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A semiclassical method based on the closed-orbit theory is applied to
analysing the dynamics of photodetached electron of H$^- $ in the
parallel electric and magnetic fields. By simply varying the magnetic
field we reveal spatial bifurcations of electron orbits at a fixed
emission energy, which is referred to as the fold caustic in
classical motion. The quantum manifestations of these singularities
display a series of intermittent divergences in electronic flux
distributions. We introduce semiclassical uniform approximation to
repair the electron wavefunctions locally in a mixed phase space and
obtain reasonable results. The approximation provides a better
treatment of the problem. 相似文献
53.
Accuracy and effectiveness of self-gating signals in free-breathing three-dimensional cardiac cine magnetic resonance imaging 下载免费PDF全文
Conventional multiple breath-hold two-dimensional(2D) balanced steady-state free precession(SSFP) presents many difficulties in cardiac cine magnetic resonance imaging(MRI). Recently, a self-gated free-breathing three-dimensional(3D) SSFP technique has been proposed as an alternative in many studies. However, the accuracy and effectiveness of selfgating signals have been barely studied before. Since self-gating signals are crucially important in image reconstruction, a systematic study of self-gating signals and comparison with external monitored signals are needed.Previously developed self-gated free-breathing 3D SSFP techniques are used on twenty-eight healthy volunteers. Both electrocardiographic(ECG) and respiratory bellow signals are also acquired during the scan as external signals. Self-gating signal and external signal are compared by trigger and gating window. Gating window is proposed to evaluate the accuracy and effectiveness of respiratory self-gating signal. Relative deviation of the trigger and root-mean-square-deviation of the cycle duration are calculated. A two-tailed paired t-test is used to identify the difference between self-gating and external signals. A Wilcoxon signed rank test is used to identify the difference between peak and valley self-gating triggers.The results demonstrate an excellent correlation(P = 0, R 0.99) between self-gating and external triggers. Wilcoxon signed rank test shows that there is no significant difference between peak and valley self-gating triggers for both cardiac(H = 0, P 0.10) and respiratory(H = 0, P 0.44) motions. The difference between self-gating and externally monitored signals is not significant(two-tailed paired-sample t-test: H = 0, P 0.90).The self-gating signals could demonstrate cardiac and respiratory motion accurately and effectively as ECG and respiratory bellow. The difference between the two methods is not significant and can be explained. Furthermore, few ECG trigger errors appear in some subjects while these errors are not found in self-gating signals. 相似文献
54.
用有限温度场论中重整化群方法,研究了QCD的有效耦合常数的温度依赖性,发现高温时耦合常数在全动量区都很小,体现出了QCD的退禁闭特征。 相似文献
55.
Phase-sensitive fat suppression steady-state free procession sequence with phase correction 下载免费PDF全文
Robust and fast fat suppression is a challenge in balanced steady-state free precession (SSFP) magnetic resonance imaging. Although single-acquisition phase-sensitive SSFP can provide fat-suppressed images in short scan time, phase errors, especially spatially-dependent phase shift, caused by a variety of factors may result in misplacement of fat and water voxels. In this paper, a novel phase correction algorithm was used to calibrate those phase errors during image reconstruction. This algorithm corrects phase by region growing, employing both the magnitude and the phase information of image pixels. Phantom and in vivo imagings were performed to validate the technique. As a result, excellent fat-suppressed images were acquired by using single-acquisition phase-sensitive SSFP with phase correction. 相似文献
56.
This paper presents theoretical computations of the ionization rate of Rydberg lithium atom above the classical ionization threshold using semiclassical approximation. The yielded random pulse trains of the escape electrons are recorded as a function of emission time such that they can be related to the terms of the recurrence periods of the photoabsorption. This fact illustrates that it is ionic core scattering processes which give rise to chaos in autoionization dynamics and this is verified by comparison of our results with the hydrogen atom situation. In order to reveal the chaotic properties in detail, the sensitive dependence of the ionization rate upon the scaled energy is discussed for different scaled energies. This approach provides a simple explanation for the chaotic character in autoionization decay of Rydberg alkali-metal atoms. 相似文献
57.
Correspondence between classical dynamics and recurrence spectra of Rydberg hydrogen atom near a metal surface 下载免费PDF全文
The chaotic behaviours of the Rydberg hydrogen atom near a metal surface are presented. A numerical comparison of Poincare surfaces of section with recurrence spectra for a few selected scaled energies indicates the correspondence between classical motion and quantum properties of an excited electron. Both results demonstrate that the scaled energy dominates sensitively the dynamical properties of system. There exists a critical scaled energy εc, for ε 〈 εc, the system is near-integrable, and as the decrease of ε the spectrum is gradually rendered regular and finally turns into a pure Coulomb field situation. On the contrary, if ε 〉 εc, with the increase of ε, the system tends to be non-integrable, the ergodic motion in phase space presages that chaotic motion appears, and more and more electrons are adsorbed on the metal surface, thus the spectrum becomes gradually simple. 相似文献
58.
Interleaving Gradient Magnetic Field Method for Diffusion Weighted Spectroscopy 总被引:1,自引:0,他引:1 下载免费PDF全文
Diffusion-weighted magnetic resonance spectroscopy (DWS) has considerable potential in clinical and research applications. However, it is seldom implemented in conventional magnetic resonance imaging (MRI) scanners due to the strict hardware requirements. We propose an interleaving gradient magnetic field (IGMF) method based on point resolved spectroscopy (PRESS). Four interlaced powerful diffusion sensitive gradient magnetic fields are positioned around the two π refocusing rf pulses in the PRESS sequence. This method utilizes the interval time in the PRESS pulse sequence, doubles the duration time of the diffusion sensitive gradient magnetic field and decreases the detrimental effect of the induced eddy current. The results of theoretical analysis and experimental observation demonstrate that the IGMF method is suitable for conventional MRI scanners. 相似文献
59.
建立离子色谱抑制电导检测法同时测定饮水中F-、Cl O2-、Cl-、NO2-、Cl O3-、Br-、NO3-、H2PO4-和SO42-9种阴离子的方法。采用SH-AP-2型阴离子交换柱为分离柱,以4.0 mmol/L Na2CO3-4.0 mmol/L Na HCO3溶液为淋洗液,流量为0.80 m L/min,采用等度洗脱方式将9种阴离子完全分离,利用抑制电导检测。Cl O2-、Cl O3-的质量浓度在0.05~2.0 mg/L范围内,SO42- 相似文献