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We have recently shown that utilizing double frequency sweeps (DFSs) instead 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 frequencies symmetrically around the rf-carrier frequency. Convergent or divergent DFSs can be generated by appropriate time-dependent cosine modulation of the rf field. Population and coherence transfer induced by sweeping the modulation frequency through the quadrupolar satellite transitions is investigated in detail. The time dependence of such passages determines the adiabaticity of the transfer processes. Insight into the involved spin dynamics is 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 coherence transfer in spin-3/2 systems induced by the combined action of simple time independent cosine amplitude modulation (CAM) of the rf field and sample spinning [Madhu et al., 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 covering the whole adiabaticity range resulting in an accurate description of actual experiments. In this context it will be shown that both population and coherence transfer are governed by the same principles and one unique adiabaticity parameter for each pair of spectral satellites. The transfer phenomena derived for spin-3/2 systems will be studied and quantified experimentally for 23Na in a single crystal of NaNO3. In a static and spinning sample the combination with DFS and CAM irradiation will be studied showing the equivalence of the transfer in all these situations. Further we will demonstrate the greater flexibility of a DFS compared to a CAM pulse to manipulate the adiabaticity and thus to maximize the transfer efficiency. Finally, the 27Al resonance in an α-Al2O3 single crystal will be inspected to demonstrate that the efficiency of DFS-induced population and coherence transfer in spin-5/2 systems depends on the direction of the DFS.

Enhancement of the sensitivity of magnetic resonance force microscopy allowed the extension of the technique to observe half-integer quadrupolar nuclei. This is demonstrated for various different compounds and nuclei with different spin numbers. The possibility of obtaining spatially localized spectral information through the quadrupole interaction is implemented by using nutation NMR. This enables us to superimpose a contrast function on the image of materials depending on their local lattice structure. This opens up new possibilities for both surface and subsurface studies in materials chemistry.

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