Selective Excitation of Multiple-Quantum Coherence in Nuclear Magnetic Resonance

Wideband selective n-quantum excitation in the NMR of coupled spins is demonstrated for the first time. By a combination of multiple pulse averaging and phase shifts a pure n-quantum excitation operator can be produced (n=2). This allows enhancement of normally weak n-quantum transitions. Selective excitation of the zero- and four-quantum transitions in benzene illustrates this approach. Extensions to selective absorption of only groups of n photons in other regimes of spectroscopy are straight-forward, in principle.