共查询到20条相似文献,搜索用时 0 毫秒
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G. Hinze R. Böhmer G. Diezemann H. Sillescu 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,131(2):218-223
Four-time stimulated echo experiments have been used to monitor the temporal evolution of the reorientation rates of deuterated molecules or colloidal suspensions. We present extended phase cycles for this seven-pulse experiment. In order to test its performance three vastly different materials are chosen. These include a crystal in which the molecules carry out well-defined 180° flips and a supercooled liquid characterized by a distribution of jump angles centered around 10°. As an example for rotational diffusion, data on a concentrated suspension of polystyrene spheres in a viscous medium are presented. 相似文献
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Alexandre A. Khrapitchev Paul T. Callaghan 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2001,152(2):259
When two pairs of position-encoding pulses are used in a pulsed gradient spin echo (PGSE) NMR experiment, it is possible to examine velocity fluctuations. The one-dimensional version of double PGSE NMR uses identical pulse pairs whose amplitudes are stepped simultaneously. In the two-dimensional version (VEXSY) the pulse pairs are stepped independently, resulting in a velocity exchange spectrum. A key limitation in such experiments is transverse relaxation, so that stimulated echoes are often used as the method of choice. It is shown here that the use of stimulated echoes results in a superposition of signals arising from different magnetization pathways such that the spin phases may reflect both the sum and difference of displacements over the pulse pair encoding times, as well as the displacement over the exchange time between the pulse pairs. A phase cycle scheme that selects desired encodings as required is demonstrated. 相似文献
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N. I. Shamrov 《Journal of Applied Spectroscopy》2000,67(6):981-989
Statistical properties of the Raman scattering of an intense step pulse are investigated by numerical solution of the equations of a semi-classical theory with assignment of a random Gaussian distribution of the initial polarization over the sample. The probability density functions of the intensity, energy, delay time, and width of a Stokes pulse and also of the position, height, and width of the spectral peaks of the first incidental Stokes component with the preservation of the phase memory of scattering centers are calculated. The influence of collisional dephasing on the fluctuation of these characteristics is considered. 相似文献
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The study of anisotropies in the Cosmic Microwave Background radiation is progressing at a phenomenal rate, both experimentally and theoretically. These anisotropies can teach us an enormous amount about the way that fluctuations were generated and the way they subsequently evolved into the clustered galaxies which are observed today. In particular, on sub-degree scales the rich structure in the anisotropy spectrum is the consequence of gravity-driven acoustic oscillations occurring before the matter in the universe became neutral. The frozen-in phases of these sound waves imprint a dependence on many cosmological parameters, that we may be on the verge of extracting.This essay received the third award from the Gravity Research Foundation, 1995-Ed. 相似文献
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Erika G. Kisvarsanyi N.S. Sullivan 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,127(2):192-198
The theory of the formation of NMR multiple echoes for isotopic impurities (HD) in solid para-H2has been developed for the case of weak quantum tunneling of the impurities. The results show that even for low tunneling rates (100–1000 Hz), NMR multiple echoes can be observed for experimentally accessible nuclear spin polarizations and can be used to provide an independent test for the existence of tunneling in the solid hydrogens. 相似文献
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Nonlinear Theory of Spectra of Thermally Stimulated Currents in Complex Crystals with Hydrogen Bonds
Tonkonogov M. P. Ismailov Zh. T. Timokhin V. M. Fazylov K. K. Kalitka V. A. Baimukhanov Z. K. 《Russian Physics Journal》2002,45(10):1008-1018
The nonlinear theory of thermally stimulated depolarization currents is developed. The theory explains the processes of hetero- and homocharge relaxation in complex crystals with hydrogen bonds and allows the relaxation oscillator parameters to be calculated using the quadratic approximation for the external electric field. 相似文献
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Stokes dynamics in stimulated Brillouin scattering generated in optical fibers is analyzed over a wide range of nonlinear refractive indices under a no feedback condition. The Stokes fluctuation becomes complicated as the nonlinear refractive index increases. Chaotic behavior appears without external feedback at a large nonlinear refractive index of 1×10-19 m2/V2 with pump power larger than 0.3 W. 相似文献
