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 $\varphi$ a pure $n$-quantum excitation operator can be produced ($n=\frac{2\pi }{\varphi }$). 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.

• Received 24 September 1979

DOI:https://doi.org/10.1103/PhysRevLett.43.1791