Multiple-pulse sequence design principles

Homonuclear or heteronuclear Hartmann-Hahn mixing periods are versatile experimental building blocks that form the basis of a large number of combination experiments (see Section XIII). In practice, the actual multiple-pulse sequence that creates Hartmann-Hahn mixing conditions can usually be treated as a black box with characteristic properties. In this section, design principles and practical approaches for the development of Hartmann-Hahn mixing sequences are discussed. 

Special classes of multiple-pulse sequences were designed by Levitt et al, using the symmetry principles of internal spin Hamiltonians [77,86-88] where an integer number of RF pulse units spans a certain number of rotor cycles. These sequences are classified as CN and RN sequences [77]. 

Time-domain spectroscopies entail a major shift in emphasis from traditional spectroscopies, since the experimenter can control, in principle, the duration, shape, and sequence of pulses. One may say that traditional, CW spectroscopy, is passive—the experimenter attempts to study static properties of a particular molecule. Coherent pulse experiments are active in that, given a set of molecular properties (which may in fact be known from various spectroscopies), one tries to arrange for a desired chemical product, or to design a pulse sequence that will probe new molecular properties. The time-dependent quantum mechanics-wavepacket dynamics approach developed here is a natural framework for formulating and interpreting new multiple pulse experiments. Femtosecond experiments yield to a particularly simple interpretation within our approach. 

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