用TPPI方法消除2D COSY和MQSY实验中与辐射阻尼有关的谐波峰

在二维相关(COSY)和多量子(MQSY)实验中,TPPI(Time-proportional-phase-in-crement)方法被用于消除与辐射阻尼相关的谐波峰.二维实验中,^c/2混合脉冲常常非线性地改变磁化强度的进动相位.这一非线性变化和辐射阻尼一起导致依赖于化学位移偏置的谐波.然而,如果混合脉冲的相位正比于演化时间t₁,谐波消失,但水峰保持不变.     

CT-6B托卡马克装置等离子体电子迴旋辐射中热辐射和非热辐射的观测

本文中利用在基波(35GHz)或二次谐波(70GHz)混频工作的等离子体微波辐射(PME)测量仪,进行了CT-6B托卡马克装置等离子体电子迴旋辐射的实验。实验结果表明:当装置低逃逸放电时,二次谐波混频接收到的信号是与电子温度有关的热辐射信号。装置逃逸放电时,基波混频接收到的信号是与逃逸电子行为有关的非热辐射信号。 关键词：     

激光平面靶3ω0/2谐波空间精细结构的时间和光谱特性

我们从实验上观察到3ω₀/2谐波沿90°辐射精细结构,并从运动细丝底部附近n_c/4处辐射3ω₀/2谐波来解释3ω₀/2谐波细丝结构的存在;从运动的细丝底部对TPD带来的Doppler修正解释3ω₀/2谐波的双峰结构以及红强蓝弱的特点,数量上对红移、蓝移的估算与实验大体相符。用宽带激光打靶,由于靶面照明均匀及宽频带作用,细丝不易形成,对抑制TPD产生的超热电子可能是有益的。 关键词：     

空气中激光等离子体通道的三次谐波辐射研究

对超强飞秒激光脉冲在空气中形成的长等离子体通道的三次谐波辐射进行了研究．实验研究发现三次谐波的转换效率在通道范围内基本保持不变.还对等离子体通道锥角辐射中的三次 谐波的辐射角进行了研究，结果表明其辐射角接近6mrad，与理论计算结果符合得很好． 关键词： 三次谐波 等离子体通道     

在强辐射阻尼存在下完全反转态的线型

在180°脉冲加z方向脉冲梯度场作用后,采样得到90%水样的自由感应衰减信号,其具有几乎完整的双曲正割函数特征,并通过付立叶变换得到其频谱,发现其具有强烈的震荡特征,这与通常用不完善的180°单脉冲观测到的相位畸变有所不同,并且发现这种强震荡只有在强辐射阻尼存在下以及θ₀非常接近180°时才能观测到.在辐射阻尼线型理论基础上,合理的理论计算已进行,我们得到了与实验观测相当一致的结果.     

超热电子产生的靶后相干渡越辐射光谱实验研究

利用OMA光学多道分析仪测量了激光与薄膜靶相互作用中产生的辐射光谱，在靶后观察到红移的二次谐波发射. 这种二次谐波是v×B加热产生的、具有微脉冲结构的超热电子束在等离子体-真空边界产生的相干渡越辐射(CTR). 随着激光能量的增大，红移峰向长波方向移动，光谱同时发生展宽. 分析认为，等离子体临界面的迅速膨胀是导致二次谐波红移的主要原因. 随着预脉冲能量的增大，临界面膨胀速度增大，导致了发射峰更大的红移. 实验还测量了靶面法线方向的辐射光谱，观察到基频辐射的红移和展宽. CTR为诊断临界面的运动方向和速度提供了一种新的方法. 关键词： 相干渡越辐射 超热电子 超短超强激光 等离子体相互作用     

对超短脉冲强激光在大气通道中产生的三次谐波偏振特性及白光光谱调制特性的研究

研究了超短脉冲强激光在大气中传输产生电离通道后辐射的三次谐波强度随激光偏振的变化.此外，还研究了产生的白光辐射在短波长上的有规律的谱调制，分析了白光谱调制的产生机理. 关键词： 超短脉冲激光 大气通道 超连续辐射 三次谐波     

相对论谐波辐射对激光参数与等离子体参数的依赖特性

分析了相对论谐波辐射对入射激光参数和等离子体参数的依赖特性.结果表明：入射激光强度、脉冲宽度、脉冲形状和等离子体密度等参数对相对论谐波辐射有重要的影响.所得结果为实验提供了参考和依据. 关键词：     

在辐射阻尼存在下有效横向弛豫时间T*2的快速测定

利用CPMG自旋回波技术,通过一连串180°脉冲抑制辐射阻尼效应,让磁化矢量经过横向弛豫T₂过程大幅度衰减后再检测回波信号的线宽,该方法可直接获得有效横向弛豫时间T*₂,并可在CPMG测试T₂值的同时进行. 另外,当饱和恢复法用于估计T*₂值时,信号检测必须使用小角度脉冲.所有结果已进行了实验验证.     

