Free-induction decay in a fluctuating two-level system

We study theoretically free-induction decay (FID) in an inhomogeneously broadened system of two-level atoms whose transition frequencies fluctuate according to the Ornstein-Uhlenbeck process. A numerical solution of the model shows that at low intensities the saturation behaviour of FID may depend strongly on the statistics of the fluctuations, but at high intensities the fluctuations play a minor role. The results mimic qualitatively the recent experimental data of DeVoe and Brewer, but a full quantitative agrrement with experiments is not reached.     

Chirped-pulse millimeter-wave spectroscopy of Rydberg-Rydberg transitions

Transitions between Rydberg states of Ca atoms, in a pulsed, supersonic atomic beam, are directly detected by chirped-pulse millimeter-wave spectroscopy. Broadband, high-resolution spectra with accurate relative intensities are recorded instantly. Free induction decay (FID) of atoms, polarized by the chirped pulse, at their Rydberg-Rydberg transition frequencies, is heterodyne detected, averaged in the time domain, and Fourier transformed into the frequency domain. Millimeter-wave transient nutations are observed, and the possibility of FID evolving to superradiance is discussed.     

Selective excitation in pulsed EPR of a spin-correlated triplet-radical pair

Experiments are described in which a low-amplitude microwave pulse excites only one out of three allowed transitions of the quinone radical (Q(A)(-)) in a spin-correlated triplet-radical pair 3PQ(A)(-) of the bacterial photosynthetic reaction center. A second high-amplitude pulse produces a FID whose temporal shape is strongly modulated with frequencies determined by electron-electron dipolar interaction in the pair. The FID is detected in both the in-phase and the out-of-phase channels. The out-of-phase FID is a result of switching off the magnetic dipolar interaction between 3P and Q(A)(-) due to decay of 3P during the time interval between the two pulses. Refocusing of FID by an additional non-selective pulse allows a dead-time free measurement of this modulation. The influence of the dead-time problem on the distance determination is discussed.     

Sound propagation and speech transmission in a branching underground tunnel

The characteristics of sound propagation and speech transmission along a tunnel with a "T" intersection were investigated. At receivers within sight of the sound source, low frequencies were mainly attenuated around the intersection than high frequencies. At receivers out of sight of the source, high frequencies were extensively attenuated. The overall pattern of sound attenuation along the different sections of tunnel, which was calculated by the conical beam method, agreed well with the measurements in this study. Numerical calculations of reflected and diffracted waves with minimum transmission paths in a two-dimensional plane showed that reflected waves were the primary contributors to sound fields out of sight of the source. The articulation scores measured at receivers within sight of the source were high, and most of the confusion concerned syllables that could easily be misheard, even if there were a high signal-to-noise ratio. The types of syllable confusions observed at the receivers out of sight of the source appeared to have been caused by the greater deterioration in speech signals along this part of the tunnel, especially at high frequencies. The evaluation by rapid speech transmission indices (RASTI) appeared to be overestimated at the receivers out of sight of the source. Taking into account the early decay times of impulsive sound and the calculation procedures used in RASTI, it is concluded that speech intelligibility may not have been evaluated correctly by RASTI.     

Eigenmodes in the long-time behavior of a coupled spin system measured with nuclear magnetic resonance

The many-body quantum dynamics of dipolar coupled nuclear spins I=1/2 on an otherwise isolated cubic lattice are studied with nuclear magnetic resonance. By increasing the signal-to-noise ratio by 2 orders of magnitude compared with previous reports for the free induction decay (FID) of (19)F in CaF(2) we obtain new insight into its long-time behavior. We confirm that the tail of the FID is an exponentially decaying cosine, but our measurements reveal a second decay mode with comparable frequency but twice the decay constant. This result is in agreement with a recent theoretical prediction for the FID in terms of eigenvalues for the time evolution of chaotic many-body quantum systems.     

Study of Water States in Different Hydrated and Consolidated Clay Using Nuclear Magnetic Resonance

ABSTRACT

The aim of the present work was to test low-resolution NMR to characterize and quantify the state of water in some clay materials such as illite, montmorillonite, and bentonite with water content and in a compacted state. Low-field ¹H-NMR spectroscopy (0.54 T) was used to determine the most mobile fraction of water in different types of clay materials by analyzing free induction decay (FID) and Carr-Purcell-Meiboom-Gill (CPMG) relaxation curves at 20°C, ?7°C, and ?25°C. Results have shown that a fast exchange occurs between the first bound monolayer of water and the following ones for illite, montmorillonite, and bentonite; a multiexponential decay of some CPMG relaxation curves was also observed. FID measurements at ?25°C showed that 83% of the water in illite (41% water content), 44% in montmorillonite (31% w.c.), and 52% in bentonite (27% w.c.) is frozen at this temperature. Treatment of signals obtained with bentonite compacted in capillaries (28% w.c.) was complicated by a signal coming from the capillaries themselves and an increase in internal magnetic field gradients, which led to a dual quantification whose most likely result is 39% of freezable water.     

