Amplitudes of gamma detected NMRON spin echo signals

The amplitudes of the spin echo and the baseline components of gamma detected NMRON signals have been calculated for the pure Zeeman case using a density matrix approach. Optimum turn angles and the separate and combined influences of line broadening and skin effect, appropriate to bulk ferromagnetic metals, are examined. For narrow line, zero skin effect samples, the triple pulse sequence (cos⁻¹(1/√3))−180°−(180°−cos⁻¹(1/√3)) affords the largest principal echo. However, its advantage is diminished with the introduction of either line broadening or skin effect and is subsequently lost when both these effects are included or either becomes severe. The skin effect distortion of the spin echo signal can dominate over intermediate-to-broad line distortions, for that range of experimental conditions generally applicable tometallic ferromagnetic hosts.     

Gamma detected NMRON spin echo profiles for equal turn angle pulses-zero skin effect

The theoretical profiles for both principal (τ₂=τ₁) and harmonic (τ₂=1/2τ₁, 2τ₁) spin echoes in gamma detected triple pulse NMRON experiments are systematically calculated for a wide range of the parameter for inhomogeneous broadening using analytic expressions derived from physical ensemble rotation considerations, specific to a pure Zeeman Hamiltonian. In the absence of skin effects, and assuming equal turn angle pulses, these calculations provide the optimum turn angle conditions for maximum spin echo amplitude.     

Observation of a new coherent transient in NMR — Two-pulse nutational stimulated echo in the angular distribution of gamma-radiation from oriented nuclei

A new coherent transient in NMR, the two-pulse nutational stimulated echo is reported for the ferromagnetic system⁵⁰CoFe, observed by monitoring the nuclear spin dynamics as a function of the second pulse duration via anisotropic gamma quanta from thermally oriented radioactive nuclei,⁶⁰Co. The mechanism of echo formation under strong Larmor inhomogeneous broadening and the secondary but important role of inhomogeneity associated with the rf amplitude (Rabi freqeuncy) due to skin-effect are investigated via the method of concatenation of perturbation factors in the statistical tensor formalism. For those experiments performed on time scales short compared with irreversible relaxation the theoretical predictions and subsequent experimental time-domain signals are in excellent accord. Remarkable constancy of amplitude of the new gamma-detected two-pulse echo with increase of interpulse time interval is observed, the longitudinal relaxation being manifest in the off-echo signals. Comparisons are made with NMRON four-pulse stimulated off-echo decay (analogous with conventional NMR three pulse stimulated echo) which is also sensitive to longitudinal relaxation, and with three-pulse on-echo decay (analogous with conventional NMR two-pulse Hahn echo) which measures transverse relaxation.     

Photon Echo from Localized Excitons in Semiconductor Nanostructures

An overview on photon echo spectroscopy under resonant excitation of the exciton complexes in semiconductor nanostructures is presented. The use of four-wave-mixing technique with the pulsed excitation and heterodyne detection allowed us to measure the coherent response of the system with the picosecond time resolution. It is shown that, for resonant selective pulsed excitation of the localized exciton complexes, the coherent signal is represented by the photon echoes due to the inhomogeneous broadening of the optical transitions. In case of resonant excitation of the trions or donor-bound excitons, the Zeeman splitting of the resident electron ground state levels under the applied transverse magnetic field results in quantum beats of photon echo amplitude at the Larmor precession frequency. Application of magnetic field makes it possible to transfer coherently the optical excitation into the spin ensemble of the resident electrons and to observe a long-lived photon echo signal. The described technique can be used as a high-resolution spectroscopy of the energy splittings in the ground state of the system. Next, we consider the Rabi oscillations and their damping under excitation with intensive optical pulses for the excitons complexes with a different degree of localization. It is shown that damping of the echo signal with increase of the excitation pulse intensity is strongly manifested for excitons, while on trions and donor-bound excitons this effect is substantially weaker.     

Oscillatory free-induction decay signals in the angular distribution of nuclear radiation in ferromagnetic metals under strong inhomogeneous broadening

The problem of the oscillatory free induction decay in the angular distribution of nuclear radiations is theoretically examined for inhomogeneous linewidths, with and without the influence of the skin effect, utilizing the method of concatenation of perturbation factors. The second rank detection tensor applicable to gamma detected pulsed NMRON signals from ferromagnetic metals is examined for axial geometry. Important extensions to the pulse area theorem originally developed only for the first rank detection tensor are proposed for second rank tensors.     

