Hartmann-Hahn spectroscopy principle

Polarization can be transferred simultaneously within several spin pairs without cross-talk if different rf amplitudes are used for each pair of spins in the multiply selective irradiation scheme. This principle has been used in Hartmann-Hahn Hadamard spectroscopy (HAHAHA), where a phaseencoding scheme is used to separate simultaneously acquired spectra that represent individual doubly selective Hartmann-Hahn transfer experiments (Kupce and Freeman, 1993e). 

General symmetry principles for rotor-synchronized pulse sequences in MAS solid-state NMR have been presented. The synunetry theory has been extended to the case of generalized Hartmann-Hahn sequences, in which rotor-synchronized r.f. irradiation is applied simultaneously to two isotopic spin species. The symmetry theory has been used to design pulse sequences which implement heteronuclear dipolar recoupling at the same time as decoupling homonuclear spin-spin interactions, and which also suppress CSAs. Experimental demonstrations of heteronuclear 2D correlation spectroscopy, heteronuclear MQ spectroscopy, and the estimation of intemuclear dipolar couplings have been given. 

The photon echo phenomenon is perhaps the most dramatic manifestation of using physical principles to overcome what appears to be the erasure of molecular information caused by inhomogeneous broadening. The effect was first demonstrated by Hahn in NMR spectroscopy (Phys. Rev. 80, 580 (1950)) and extended to optical spectroscopy by Kemit, Abella and Hartmann (Phys. Rev. Letters, 13, 567 (1964)).21 Here we only outline its basic physical principle. 

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