Nuclear Spin Absorption Spectra in Solids

Nuclear magnetic spin absorption spectra in both parallel and perpendicular ac and dc magnetic fields are reported for lithium nuclei in lithium metal. This is a case where the predominant interactions are the Zeeman interaction of the nuclear magnetic moment with the dc field and the magnetic dipole-dipole interaction between nuclei in fixed lattice positions. For an applied dc field, H₀, the spectrum consists of absorption around zero frequency, around ω=γH₀, and around ω=2γH₀. Linewidths and relative amplitudes observed experimentally are in good agreement with the theory, which predicts linewidths of approximately the same magnitude for all lines and line amplitudes which vary as H₀^-4, H₀⁰, or H₀^-2. At applied dc fields which are large compared with the internal magnetic dipole-dipole fields an additional absorption is observed for the parallel-field case at very low frequencies; this is attributed to the relatively long time required for establishment of internal equilibrium at these fields.Absorption spectra are reported for lithium-magnesium alloys. In these alloys addition of magnesium produces electric quadrupole broadening of the lithium absorption spectra. The zero-field absorption spectrum of copper is also reported.