调频激励增强微气泡的次谐波理论和实验研究

超声造影剂的次谐波成像可以提高造影组织比，提供更好的图像质量. 提出一种利用调频脉冲激励以增强造影剂微气泡产生的次谐波新方法. 基于修正的Church方程，从理论上讨论了次谐波的产生与调频激励声压的关系及产生阈值，并且实验证实了优化调频信号的带宽及调频时间可以提高次谐波信号幅度及改善主瓣和旁瓣特性. 理论与实验表明，与传统脉冲信号激励相比，调频信号激励产生的次谐波幅度可提高约22dB. 关键词： 调频激励 超声造影剂 微气泡 次谐波     

Signal Enhancement Using 45° Water Flipback for 3D N-Edited ROESY and NOESY HMQC and HSQC

A novel implementation of the water flipback technique employing a 45° flip-angle water-selective pulse is presented. The use of this water flipback technique is shown to significantly enhance signal in 3D ¹⁵N-edited ROESY in a 20 kDa complex of the vnd/NK-2 homeodomain bound to DNA. The enhancement is seen relative to the same experiment using weak water presaturation during the recovery delay. This enhancement is observed for the signals from both labile and nonlabile protons. ROESY and NOESY pulse sequences with 45° water flipback are presented using both HMQC and HSQC for the ¹⁵N dimension. The 45° flipback pulse is followed by a gradient, a water selective 180° pulse, and another gradient to remove quadrature images and crosspeak phase distortion near the water frequency. Radiation damping of the water magnetization during the t₁ and t₂ evolution periods is suppressed using gradients. Water resonance planes from NOESY–HMQC and NOESY–HSQC spectra show that the HMQC version of the pulse sequences can provide stronger signal for very fast exchanging protons. The HSQC versions of the ROESY and NOESY pulse sequences are designed for the quantitative determination of protein–water crossrelaxation rates, with no water-selective pulses during the mixing time and with phase cycling and other measures for reducing axial artifacts in the water signal.     

Solvent Magnetization Artifacts in High-Field NMR Studies of Macromolecular Hydration

With the use of high magnetic fields and improved quality factor ratings of the probeheads in modern NMR spectrometers, radiation damping becomes more and more important. In addition, the demagnetizing field effect from protonated solvents gains significance with the increase of the magnetic field strength. During a typical NMR pulse sequence the magnetic fields caused by these effects become time-dependent, which makes the system nonlinear and may, for example, measurably influence the precession frequencies of all nuclei in the sample. Since radiation damping can affect signals that are several kilohertz away from the solvent resonance, the amplitude, phase, and frequency of the desired signals can be disturbed so as to give rise to spectral artifacts. In particular when difference methods are used to obtain the final spectrum, the data sets may be severely deteriorated by such artifacts. This paper investigates effects from the demagnetizing field and from radiation damping with a selection of pulse sequences in use for studies of macromolecular hydration, and strategies are described for the detection and elimination of the ensuing artifacts.     

Characterization and Suppression Techniques for Degree of Radiation Damping in Inversion Recovery Measurements

Radiation damping is a phenomenon in which transverse nuclear magnetization couples with the current in a coil used in nuclear magnetic resonance experiments. This results in an additional magnetic field that increases the relaxation pathway for the magnetization, which then relaxes back to equilibrium more quickly. Radiation damping has been shown to affect longitudinal relaxation time (T₁) measurement in inversion recovery experiments. In this work, we demonstrate that the extent of radiation damping depends upon the T₁ of the sample. Radiation damping difference spectroscopy is used to characterize the severity of radiation damping, while gradient inversion recovery is used for radiation damping suppression in T₁ measurements. At field strength of 9.4 T, for the radiation damping characteristic time (T_rd) of 50 ms, these investigations show non-negligible radiation damping effects for T₁ values greater than T_rd, with severe distortions for T₁ longer than about 150 ms, showing reasonable agreement with the predicted T_rd. We also report a discrepancy between published expressions for the characteristic radiation damping time.     

Quantum mechanical calculation of Suryan’s line broadening in nuclear magnetic resonance

The first quantum theory of the classical radiation damping in nuclear magnetic resonance is presented. Relaxation times and life times arising from the interaction of nuclear spin with the radio-frequency radiation field are calculated. Second-order line shifts are predicted and the existence ofI _z andI _z ² -type operators due to photons is pointed out. The predicted line shifts as well as relaxation are found to be measurably large. Numerical estimates are given for protons in water.     

