We present NMR signals from a strongly coupled homonuclear spin system, H 1 nuclei in
adamantane, acquired with simultaneous two-photon excitation under conditions of the Lee-
Goldburg experiment. Small coils, having inside diameters of 0.36 mm⁠, are used to
achieve two-photon nutation frequencies of∼ 20 kHz⁠. 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 …