GaAs中的飞秒自由感应衰减

用双光束诱导半导体ＧａＡｓ的激子相干态,记录液氮温度样品的光电导信号随双脉冲之间延时的变化。实验与理论分析结果表明：对于均匀增宽的二能级体系,这种中以直接反映相干态的自由感应衰减。求解均匀增宽二能级体系,这种测量方法可以直接反映相干态的自由感应衰减。求解均匀增宽二能级体系的密度矩阵方程,拟合实验结果得到载流子相干态的退相时间Ｔ２＝１１５ｆｓ。     

Artifacts introduced by zero order phase correction in proton NMR spectroscopy and a method of elimination by phase filtering

In in-vivo applications of proton NMR spectroscopic imaging, an oscillatory "ringing" artifact has been observed in some of the spectra. The source of this artifact was found to be the presence of a harmonic "beating" effect in the amplitude of the water reference free induction decay (FID) which was used for zero order phase correction for B0 inhomogeneity and eddy current compensation. The source of the beats was found to be the presence of distinct populations of spins resonating at slightly different frequencies. When the common method of zero order phase correction was implemented using such an FID, the resulting phase-corrected, water-suppressed spectra displayed ringing. Examination of the unwrapped phase correction angle revealed unexpected jumps in phase at points in time corresponding to nodes in the amplitude of the FID. Low-pass filtering of the phase correction angle of the reference FID was found to smooth out these unanticipated phase jumps. When used as a reference for phase correction, the filtered phase information gave a phase-corrected, water-suppressed spectrum free from ringing.     

Gaussian approximation in the theory of MR signal formation in the presence of structure-specific magnetic field inhomogeneities

A detailed theoretical analysis of the free induction decay (FID) and spin echo (SE) MR signal formation in the presence of mesoscopic structure-specific magnetic field inhomogeneities is developed in the framework of the Gaussian phase distribution approximation. The theory takes into account diffusion of nuclear spins in inhomogeneous magnetic fields created by arbitrarily shaped magnetized objects with permeable boundaries. In the short-time limit the FID signal decays quadratically with time and depends on the objects' geometry only through the volume fraction, whereas the SE signal decays as 5/2 power of time with the coefficient depending on both the volume fraction of the magnetized objects and their surface-to-volume ratio. In the motional narrowing regime, the FID and SE signals for objects of finite size decay mono-exponentially; a simple general expression is obtained for the relaxation rate constant deltaR2. In the case of infinitely long cylinders in the motional narrowing regime the theory predicts non-exponential signal decay lnS approximately -tlnt in accordance with previous results. For specific geometries of the objects (spheres and infinitely long cylinders) exact analytical expressions for the FID and SE signals are given. The theory can be applied, for instance, to biological systems where mesoscopic magnetic field inhomogeneities are induced by deoxygenated red blood cells, capillary network, contrast agents, etc.     

Application of double spin echo spiral chemical shift imaging to rapid metabolic mapping of hyperpolarized [1-¹³C]-pyruvate

Undersampled spiral CSI (spCSI) using a free induction decay (FID) acquisition allows sub-second metabolic imaging of hyperpolarized 13C. Phase correction of the FID acquisition can be difficult, especially with contributions from aliased out-of-phase peaks. This work extends the spCSI sequence by incorporating double spin echo radiofrequency (RF) pulses to eliminate the need for phase correction and obtain high quality spectra in magnitude mode. The sequence also provides an added benefit of attenuating signal from flowing spins, which can otherwise contaminate signal in the organ of interest. The refocusing pulses can potentially lead to a loss of hyperpolarized magnetization in dynamic imaging due to flow of spins through the fringe field of the RF coil, where the refocusing pulses fail to provide complete refocusing. Care must be taken for dynamic imaging to ensure that the spins remain within the B?-homogeneous sensitive volume of the RF coil.     

新药磺胺嘧啶锌中有机溶剂残留量测定

建立了国家二类新药磺胺嘧啶锌中有机溶剂甲醇和丙酮残留量的毛细管顶空气相色谱分析法。采用PEG-20M弹性石英毛细管色谱柱,F ID检测器,程序升温方法,以氮气为载气。两种有机溶剂完全分离,在浓度范围内具有良好的线性（r均大于0.999）,丙酮和甲醇的检出限分别为3.7m g/L和2.9m g/L,精密度RSD均小于1%,平均回收率分别为101.1%和100.8%.经实验方法学验证,该方法灵敏、准确可靠,适用于磺胺嘧啶锌原料药中有机溶剂残留量的测定。     

Rapid and accurate measurement of transverse relaxation times using a single shot multi-echo echo-planar imaging sequence

Methods for making rapid and accurate measurements and maps of the transverse relaxation time from a single free induction decay (FID) are proposed. The methods use a multi-echo sequence in combination with B1 insensitive (hyperbolic secant or BIREF2b) refocusing pulses and rapid echo-planar imaging techniques. The results were calibrated against a single spin echo echo-planar imaging sequence using a phantom containing a range of CuSO4 concentrations. The mean percentage absolute difference between the multi-echo and single-echo results was 3% for the multi-echo sequence using the hyperbolic secant refocusing pulse, and 7% for the multi-echo sequence using the BIREF2b refocusing pulse, compared to 13% for a multi-echo sequence using a nonselective sinc refocusing pulse. The use of the sequences in vivo has been demonstrated in studies of gastric function, i.e., the measurement of gastric dilution and monitoring of formation of a raft of alginate polysaccharide within the stomach.     

