ROTOR-SYNCHRONIZED AMPLITUDE-MODULATED NUCLEAR-MAGNETIC-RESONANCE SPIN-LOCK SEQUENCES FOR IMPROVED CROSS-POLARIZATION UNDER FAST MAGIC-ANGLE SAMPLE-SPINNING

We demonstrate that in solid-state nuclear magnetic resonance the Hartmann-Hahn cross-polarization efficiency in the presence of fast magic angle sample spinning can be improved by the application of rotor-synchronized amplitude-modulated radio-frequency irradiation during the mixing time (AMCP pulse sequences). The modulation reintroduces time-independent coupling elements between dressed eigenstates of the Floquet Hamiltonian which lead to flip-flop transitions. By numerical optimization, an optimal AMCP pulse sequence is designed which provides a significant improvement over conventional cross polarization. The matching condition can be broadened by rotor-synchronized 180° phase shifts to render the transfer less sensitive to exact matching of the applied radio-frequency field strengths. © 1995 American Institute of Physics.