NMR EXCITATION OF QUADRUPOLAR ORDER USING ADIABATIC DEMAGNETIZATION IN THE ROTATING-FRAME

Adiabatic demagnetization and remagnetization in the rotating frame (A DRF and ARRF) are shown to be practical and efficient techniques for e xciting and observing quadrupolar order, T-2,T-0, in NMR of quadrupola r nuclei such as H-2 or Na-23. A detailed theoretical description of A DRF and ARRF of spin I=1 and 3/2 nuclei. based on the well-known vecto r model of NMR, is presented and used to derive analytically a variety of pulse shapes for ADRF and ARRF. This theory is also used to calcul ate the state of the spin I=1 and 3/2 density operators following an A DRF or ARRF pulse and it is shown that the desired coherence transfer processes have the maximum amplitudes allowed by the well-known ''univ ersal bound'' theorem. In principle, therefore, ADRF is shown to be su perior as a method of exciting quadrupolar order to the Jeener-Broekae rt experiment since the latter fails to excite the maximum T-2,T-0 amp litude for spin I=3/2 nuclei. The performance of ADRF is investigated using computer calculations and simulations and the conditions under w hich it yields broadband (i.e., non-oscillatory) excitation of quadrup olar order are derived. Using both H-2 (I=1) and Na-23 (I=3/2) NMR of liquid crystalline and biological samples, ADRF and ARRF rut demonstra ted experimentally. The predicted broadband excitation behavior is obs erved far both ADRF and ARRF, whereas both the Jeener-Broekaert and do uble-quantum filtration experiments show excitation profiles that osci llate sinusoidally as a function of either the quadrupolar splitting p arameter, omega(Q), or the duration of the pulse sequence, tau. Finall y, a more general discussion of ADRF and ARRF of quadrupolar nuclei is presented and it is shown that the maximum coherence transfer amplitu des are achieved for nuclei of any spin quantum number I. (C) 1998 Ame rican Institute of Physics. [S0021-9606(98)00103-2].