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111.
Kälin M Gromov I Schweiger A 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,160(2):166-182
When the modulation frequency used in continuous wave electron paramagnetic resonance (cw EPR) spectroscopy exceeds the linewidth, modulation sidebands appear in the spectrum. It is shown theoretically and experimentally that these sidebands are actually multiple photon transitions, sigma(+)+kxpi, where one microwave (mw) sigma(+) photon is absorbed from the mw radiation field and an arbitrary number k of radio frequency (rf) pi photons are absorbed from or emitted to the modulation rf field. Furthermore, it is demonstrated that both the derivative shape of the lines in standard cw EPR spectra and the distortions due to overmodulation are caused by the unresolved sideband pattern of these lines. The single-photon transition does not even give a contribution to the first-harmonic cw EPR signal. Multiple photon transitions are described semiclassically in a toggling frame and their existence is proven using second quantization. With the toggling frame approach and perturbation theory an effective Hamiltonian for an arbitrary sideband transition is derived. Based on the effective Hamiltonians an expression for the steady-state density operator in the singly rotating frame is derived, completely describing all sidebands in all modulation frequency harmonics of the cw EPR signal. The relative intensities of the sidebands are found to depend in a very sensitive way on the actual rf amplitude and the saturation of single sidebands is shown to depend strongly on the effective field amplitude of the multiple photon transitions. By comparison with the analogous solutions for frequency-modulation EPR it is shown that the field-modulation and the frequency-modulation technique are not equivalent. The experimental data fully verify the theoretical predictions with respect to intensities and lineshapes. 相似文献
112.
M. C. Calderer 《Mathematical and Computer Modelling》2001,34(12-13)
We discuss mathematical and physical aspects of the phase transition from nematic to smectic A liquid crystals. The first approach deals with analyzing a model obtained from the Maier-Saupe theory of nematic by taking into account that elongated liquid crystal molecules present distinguishable ends. Moreover, we represent long range microscopic interactions by means of nonlocal free energy functionals. The smectic configurations emerge as solutions of the extended nematic theory, through a modulation process. The second part of the article deals with energy minimization of the de Gennes free energy for smectic A* liquid crystals, and with the study of uniform twist grain boundary (TGB) structures. The goal is to mathematically justify parameter regions of the phase diagram of the transition between nematic and smectic A liquid crystals. Both approaches complement each other from the point of view that, while the first one deals with mechanisms causing layer arrangements, the second approach focuses on how chirality and layer effects interact, in a system with preassumed periodicity. The A* notation refers to chiral liquid crystals. 相似文献
113.
114.
A.B. Utkin R. Vilar A.J. Smirnov 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,17(2):145-148
The consideration is carried out in its general formulation: the wave aberration function is represented in terms of classical
aberrations (the Zernike polynomials), the phase transfer function (argument of the complex optical transfer function) is
defined by a chain of transformations originating from the generalized pupil function. Quasi-analytical quadrature formulas
are derived that link the optical transfer function and the phase transfer function with the aberration terms. It is shown
that the phase transfer function contains information on the odd-order aberrations, which can be retrieved from coefficients
to the Taylor expansion of the derived quadrature relation.
Received 16 June 2001 相似文献
115.
采用光外差-磁旋转-速度调制吸收光谱技术, 在可见光波段范围16800~17573 cm-1, 对N2+的A 2Πu-X 2Σ+g(12,6)、(11,5)、(7,2)带和B 2Σ+u-X 2Σ+g (1,5)带进行了测量和分析,推导了双原子分子振转能级在受到微扰作用时的有效哈密顿量形式,并分析了N2+的A 2Πu-B 2Σ+u之间存在的微扰相互作用,通过与实验数据的拟合得到了精确的电子态微扰常数ξe、ηe . 相似文献
116.
Rong LinYuanmei Gao 《Physics letters. A》2011,375(36):3228-3231
We report on the experimental observation of the modulation instability of uniform beam and frequency-doubling of interference fringes spatial frequency in self-defocusing photorefractive crystal LiNbO3:Fe by the same experimental setup. Frequency-doubling appears if the nonlinearity of the c-axis is bigger than the modulation of the cylindrical lenses, otherwise the modulation instability appears. 相似文献
117.
118.
Modulations present in vibration signals generated by rotating machinery might carry a lot of useful information about objects’ technical condition. It has been proven that both gearboxes and rolling element bearing (REB) faults manifest themselves as modulations. The paper describes a technique for detection of modulations in vibroacoustic signals, called modulation intensity distribution (MID), which is a function that combines multiple spectral correlation densities in one way or another, depending on the application. Additionally, the paper describes a functional obtained by integrating an MID (denoted by IMID) that has the advantage of being a function of only one frequency variable instead of two. The paper investigates the utility of the MID as an indicator for detection of the presence of rolling element bearing faults in high noise environments. For the purpose of testing, a wind turbine that suffered both advanced gearbox fault and early stage of bearing fault was chosen. Additionally, the paper undertakes the problem of application of the proposed tool in an industrial condition-monitoring system. In order to show the behavior of cyclic components generated by the turbine under study over a long period of time, the set of MIDs integrated over full range of potential carrier signals was presented as a cascade plot. 相似文献
119.
120.
在高重复频率的光窄脉冲源中,为了获得更窄的脉冲,需要增加泵浦功率|但泵浦功率的提高会使光放大器的增益趋于饱和,放大的自发辐射噪音增强,脉冲光信噪比下降.为此,提出了利用啁啾管理和光纤光参量放大相结合的脉冲压窄方案.将强度调制后的泵浦光通过相位调制引入线性负啁啾,由于经参量过程后在新的闲频光上啁啾会加倍,从而进一步增大了信号的谱宽,并通过色散补偿介质实现了光脉冲压缩.实验给出了10 GHz速率下的结果,通过泵浦光的相位调制在0.5 W平均泵浦光功率条件下得到了脉宽19 ps的光脉冲,结果与理论分析基本吻合. 相似文献