Free-induction decay in molecular iodine measured with an extended-cavity diode laser

The free-induction decay (FID) technique combined with broadly tunable lasers offers a convenient way to survey molecular dephasing times. We measured FID's in molecular iodine by frequency switching an extended-cavity diode laser with a small current modulation. These decay times provide valuable information on the Doppler-free linewidths for frequency-standard applications.     

Static nuclear spin polarization induced in a liquid by a rotating magnetic field

We demonstrate that protons in a liquid acquire a static polarization perpendicular to the plane of a rotating magnetic field. The rotating field was reduced adiabatically to zero, transforming the static polarization in the rotating frame to the laboratory frame. The application of a small magnetic field perpendicular to the polarization induced a free induction decay (FID) that was detected by a superconducting quantum interference device. The results agree with the predictions of the modified Bloch equations. The FID remained observable in the presence of magnetic material, suggesting that this technique may find practical applications.     

Coherent control of spin precession motion with impulsive magnetic fields of half-cycle terahertz radiation

Coherent control of the precession motion of magnetizations in a single crystal YFeO3 with double half-cycle pulse terahertz waves was demonstrated. Quasiferromagnetic (0.299 THz) and quasiantiferromagnetic (0.527 THz) precession modes were selectively excited by choosing an appropriate interval of two pulses and were observed as free induction decay (FID) signals from the spin system. By observing the circularly polarized FID signals due to ferromagnetic resonance, we also succeeded in confirming directly the energy storage in the spin system and recovery from that to the electromagnetic radiation.     

Modulation of optical FID and structure of absorption line due to superhyperfine interaction in ruby

Modulation of optical free induction decay (FID) and structure of R₁ absorption line due to superhyperfine interaction in ruby have been observed by using Stark-switching and Stark-sweeping techniques, respectively. The inhomogeneous broadening is effectively eliminated by a narrow optical hole burning. Frequency and phase characteristics of the FID signal have also been examined. The agreement between experimental results and theoretical calculations is quite satisfactory.     

Phase relaxation in disordered nuclear paramagnets

A new method for the construction of main correlation functions of nuclear paramagnets has been developed on the basis of the combination of the projection technique and cumulant expansions. It gives satisfactory results for the free induction decay and resonance line shape function in magnetically concentrated crystals. Polarization transfer in magnetically dilute crystals has been taken into account. It significantly retards a decrease in the FID at times longer than the phase relaxation time T ₂. The FID remains mainly monotonic in contrast to magnetically concentrated crystals, where the FID oscillates. The results have been compared to the existing experimental data.     

Molecular decay rates and emission frequencies in the vicinity of an anisotropic metamaterial

The decay rates and emission frequencies of a fluorescing molecule in the vicinity of an anisotropic metamaterial are studied within a phenomenological model. A special kind of indefinite medium recently shown to have a broadband all-angle negative refraction is studied for its interaction with the emitting dipole in the long wavelength limit at close molecule-substrate distances. In addition, materials of alternating metal-dielectric stratified layers are also studied. For the first kind which consists of a composite of metallic nanowires and a dielectric host, the results show that large molecular decay rates (hence exceedingly short lifetimes) occur when negative refraction takes place within the medium; and large blue-shifts can occur for the emission frequencies of the admolecues. For the stratified layered system, abrupt changes in both the emission lifetimes and frequencies can also take place upon negative refraction, although to a much less extent. These abrupt changes can thus provide signatures for probing the transition to such unusual optical property for the medium via surface-fluorescence experiments.     

Optical coherent transient effects in a magnetized semiconductor quantum wire

Effects of an external magnetic field on free induction decay (FID) and the Stark effect in GaAs/GaAlAs quantum wires have been investigated analytically. Our results show that both FID and the Stark effect become enhanced due to the presence of a magnetic field. We have also seen that the magnetic field plays an important role in wider wires while in thinner wires, the geometric confinement dominates over the magnetic effects. The results are found to be in good qualitative agreement with that available in the literature.     

基于过抽样Gabor变换的核磁共振FID信号增强算法

基于作者先前提出的过抽样实值离散Gabor变换,本文提出了一有效的算法用于核磁共振自由感应衰减（NMR FID）信号的减噪. 由于NMR FID信号在时域中是一短暂的振荡衰减信号,使得变换后的NMR FID信号能量在时频域中集中在少数变换系数上,而噪声则遍布在整个变换系数上,因此通过对变换系数幅度进行阈值限制方法可达到明显地增强NMR FID信号的目的. 文中在理论和模拟实验上分析表明,过抽样Gabor变换比临界抽样Gabor变换更适宜于NMR FID信号的减噪,因为在过抽样条件下比在临界抽样条件下的综合窗及其对应的分析窗,无论是在时域中还是在频域中都可具有更好的局域分布集中性,同时,Gabor变换在过抽样条件下也比在临界抽样条件下具有更高的时频精度.