| Abstrakt: |
The response of spin-1 nuclei to sequences of high-power, non-selective r.f. pulses is calculated in detail using the Redfield density matrix formalism and Wigner rotation matrices. The spin system considered is subject to both a large, static Zeeman interaction and a small, first order, secular quadrupole coupling. It is shown that an in-phase pulse pair, 90°x-τ-90°x, applied such that the second pulse occurs at a zero-crossing of the on-resonance free-induction decay signal following the first pulse, creates a state of pure double quantum coherence and is thus equivalent to the soft double-quantum 90° pulse described by Pines and co-workers [6-10, 23]. In addition, in the presence of a resonance offset, Δ, it is shown that this state may be created either by an appropriate choice of Δ or by using the modified pulse sequence, 90°x-τ/2-180°x-τ/2-90°x. The response of the state of double-quantum coherence to resonance offsets and subsequent r.f. pulses is also investigated in some detail. In particular the effect of a general pulse, Θα, applied to the system after the double-quantum coherence has evolved for a period under the influence of a resonance offset, Δ, is calculated and the conditions required to effect total transfer of the order back into transverse magnetization (single-quantum coherence) are evaluated. Similarly, the conditions which effect a transfer of the double-quantum coherence into a mixed state of quadrupole (zero-quantum) and double-quantum order are discussed. The results clearly show the relationships between the phases of the r.f. pulses, the magnetization vectors, and the state of double-quantum coherence necessary to effect these transfers and thus lead naturally to a set of pictorial models for these processes. These involve consideration of both single-quantum and double-quantum frames of reference and agreement between the models and theory is quantitative, even to the derivation of operators corresponding to the cartesian components of the double-quantum coherence vector in the double-quantum frame. These operators are identical to those described by Vega and Pines [9] and Wokaun and Ernst [13].Experimental verification of the main features of the theory is presented in the form of measurements on 2H nuclei in the magnetically orientable mesophase of the caesium perfluoro-octanoate/D2O system. |
