Mixed echo

Principles of the Method. The method is based on the mixed echo composed of the Hahn and the magic echo (33,71,72). The Hahn echo explores the magic echo shape generated by an accordion magic sandwich (cf Fig. 7). 

Fig. 7. AIMS pulse sequence used for generation of a mixed echo composed of a magic and a Hahn echo. A dephasing gradient pulse is applied at the end of the sequence. Reproduced from Ref 70, with permission from Elsevier.

MS), ie, 90S - (t + nh.) - 90J - Burst Pulsex (2r - raA) - Burst Pulsejf (2t -n A) -90y - (r + rA) - Mixed Echo (see Fig. 7), where ra is a positive and negative integer number and A is the time decrement and increment, respectively. We call this mismatched MS composite pulse with variable n the accordion magic sandwich (hereafter the method will be called AIMS). 

Mixed Echo Decay. The mixed echo amplitude decay can be expressed in terms of the autocorrelation function of the dipolar Hamiltonian (64). Transverse relaxation processes described by a single correlation time are rarely found in amorphous polymers including elastomers. If a normalized distribution of correlation times (tc) is introduced, than the mixed echo decay is given by (64)... 

Parameters of the Dipolar Correlation Function for a Cross-Linked Elastomer Series. The mixed echo decays were measured for a series of cross-linked natural rubber (64). The model involving the distribution of correlation times (see eq. 29) was used to fit the mixed echo decay. The residual second van Vleck moments obtained from fits as a function of shear modulus G. 

Fig. 11a and b. Decay of the alignment echo height as a function of the mixing time x2 for different motional mechanisms, a Tetrahedral jumps as a model for conformational changes b Diffusive motion, the solid lines correspond to unrestricted rotational diffusion, the dashed lines to diffusion restricted to an angular region of 8°. Note the strong dependence of the decay curves on the evolution time t, in case of diffusive motion... 

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time mixing pulse, while the mixing pulse in the SECSY sequence is placed in the middle of the period. The information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

Fig. 1.19 Spin-echo based pulse sequence to each gradient pulse, A the separation between encode velocity change. The gradients are each pair of bipolar gradient pulses and tm the stepped pair-wise independently (2D VEXSY) mixing time between the bipolar gradient pairs, or simultaneously (1 D VEXSY). For a VEXSY The opposite polarity of the bipolar gradient experiment, 7q to k4 are usually applied along pair is realized by an inversion 180° pulse, the same spatial direction. 8 is the duration of...

Fig. 4.1.5 A stimulated-echo sequence for detecting spin displacements. This sequence detects spin displacements along the Zt direction. TM is mixing time.

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