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基于非线性包络方程我们研究了平面非线性介质波导中的时空不稳定性,得到了有拉曼散射效应情形下不稳定性调制的增益谱的表达式.结果表明在正常和反常两种群速度色散情形下,拉曼散射效应都会导致出现新的不稳定性区域,并且使原有的每一个谱分量的增益上升从而使谱的范围扩大.并通过分析反常色散情况下四种不同函数分布的非线性色散介质系数对不稳定性增益谱的影响,得到呈双曲函数分布的非线性色散介质最为理想. 相似文献
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Minghua Han Naichang Yuan 《International Journal of Infrared and Millimeter Waves》1998,19(8):1131-1140
Estimation of transient signal in additive noise is very important in radar object detection and recognition. This paper presents a new method for transient signal reconstruction based on bispectrum estimation techniques. The third-order cumulants of the received noisy ultra-wide band echoes are acquired first and an ARMA model is then fitted. The bispectrum of the output signal of the ARMA model will be used to reconstruct the transient signal. Simulation results show that the effect is very good even in lower signal-to-noise (SNR) situation. 相似文献
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If N atoms simultaneously interact with quasiresonant classical and quasiresonant quantized fields, the modes exchange photons. This processes exhibits cooperative properties, i.e., the number of photons in the quantized mode oscillates, and the amplitude of these oscillations is proportional to N 2. Zh. éksp. Teor. Fiz. 116, 858–869 (September 1999) 相似文献
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Summary In this paper we have studied the nonlinear interaction of a Gaussian electromagnetic beam with an electrostatic upper hybrid
wave in a collisionless magnetized plasma. On account of the interaction of the incident Gaussian electromagnetic beam with
the plasma, a time-independent component of the ponderomotive force becomes finite and leads to the modification in the background
density. This results in the excitation of the upper hybrid mode of the plasma. The excited electrostatic wave leads to the
enhanced Raman backscattering. An expression for the scattered power is derived and the effect of the external static magnetic
field on the enhancement of the density perturbation and scattered power is discussed.
To speed up publication, the authors of this paper have agreed to not receive the proofs for correction. 相似文献
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Aleinik A. N. Aryshev A. S. Bogomazova E. A. Kalinin B. N. Naumenko G. A. Potylitsyn A. P. Saruev G. A. Sharafutdinov A. F. 《JETP Letters》2004,79(7):320-323
JETP Letters - The design of free-electron lasers (FELs) based on submillimeter-range undulators involves difficulties associated with the fabrication of undulators with a very small period. To do... 相似文献
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Minghua Han Naichang Yuan 《International Journal of Infrared and Millimeter Waves》1998,19(8):1141-1149
According to the features of the echoes of the ultra-wide band radar, this paper analyses the estimating effects of choosing the AR model, MA model and ARMA model based on bispectrum. It shows that ARMA model is much better than others. Simulations verify this result, also demonstrate the estimation precision has been improved. ARMA model parametric bispectrum estimation is very efficient to echoes of ultra-wide band radar. 相似文献
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The collisionsless Boltzmann equation is solved in two ways by the method of characteristics. The solution is used for an exact treatment of higher-order temporal and spatial plasma wave echoes in the absence of steady external fields. The second-order spatial echo is treated separately and analyzed numerically. 相似文献
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Building a refrigerator based on the conversion of heat into optical energy is an ongoing engineering challenge. Under well-defined conditions, spontaneous anti-Stokes fluorescence of a dopant material in a host matrix is capable of lowering the host temperature. The fluorescence is conveying away a part of the thermal energy stored in the vibrational oscillations of the host lattice. In particular, applying this principle to the cooling of (solid-state) lasers opens up many potential device applications, especially in the domain of high-power lasers. In this paper, an alternative optical cooling scheme is outlined, leading to the radiative cooling of solid-state lasers. It is based on converting the thermal energy stored in the host into optical energy by means of a stimulated nonlinear process, rather than a spontaneous process. This should lead to better cooling efficiencies and a higher potential of applying the principle for device applications. 相似文献
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Emission spectra of CdTe at low temperature under high intensity optical pumping by means of a tunable dye laser are reported.We show that the optical gain process is enhanced when a high exciton density is achieved by means of quasi-resonant pumping. The mechanism responsible for the amplification process is found to be the well-known exciton-exciton scattering. 相似文献