Multispin dynamics of the solid-state NMR free induction decay

We present a new experimental investigation of the NMR free induction decay (FID) in a lattice of spin-1∕2 nuclei in a strong Zeeman field. Following a π∕2 pulse, evolution under the secular dipolar Hamiltonian preserves the coherence number in the Zeeman eigenbasis, but changes the number of correlated spins in the state. The observed signal is seen to decay as single-spin, single-quantum coherences evolve into multiple-spin coherences under the action of the dipolar Hamiltonian. In order to probe the multiple-spin dynamics during the FID, we measured the growth of coherence orders in a basis other than the usual Zeeman eigenbasis. This measurement provides the first direct experimental observation of the growth of coherent multiple-spin correlations during the FID. Experiments were performed with a cubic lattice of spins (¹⁹F in calcium fluoride) and a linear spin chain (¹⁹F in fluorapatite). It is seen that the geometrical arrangement of the spins plays a significant role in the development of higher-order correlations. The results are discussed in light of existing theoretical models.