Filter-exchange PGSE NMR determination of cell membrane permeability

A new PGSE NMR sequence is introduced for measuring diffusive transport across the plasma membrane of living cells. A “diffusion filter” and a variable mixing time precedes a standard PGSE block for diffusion encoding of the NMR signal. The filter is a PGSE block optimized for selectively removing the magnetization of the extracellular water. With increasing mixing time the intra- and extracellular components approach their equilibrium fractional populations. The rate of exchange can be measured using only a few minutes of instrument time. Water exchange over the plasma membrane of starved yeast cells is studied in the temperature range +5 to +32 °C.     

Analytical efficiency of the molecular rotational quantum echo

The sensitivity of the delay time of the molecular rotational quantum echo to change in the rotational constants of molecules was investigated with a model of the rotational quantum echo considered as an optically induced quantum synchronization of the Fourier rotational modes of an asymmetric molecular top. The J-echo and the K-echo were investigated for the A- and C-configurations of rotation of a top molecule. The contour maps of the dependence of the sensitivity of the delay time of these types of echo to changes in the rotational constants on the parameters of the molecular tops have been obtained for all the rotational constants. They show that the delay time of the rotational quantum echo is greatly dependent on the configuration of rotation of the top molecule and on the rotational constants. It is shown that the optical probing of the rotational quantum echo can also be performed by the optical rotation of the investigated molecule, and in this case the rotational force is dependent on the intramolecular orientation of the magnetic dipole moment. It was determined that a high sensitivity of the delay time of the echo to changes in the rotational constants can be responsible for the broadening of the echo pulse and even for its disappearance. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. Skorina Ave., Minsk, 220072, Belarus; e-mail: lssm@imaph.bas-net.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 758–764, November–December, 1999.     

Dynamic properties of P4O6S and P4O7: P spin–echo and P MAS–NMR investigations

The rotational dynamics of P₄O₆S and P₄O₇ in the solid state were studied by means of ³¹P NMR spectra of spinning and static powder samples in the temperature range of 153–295 K and 295–388 K, respectively. All spectra were simulated to confirm the type of the motion and to extract the time scales as a function of the temperature. Good agreement between experimental and theoretical data was obtained on the basis of a three-site jump model. For P₄O₆S, the activation energy and the pre-exponential factor derived from the lineshape simulations amount to 51(2) kJ/mol and 6(3)·10¹⁵ s⁻¹. For P₄O₇, the spectral analysis yields an activation energy of 67(1) kJ/mol and a pre-exponential factor of 6(2)·10¹⁴ s⁻¹. The dynamic behavior was checked independently by lineshape analyses under both MAS and static conditions. Activation energies are consistent within the errors for the lineshape analyses. Additionally, we have analyzed spin–lattice relaxation measurements, which show the correct trends for the activation energies.     

Magic Echo Solid-State NMR Imaging without a Rapidly Switchable Field Gradient

To relax the high-speed requirement imposed on the gradient system used in solid-state proton imaging, we propose two simple modifications of the magic echo imaging sequence, TREV-16TS. In the first modification, the applied gradient is inverted in the middle of the RF irradiation; the second modification utilizes a sinusoidal gradient synchronized with the RF sequence. It is estimated by experiments that as long as the RF amplitude is at least about 10 times stronger than the resonance offset induced by the gradient, the spatial resolution is not degraded significantly by the line narrowing deterioration due to the gradient applied during the on-resonance RF irradiation. The modifications allow commercially available standard gradients to be used for the magic echo imaging of solids.     

Photon-echo detected nuclear quadrupole resonance spectroscopy

Rare-earth-ion-doped crystals (REICs) have played an important role in quantum information processing due to their excellent coherent properties. In order to obtain the information regarding the hyperfine structures of the rare-earth ions in REICs, optically detected nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) techniques based on RF resonance and various optical detection methods are widely employed in previous works. Here we demonstrate a new method of NQR spectroscopy based on the photon-echo detection. The hyperfine spectra of the ground state (⁷F₀) and the optically-excited state (⁵D₀) of ¹⁵¹Eu³⁺ in Y₂SiO₅ at zero field are obtained. This method can determine the hyperfine splittings within the ground state and the optically-excited state and is shown to be robust against electrical noise. Our results provide an alternative way for optical detection of NMR and NQR with high signal-to-noise ratio.     

Linear and nonlinear optical properties of semiconductor nanorings with magnetic field and disorder - Influence on excitons and biexcitons

Linear and nonlinear optical absorption spectra are studied theoretically for semiconductor nanorings penetrated by a magnetic field. Due to the Aharanov-Bohm effect the spectral position as well as the oscillator strength of the exciton change periodically as function of the magnetic flux enclosed by the ring. In the nonlinear differential absorption spectra it is found that the magnetic field strongly modifies Coulomb many-body correlations. In particular, the magnetic-field-induced increase of the exciton binding energy is accompanied by a decrease of the biexciton binding energy. The persistence of these effects in the presence of energetic disorder is analyzed. Received 31 January 2001     

高斯回波模型在超声回波模拟中的应用及其迭代算法的讨论

本文将超声回波模拟成被高斯白噪声污染的高斯回波，并建立关于一系列估计参数的非线性模型，如回波的频带宽度，到达时间，中心频率，相位及回波幅度。这些参数对于超声无损检测中材料检测、目标探测、目标分类、及速率测量等密切相关。运用高斯-牛顿算法进行迭代计算，该算法能以较少的迭代次数计算出较为精确的参数向量，但该算法对到达时间T的初值设置较为敏感。     

A modified PGSE for measuring diffusion in the presence of static magnetic field gradients

With a proper timing of pi pulses, it is possible to reduce the effect of the static internal magnetic field gradient on the measurement of diffusion with the pulsed gradient spin echo (PGSE). A pulse sequence that in the first order eliminates the effect of weak internal static gradients in a standard PGSE experiment is introduced. The method should be applied in the cases, where strong and short magnetic gradient pulses are used to investigate the motion of liquid in heterogeneous samples with large susceptibility differences such as porous media.     

Cross-relaxation effects in stimulated-echo-type PGSE NMR experiments by bipolar and monopolar gradient pulses

Exchange of longitudinal spin polarization by dipolar cross relaxation between nonequivalent spins results in a modulation of the stimulated echo signal on increasing the encoding/decoding delays and in a multiexponential decay on increasing the diffusion time. These artifacts are suppressed by 180° pulses inserted in the middle of the gradient encoding/decoding periods. The efficiency of the gradient encoding is preserved if bipolar gradient pulses are used instead of monopolar pulses. The behavior of the different pulse sequences is demonstrated by ¹⁹F PGSE NMR experiments in a lyotropic liquid crystal in both isotropic micellar and oriented nematic phases.     

Properties of the HYSCORE spin echo signal

In hyperfine sublevel correlation spectroscopy (HYSCORE), the finite duration of the microwave pulses leads to an incomplete inversion of the electron spin magnetization by the third pulse, which results in a significant admixture of stimulated ESEEM to HYSCORE ESEEM. This virtually unavoidable contribution of stimulated ESEEM seriously hampers the analysis of the modulation amplitudes in HYSCORE. In this work, we analyze the properties of the spin echo signals contributing to the composite HYSCORE signal. Based on this analysis, we propose the strategies of HYSCORE data acquisition and processing that allow one to practically eliminate the contribution of the stimulated echo and make the HYSCORE ESEEM analyzable in quantitative terms.