Jeener-Broekaert echo

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). 

Equation (3.2.14) is the starting point for numerical simulation of dynamic wideline NMR spectra. With the Jeener-Broekaert echo the imaginary part of the exponential depending on h is measured, and with the stimulated echo the real part is measured. The lineshapes of solid-echo spectra follow from (3.2.14) with t , = 0. 

These composite pulses can be used in liquid- and solid-state multi-pulse experiments such as the //V PT experiment (insensitive nuclei enhanced by polarization transfer) and the Jeener-Broekaert echo experiment [Wiml]. 

Fig. 6. The generalized Jeener-Broekaert three pulse sequence. Note that FT of the solid echo and the alignment echo starts at times delayed by the pulse separation r, after the second and third pulse, respectively...

In the stimulated echo experiment, also shown in Fig. 6.2.3, the second pulse transfers the system into a mixture of Zeeman and double quantum order (alongandpg). Here, the relevant relaxation times are Ti (longitudinal Zeeman) and T q (double quantum), for which the 45 pulses of the Jeener-Broekaert sequence are replaced by 90v pulses. Again, two echos evolve at T] around the third pulse, and are refocussed by the fourth pulse. The two negative echo amplitudes vary as function of T2, with -[exp(-T2/Tiz) + exp(-T2/Ti3Q)], and both Ti and Tqq can be determined as separate values [14]. 

Rg.6. Schematic representation of various pidse sequences, employed in dynamic NMR of I = I spin systems Quadrupole echo sequence (QE), inversion recovery sequence (// ), saturation recovoy sequence (SR) and Jeener-Broekaert sequence (JB)... 

This is illustrated in Fig. 9. The 2D spectra refer to quadrupole echo sequences and characterize two possible reorientation mechanisms of a methyl group (three-site jumps vs continuous diffusion). Drastic spectral differences are observed. Ajqjarratly, these 2D relaxation spectra sensitively indicate the type of motion. The same is true for the corresponding normalized contour dots (see Fig. 9). We note that similar 2D spectra can be obtained from inversion recovery or Jeener-Broekaert sequences (see Fig. 6) [68]. Thus, by applying this 2D technique to different pulse sequoic, the various motions can be differentiated over an extremely wide dynamic range, extending from the fast-rotational to the ultraslow motional re me. Sin<% the different motions (see Fig. 4) modulate different kinds of molecular order (see Fig 3) these orders can be differentiated, likewise. 

Li and Be work has gained from the application of the stimulated-echo spectroscopy to study the ultra-slow dynamics of nuclear spin-3/2 probes. Apart from the dominant first-order quadrupolar interaction, the impact of the homonuclear dipolar interactions was also considered. Explicit analytical expressions describing various aspects of a coupled quadupolar pair subjected to a Jeener-Broekaert pulse sequence have been derived. Extensions to larger spin systems are also briefly discussed. These results are compared with experimental data on a single-crystalline Li ion conductor. 

The Jeener-Broekaert (J-B) pulse sequence [2.18] shown in Fig. 2.5 allows the creation of spin alignment and the observation of a stimulated echo [2.19] for a spin-1 system. The density matrix at the end of second pulse... 

The Jeener echo sequence (Jeener and Broekaert, 1967 Goldman, 1970) is... 

Jeener and Broekaert introduced, in 1%7, a three-pulse B,(r) sequence to measure the relaxation time Tm of the dipolar order of / = 1 spin systems in the presence of a conventional high Zeeman field, Bq, which is based on the decay time of the so-called Jeener echo . It was later extended by Spiess and Kemp-Harper and Wimperis to study in a similar way the quadrupolar order in / a 1 systems. The appearance of a Jeener echo depends upon the existence of interactions that are not averaged out by molecular motions on the considered time scale. The method has become of great importance in recent relaxation studies, in particular of liquid crystals because, in standard spin relaxation theories, it provides a power l means to separate and analyse the spectral densities / v) and /2) j. i4,is,2025 ggg... 

---