Two-photon Lee-Goldburg nuclear magnetic resonance: Simultaneous homonuclear decoupling and signal acquisition

We present NMR signals from a strongly coupled homonuclear spin system, $H1$ nuclei in adamantane, acquired with simultaneous two-photon excitation under conditions of the Lee-Goldburg experiment. Small coils, having inside diameters of $0.36mm$⁠, are used to achieve two-photon nutation frequencies of $∼20kHz$⁠. The very large rf field strengths required give rise to large Bloch-Siegert shifts that cannot be neglected. These experiments are found to be extremely sensitive to inhomogeneity of the applied rf field, and due to the Bloch-Siegert shift, exhibit a large asymmetry in response between the upper and lower Lee-Goldburg offsets. Two-photon excitation has the potential to enhance both the sensitivity and performance of homonuclear dipolar decoupling, but is made challenging by the high rf power required and the difficulties introduced by the inhomogeneous Bloch-Siegert shift. We briefly discuss a variation of the frequency-switched Lee-Goldburg technique, called four-quadrant Lee-Goldburg (4QLG) that produces net precession in the $x-y$ plane, with a reduced chemical shift scaling factor of $1∕3$⁠.