NMR study of heterogeneity in pyridine-N-oxide...HCl crystal

An origin of narrow ¹H NMR signals in pyridine-N-oxide (PyO)...HCl crystal has been investigated by means of MAS, SPEDAS, NOESY and COSY techniques. Spectra of crystalline samples are compared with those of solid phase obtained from liquid PyO...HCl solutions (in acetonitile/H₂O) after the heterogeneous phase separation. It has been concluded that partially resolved peaks in ¹H NMR spectra of solids are related with heterogeneity of spin system and presence of different H-bond clusters of water molecules. NOESY spectra show no cross-peaks even at very long mixing time (500 ms). This indicates there is no exchange process between spins causing different peaks, and thus the corresponding molecular aggregates are captured in “islands of mobility8 without any channels sufficient for exchange. Appearance of MAS side bands as “pseudo8 cross-peaks in 2D NMR spectra using MAS/COSY technique is reported. In the case of accidental coincidence of spinning frequency (ω _MAS ) with spectral distances between some diagonal signals, intensive non-diagonal peaks are observed at the corresponding cross-positions. A misleading conclusion concerning spin coupling is easy to avoid using various ω _MAS . This work is dedicated to Professor Robert Blinc on the occasion of his 70^th birthday.     

Single and Multiple-Selective Excitation Combined with Pulsed Field Gradients

Applications of selective, multiselective, and semiselective pulses with pulsed field gradients are described. The use of multiple-selective excitation and PFGs for coherence selection in the selective one-dimensional experiments results in spectra devoid of artifacts and with remarkable solvent suppression. Multiple-selective excitation is also employed in an experiment called Multigate, a variant of the well-known WATERGATE experiment, in order to achieve multiple solvent signal suppression. Finally, new pulse sequences are shown for recording pure absorption ω₁semiselective PFG NOESY, ROESY, and TOCSY experiments. The merits and limitations of these experiments are discussed.     

Radiation damping in an anharmonic oscillator

The effect of radiation damping in an anharmonic oscillator has been calculated using the technique of Kryloff and Bogolituboff. It is found that the intensity distribution of the emitted spectral line is asymmetric about its intensity maximum. The index of asymmetry agree with the experimental data on x-rayK _{a 1,2}lines.     

Light scattering in transition metal diselenides CoSe2 and CuSe2

Raman scattering techniques have been used to investigate the long- wavelength optical phonon spectrum of the pyrite-type transition metal diselenides CoSe₂ and CuSe₂. Of the five Raman-active phonon peaks expected by group theory only one has been observed in CoSe₂ and three in CuSe₂ at 300 and 77°K. Overtone scattering is also observed in CuSe₂. As these materials are highly conducting it is concluded that appreciable damping by free carriers obscures the observation of the missing modes.     

Radiation damping on cryoprobes

Radiation damping on 600 and 800 MHz cryoprobes was investigated. The phase angle β between a vector 90° phase shifted to the precessing magnetization and the rf field induced in the coil was found to depend markedly on whether an FID was being acquired or not. The magnitude of the radiation damping field was sufficiently strong to restore 95% of the equilibrium water magnetization of a 90% H2O sample in a 5 mm sample tube within about 5 ms following a 165° pulse. This can be exploited in water flip-back versions of NOESY and TOCSY experiments of proteins, but care must be taken to limit the effect of the radiation damping field from the water on the Ha protons. Long water-selective pulses can be applied only following corrections. We developed a program for correcting pulse shapes if β is non-zero. The WATERGATE scheme is shown to be insensitive to imperfections introduced by radiation damping.     

Nonequilibrium processes at the shock wave front in inert gases with a small amount of Fe(CO)<Subscript>5</Subscript> impurity

Nonequilibrium processes occurring in shock waves generated in argon and helium containing a small amount of Fe(CO)₅ impurity are investigated experimentally and numerically. Nonequilibrium radiation peaks are found in the relaxation zone of the shock wave, and a dependence of the radiation intensity on the shock parameters and Fe(CO)₅ concentration is obtained. Unresolved spectra of the resulting radiation in the range 400–700 nm are recorded. It is shown that the effects observed cannot be accounted for by high-energy collisions in the zone of shock translational relaxation. It is concluded that these nonequilibrium phenomena are related to electronic excitation of small iron clusters at fast condensation of supersaturated iron vapor arising due to Fe(CO)₅ decomposition. A kinetic model describing the formation of excited clusters is suggested.