Multiquantum triple-pulse spin-echo signals from quadrupole nuclei in magnetically ordered substances

We analyze theoretically the formation of NMR pulse responses from a quadrupole spin system in which the inhomogeneous broadening of a spectral line is due to both magnetic and electric quadrupole interactions. We derive formulas for the moments of formation of multiquantum echo signals in the case of three exciting pulses. For the first time we detected in experiments multiquantum spin-echo signals from copper nuclei in ferromagnetic copper sulfochromite in the cases of double-pulse and triple-pulse excitations. We find that there is good agreement between the calculated and experimentally observed moments of echo signal formation. Zh. éksp. Teor. Fiz. 115, 2106–2112 (June 1999)     

Radiative detection of NMR studies of domain nuclei in enriched ferromagnetic iron

Combined host (∼95 at% enriched stable ⁵⁷Fe) and very dilute impurity (∼0.01 at% radioactive ⁶⁰Co) NMR signals have been obtained on the one sample of polycrystalline Fe foil utilising perturbations to the gamma anisotropy from in situ thermally oriented ⁶⁰Co nuclei for both resonances. The NMR-TDNO signals on the ⁵⁷Fe sites have been followed down to applied magnetic fields well below the host magnetic saturation and exhibited two distinct components; a strong, narrow homogeneous signal, superimposed over a broader inhomogeneous signal. The impurity ⁶⁰Co⁵⁷Fe inhomogeneous resonance has been studied with three pulse NMRON and the irreversible decay of the nuclear spin echo measured as a function of applied magnetic field. This revised version was published online in August 2006 with corrections to the Cover Date.     

Pulsed NMRON studies of insulating magnetic materials

The apparatus and instrumental techniques developed for pulsed NMRON studies of insulating ordered magnets are described. Experiments have been performed on two salts,⁵⁴Mn-MnCl₂·4H₂O and⁵⁴Mn-Mn(COOCH₃)₂·4H₂O, and rotation patterns, free induction decays and spin echoes have been obtained for both single quantum and double quantum transitions. The pulsed technique can produce signals larger than those obtained in CW NMRON, and is well suited for the measurement of the very short spin-lattice relaxation times that can occur in these systems.     

Two-pulse nuclear echo signal in magnetically ordered media

An analytical expression is obtained for the amplitude of a nuclear echo signal generated in magnetically concentrated materials upon resonant excitation of the nuclear subsystem by two pulses with an equal amplitude. It is revealed that, when both the inhomogeneous broadening of the spectroscopic transition and the inhomogeneous gain distribution are taken into account in the theory simultaneously, the spin echo signal can consist of eight satellites symmetrically located around the central peak. The analytical expressions obtained are used to determine relations between the relative amplitudes and the instants of generation of satellites, on the one hand, and microscopic parameters, such as the inhomogeneous half-width of the spectral line, the halfwidth of the gain distribution function, and the average gain, on the other hand. The results of theoretical calculations and the experimental data obtained for the FeV (2 at % V) alloy are in good agreement.     

Pulsed nuclear magnetic resonance signals in magnetically ordered materials

We have obtained an analytical expression for nuclear precession and nuclear echo signals generated in magnetically ordered materials upon resonant excitation of the nuclear subsystem by two pulses of identical amplitude but different durations. We show that in a nuclear subsystem with inhomogeneous broadening of the spectroscopic transition and an inhomogeneous gain distribution, the two-pulse precession and echo signals are split into four and nine components respectively. We have analytically established a correlation between the macroscopic parameters of the components of the two-pulse signals (relative amplitudes, signal formation times) and the microscopic parameters of the magnetically ordered media (inhomogeneous half-width of the spectral line, half-width of the gain distribution function, average gain). The theoretically calculated formation times for the components of the nuclear precession and nuclear echo signals agree with the experimental data obtained for the alloy FeNiCo (70% Co). __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 529–538, July–August, 2008.     

