Anti-phase vectors

Figure 5.29. A simplified picture of multiple-quantum coherence in an AX system views this as being composed of groups of evolving anti-phase vectors that have zero net magnetisation and hence can never be directly observed.

Figure 5.44. The ID doublequantum filter. The sequence is derived from the 2D experiment by replacing the variable ti period with a fixed spin-echo optimised to produce anti-phase vectors (A = 1/27) as required for the generation of doublequantum coherence. Signal selection is then as for the 2D experiment, and gradient selection may be implemented as in Fig. 5.41.

T and H components are the same as in the yttrium oxyfluorides etc., but now they occur as strips intergrown in each layer the entire structure is divided by anti-phase boundaries perpendicular to the layers, with a slip vector R equal to half the unit-cell vector in the layer-stacking direction. These structures (often slightly monoclinic ) are CC types, with finite incommensiurate portions. They will be considered more fully in Chap. 6 below. Meanwhile, we will simply point out that anti-phase-boundary structures of this sort are strictly limited to ternaries with two cations and one anion. The related ternaries considered earlier in this section - those containing one cation and two anions - do not have these boundaries they are truly non-commensurate. This difference we take to be significant. 

Anti-Phase Domain Boundaries Burgers vector... 

Recall also that following the second pulse, some magnetisation remains associated with the original spin . Thinking back to the discussions of polarisation transfer in the INEPT experiment, it was shown that the basic requirement for the transfer of polarisation was an anti-phase disposition of the doublet vectors of the source spin, which for INEPT was generated by a spin-echo sequence. Magnetisation components that were in-phase just before the second 90° pulse would not contribute to the transfer, hence the A period was optimised to maximise the anti-phase component. The same condition applies for... 

The generation of double-quantum coherence requires an anti-phase disposition of coupling vectors, which here develop during a period A = l/2Jcc- This is provided in the form of a homonuclear spin-echo to make the excitation independent of chemical shifts. The ti period represents a genuine double-quantum evolution period in which these... 

In a model [197]. the superstructure is assumed to consist of identical slabs of basic structure with thickness D limited by planes normal to e . Usually the slab thickness is equal to an integer number n of unit cell parameters a of the basic structure i.e.. D = na, but this need not be the case. Successive. slabs are separated by planar interfaces (stacking faults, anti-phase boundaries, discom-mensuration walls, etc.) with a di.splacement vector R and a unit normal e . 

Figure 1 Schematic representation of a time-resolved coherent Raman experiment, (a) The excitation of the vibrational level is accomplished by a two-photon process the laser (L) and Stokes (S) photons are represented by vertical arrows. The wave vectors of the two pump fields determine the wave vector of the coherent excitation, kv. (b) At a later time the coherent probing process involving again two photons takes place the probe pulse and the anti-Stokes scattering are denoted by subscripts P and A, respectively. The scattering signal emitted under phase-matching conditions is a measure of the coherent excitation at the probing time, (c) Four-photon interaction scheme for the generation of coherent anti-Stokes Raman scattering of the vibrational transition.

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