. Then analytical expressions of Ionization potentialalong Boron and Carbon isoelectronic sequences are obtained.     

Systematic multi-configuration Dirac-Fock calculations of the Kα and Kβ X-ray spectra of silicon ions

The transition energies, absorption oscillator strengths, line strengths and transition probabilities between computed levels are reported for the He-like to Ne-like Silicon ion sequences. Wavefunctions were determined relativistic configuration interaction (RCI) and multi-configuration Dirac-Fock (MCDF) technique included the Breit interaction, quantum electrodynamic (QED) corrections and nuclear mass corrections. The calculated values are in good agreement with the available experimental data and the recent theoretical values obtained from other methods. These data provide reference values for the level lifetimes, charge state distributions, and average charge of silicon plasmas.     

High-resolution 2D NMR spectra in inhomogeneous fields based on intermolecular multiple-quantum coherences with efficient acquisition schemes

High-resolution 2D NMR spectra in inhomogeneous fields can be achieved by the use of intermolecular multiple-quantum coherences and shearing reconstruction of 3D data. However, the long acquisition time of 3D spectral data is generally unbearable for in vivo applications. To overcome this problem, two pulse sequences dubbed as iDH-COSY and iDH-JRES were proposed in this paper. Although 3D acquisition is still required for the new sequences, the high-resolution 2D spectra can be obtained with a relatively short scanning time utilizing the manipulation of indirect evolution period and sparse sampling. The intermolecular multiple-quantum coherence treatment combined with the raising and lowering operators was applied to derive analytical signal expressions for the new sequences. And the experimental observations agree with the theoretical predictions. Our results show that the new sequences possess bright perspective in the applications on in vivo localized NMR spectroscopy.     

Enhanced sensitivity in RIACT/MQ-MAS NMR experiments using rotor assisted population transfer

The rotor assisted population transfer (RAPT) sequence is used to enhance the sensitivity of the RIACT(II) experiment for spin-3/2 quadrupolar nuclei. A detailed theoretical analysis of the polarizations that contribute to different types of MQ-MAS experiments is provided. In particular, two polarization pathways are distinguished for the creation of triple-quantum coherence. The existence of these pathways is experimentally demonstrated by comparing the sensitivities of different sequences with and without RAPT preparation.