Quadrupole echo decay

Morrow et al. measured the spin-lattice relaxation time Ti and quadrupole echo decay times T ) of headgroup deuterated d4-DMPC as a function of temperature and pressure to yield additional information about changes in the headgroup dynamics. Generally, motions in a LC phospholipid bilayer can... 

At the present time it is unclear whether the spin echo decay spectroscopy approach successfully demonstrated for Na is generally transferable to other alkali ion nuclei such as Li and Cs. The latter two isotopes have moderately small electric quadrupole moments causing first order quadrupolar splittings that are comparable to the radio frequency excitation window. As a result, the 7i-pulse length is ill-defined in many situations and the contribution of dipolar coupling of the observed spins to nuclei in the outer Zeeman levels is difficult to quantify. Nevertheless, some promising initial results on cesium borate... 

Hollander and Prins investigated the effect of pressure on the glass-transition temperature Tg in atactic polypropylene in the pressure range up to 5000 bar. The decay rate of the deuteron-NMR quadrupole echo was used to monitor the glass transition. In further papers the same authors also studied the methyl group, segmental and chain motions of this molecular system. 

To bypass receiver deadtime effects, wideline spectra are derived by Fourier transformation of the decay of an echo. By use of the Hahn echo and the stimulated echo (Section 2.2.1), wideline spectra of and other spin-5 nuclei can be measured, for example, but not the spectra of dipolar coupled spins and of quadrupolar nuclei like H. The magnetization of nuclei with spin / = 1 can be refocused by the quadrupole echo or the solid echo, and by the Jeener-Broekaert echo or the alignment echo [Slil] (Fig. 3.2.6). 

A quadrupole echo is generated similar to a Hahn echo by two pulses which are separated by half the echo time t = t /2 (Fig. 3.2.6(a)). However, the second pulse is a 90° and not a 180° pulse, and it is shifted in phase by 90° with respect to the first. The quadrupole echo appears at a time t2 = t j2 after the second pulse. The spectra shown in Fig. 3.2.5 have been simulated for the quadrupole echo technique with acquisition of the echo decay during h — t. Clearly, the lineshape strongly depends on type and time scale of the motion. In this way, molecular reorientation with correlation times in the range of 10 s < Tc < 10 s can be characterized by H NMR [Laul, Wehl]. Faster motion with correlation times in the range of 10 s < < 10" s can be investigated... 

In general the Cooper pairs in conventional superconductors induced by phonons have. -symmetry where the gap opens uniformly on the Fermi surface and the temperature dependence of physical quantities below Tc is exponential. On the other hand, when the attractive force originates from spin or electron charge fluctuations, the Cooper pair has p- or d-wavc symmetry where the gap disappears on lines or points on the Fermi surface and the physical quantities have power-law temperature dependences. The quantities that are measured by NMR and nuclear quadrupole resonance (NQR) are the nuclear spin-lattice relaxation rate, 1 / T, the Knight shift, K, the spin echo decay rate, 1/T2 and the NQR frequency, vq. The most important quantities, K and 1/77 for the determination of the symmetry of the Cooper pairs are reviewed in the following sections. 

Before the advent of Fourier transform spectrometers, wide-line NMR was done by sweeping the magnetic field and observing the dispersion signal, or by pulsing the radiofrequency and observing the free Induction decay without transformation. The very broad spectral widths have caused problems with baselines and faithful representations of the entire llneshapes. Various techniques, such as the quadrupole echo (lA), progressive phase alternation of the excitation pulse and detector, short spectrometer dead times, and post-acquisition spectral correction (15) have circumvented most of these. 

Spin-lattice relaxation times (Tx) were obtained by the saturation-recovery method. Transverse relaxation times (T2) for the narrow central component of the line shapes were measured directly from the Lorentzian line shapes. The line shapes are Fourier transforms (FT) of quadrupole echo (QE) or free induction decay (FID) transients. Complete line shapes and long Tx measurements were obtained with a composite pulse QE sequence with a 3.5 psec tt/2 pulse length. Long Tx and weak signal strength made data accumulation tedious A single Tx measurement or spectral line shape often required over 12 hours of data acquisition. 

ESEEM is a pulsed EPR technique which is complementary to both conventional EPR and ENDOR spectroscopy(74.75). In the ESEEM experiment, one selects a field (effective g value) in the EPR spectrum and through a sequence of microwave pulses generates a spin echo whose intensity is monitored as a function of the delay time between the pulses. This resulting echo envelope decay pattern is amplitude modulated due to the magnetic interaction of nuclear spins that are coupled to the electron spin. Cosine Fourier transformation of this envelope yields an ENDOR-like spectrum from which nuclear hyperfine and quadrupole splittings can be determined. 

A related RF technique to NMR is nuclear quad-rupole resonance (NQR). In NQR, transitions between nuclear quadrupole levels of nuclei in a solid material are induced by the applied radiation. The electric field gradients in the solid orient the quad-rupolar nuclei I>1/2) and give rise to quantized energy levels that yield transitions in the MHz range. FT-NQR spectroscopy measures these splittings and the relaxation times by free induction decay or various pulse echo experiments. FT-NQR spectroscopy provides information about the local environment around the quadrupolar nucleus in a crystal. 

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