Formation of the pulsed electron-electron double resonance signal in the case of a finite amplitude of microwave fields

A theoretical study of the effect of microwave (MW) fields of finite amplitude on the process of pulsed electron-electron double resonance (PELDOR) signal formation is carried out. It is shown that the behavior of the experimentally observed values can be described by four vectors of partial magnetizations whose motion is reduced to precession in effective magnetic fields. In the case of strong spin-spin interaction, the PELDOR effect can be observed when a sufficiently powerful MW field is applied at pumping frequency to affect both components of the Pake doublet. The possibility of a “two-frequency” spin echo to appear under the action of two pulses with different carrier frequencies in the system where the inhomogeneous broadening of the electron spin resonance line contour is mainly determined by the dipole-dipole interaction is demonstrated.     

Homonuclear vanadium-51 dipolar couplings in inorganic solids obtained via hahn spin echo decay NMR spectroscopy

Dipolar dephasing of the magnetization following a Hahn spin echo pulse sequence potentially provides a quantitative means for determining the dipolar second moment in solids. In this work, the possibility of employing Hahn spin echo decay spectroscopy to obtain quantitative ⁵¹V–⁵¹V dipolar second moments is explored. Theoretical spin echo response curves are compared to experimental ones for a collection of crystalline vanadium-containing compounds. This work suggests that ⁵¹V dipolar second moments can be obtained by selectively exciting the central m = 1/2 → −1/2 by a Hahn echo sequence for vanadate compounds with line broadening no greater than approximately 220 ppm. For vanadates with greater broadening of the central transition due to chemical shift, second-order quadrupolar, and dipolar interactions, off-resonance effects lead to an oscillatory time dependence of the spin echo. Experimentally determined second moments of the normalized echo decay intensities lie within 10–33% of the calculated values if the second moments are extrapolated to zero evolution time due to the time scale dependence of spin exchange among neighboring vanadium nuclei. Alternatively, the second moments can be obtained to within 10–25% of the calculated values if the broadening of the central transition due to chemical shift and second-order quadrupolar effects can be estimated.     

Two-frequency multiple-pulse sequences in nitrogen-14 NQR

The theoretical investigation of the multiple-pulse sequences in which the radio-frequency pulses are applied simultaneously at two transitions are presented. The time-independent solution of the quantum Liouville equation for the average Hamiltonian of inhomogeneous broadening in the sequence is obtained by the projection operator technique. This solution predicts the multiple echo signals on the third transition that is not irradiated by radio-frequency pulses.     

脉冲激光探测平面目标特性对测距分布的影响

针对脉冲激光近程探测系统平面目标特性影响测距分布的问题,推导了平面目标脉冲响应方程和脉冲激光回波方程,分析了不同平面目标倾斜角、激光发射发散角和激光发射脉宽对激光回波展宽的影响.基于脉冲激光回波方程和恒阈值时刻鉴别方法,推导了平面目标的测距概率密度函数解析式,并理论仿真分析了不同平面目标倾斜角、激光发射功率、激光发射发散角及阈噪比对测距统计特性分布的影响.运用蒙特卡罗算法进行全波形模拟测距实验;搭建脉冲激光测距实验环境,进行20 m测距实验.实验结果表明:理论仿真、蒙特卡罗模拟实验与实际实验的测距概率密度分布基本一致,随着平面目标倾斜角的增大,测距均值和测距方差增大;当倾斜角度为0°,20°,40°,60°时,回波信噪比高于阈噪比,测距分布呈现高斯分布;当倾斜角度为70°时,回波信噪比低于阈噪比,分布不再呈现高斯分布,呈现上升沿缓下降沿陡的分布特性.研究结果为研究目标平面特性对脉冲激光探测测距分布的影响提供了理论依据.     

Effect of Pumping Pulse Duration on Echo Signal Amplitude in Four-Pulse PELDOR

The impact of pumping pulse duration on four-pulse pulsed electron?Celectron double resonance (PELDOR) data was experimentally studied. For biradicals with known distances between two spin labels, it is shown that refocused echo amplitude decreases with increasing the pumping pulse duration and decreasing the distance between spin labels. The effect becomes substantial when the pumping pulse duration is comparable or exceeds the inverse value of the dipole?Cdipole interaction between spin labels. This effect is essential for determination of distance distribution between labels in double-labeled molecules and for determination of the number of labels in clusters of spin-labeled molecules. PELDOR signal distortion was observed when the pumping pulse position in the time scale coincided with those of the detecting pulses. An approach of signal correction to eliminate this distortion is proposed.     

