Population and coherence transfer in half-integer quadrupolar spin systemsinduced by simultaneous rapid passages of the satellite transitions: A static and spinning single crystal nuclear magnetic resonance study

We have recently shown that utilizing double frequency sweeps (DFSs) instea d of pulses can lead to increased efficiencies in population and coherence transfer in half-integer quadrupolar spin systems. Cosine modulation of the carrier amplitude corresponds to the simultaneous irradiation of two frequ encies symmetrically around the rf-carrier frequency. Convergent or diverge nt DFSs can be generated by appropriate time-dependent cosine modulation of the rf field. Population and coherence transfer induced by sweeping the mo dulation frequency through the quadrupolar satellite transitions is investi gated in detail. The time dependence of such passages determines the adiaba ticity of the transfer processes. Insight into the involved spin dynamics i s of utmost importance in the design and optimization of experiments based on amplitude modulation, such as DFS enhanced multiple-quantum magic angle spanning, where multiple to single-quantum conversion is performed by a DFS . Vega and co-workers have provided a theoretical basis of adiabatic cohere nce transfer in spin-3/2 systems induced by the combined action of simple t ime independent cosine amplitude modulation (CAM) of the rf field and sampl e spinning [Madhu , J. Chem. Phys. 112, 2377 (2000)]. In our report we will extend this theory to DFS induced adiabatic transfer phenomena in spin-3/2 and spin-5/2 systems. A fully analytical description will be presented cov ering the whole adiabaticity range resulting in an accurate description of actual experiments. In this context it will be shown that both population a nd coherence transfer are governed by the same principles and one unique ad iabaticity parameter for each pair of spectral satellites. The transfer phe nomena derived for spin-3/2 systems will be studied and quantified experime ntally for Na-23 in a single crystal of NaNO3. In a static and spinning sam ple the combination with DFS and CAM irradiation will be studied showing th e equivalence of the transfer in all these situations. Further we will demo nstrate the greater flexibility of a DFS compared to a CAM pulse to manipul ate the adiabaticity and thus to maximize the transfer efficiency. Finally, the Al-27 resonance in an alpha -Al2O3 single crystal will be inspected to demonstrate that the efficiency of DFS-induced population and coherence tr ansfer in spin-5/2 systems depends on the direction of the DFS. (C) 2001 Am erican Institute of Physics.