Observation of a new coherent transient in NMR -- nutational two-pulse stimulated echo in the angular distribution of \gamma-radiation from oriented nuclei

A new coherent transient in pulsed NMR, the two-pulse nutational stimulated echo, is reported for the ferromagnetic system ⁶⁰CoFe using resonant perturbations on the directional emission of anisotropic \gamma-radiation from thermally oriented nuclei. The new spin echo is a result of non-linear nuclear spin dynamics due to large Larmor inhomogeneity active during radiofrequency pulse application. It is made readily observable through the gross detuning between NMR radiofrequency excitation and gamma radiation detection, and inhomogeneity in the Rabi frequency caused by metallic skin-effect. The method of concatenation of perturbation factors in a statistical tensor formalism is quantitatively applied to successfully predict and then fit in detail the experimental time-domain data. This revised version was published online in August 2006 with corrections to the Cover Date.     

Double PGSE NMR with Stimulated Echoes: Phase Cycles for the Selection of Desired Encoding

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.     

Raman amplification of ultrashort light pulses in inhomogeneously broadened three-level systems

In the limit of pulse durations that are shorter than the characteristic times of atomic decays, the dynamics of the propagation of a probe light pulse under the action of a driving pulse in the form of a self-induced transparency soliton applied to the adjacent transition of the V-scheme is considered. For a three-level absorber, it is shown that, in the absence of two-photon inhomogeneous broadening, the effect of soliton-induced transparency (SOIT) for the probe field takes place, which consists of the probe pulse undergoing forced localization by the driving pulse and propagates further without a change in the energy or shape. This type of transparency has no threshold with respect to both the energy and the area and is observed for arbitrarily weak pulses. The SOIT effect disappears in the presence of a two-photon inhomogeneous broad-ening. The effect of Raman amplification in a three-level amplifier with the Raman population inversion is considered. It is demonstrated that, under the action of the two-photon inhomogeneous broadening, the effect of Raman amplification becomes a threshold effect. The broader the two-photon inhomogeneous contour, the larger the Raman inversion required to initiate generation.     

Effect of a pulse entrapping on weak localization of waves in a resonant random medium

We consider theoretically a new physical effect in coherent backscattering enhancement (CBE) of electromagnetic or acoustic non-stationary waves from a discrete random medium under condition of Mie resonant scattering. The effect manifests itself as an angle-cone broadening of a short pulsed signal CBE from the resonant random medium, compared with the case of a non-resonant random medium. The cone broadening is associated with a pulse-entrapping effect when the pulse, while propagating within the resonant random medium, spends most of the time being 'entrapped' inside scatterers. A theory for the predicted effect is based on, first, the well known relation between the contributions of the ladder and cyclical diagrams to the time spectral density of the wave electric field coherence function and, second, a recently derived radiative transfer equation with three Lorentzian kernels of delay describing a pulse entrapping in an ensemble of resonant point-like scatterers. Using the generalized Chandrasekhar H-function, we obtain an exact analytic expression for the non-stationary albedo of the semi-infinite resonant random medium, taking into account the phenomena of a pulse CBE and entrapping. A simple analytic asymptotics is found for the albedo of the later part of the scattered pulse. This asymptotics shows quantitatively how the entrapping affects the peak amplitude and peak line shape of the CBE of a short pulse.     

Impact of echo broadening effect on active range-gated imaging

Analytical formulas and experimental proof of the echo broadening effect in active range-gated imaging, including atmospheric interference, currently exist. We investigate the impact of this effect on target detection. Our research demonstrates that the echo broadening effect affects the energy profile of the depth of view and collects only part of the signals of targets in head and tail zones. Under bad weather conditions, the effect weakens the signal-to-noise ratio (SNR) of images, especially in cases with large laser pulse width. Fortunately, by modifying the laser pulse width, the effect can be controlled. These results are valuable to the applications of active range-gated imaging.