Modulated adiabatic passage of oriented nuclei. I. The theory

Abstract

The anisotropic \ensuremath{\gamma}-ray distribution from oriented nuclei (ON) may be used to detect changes in the ensemble spin polarization and, in particular, those changes arising from nuclear magnetic resonance (NMR ON). A feature of NMR ON of dilute nuclear impurities in ferromagnets is the severe inhomogeneous broadening resulting from a distribution of magnetic hyperfine fields over the nuclear ensemble. In conventional NMR ON spectroscopy fine details of the hyperfine interaction, such as the electric quadrupole interaction, are buried in the inhomogeneous broadening. It is shown here that the quadrupole splitting may, however, be directly measured if the rf field is amplitude modulated during an adiabatic passage. Such a modulation implies a three-level coupling which is sensitive to rank-2 terms in the hyperfine Hamiltonian, and the modulation frequency corresponds to the spectral domain of the quadrupole splitting. This theory is developed and applied to nuclear ensembles with integral and half-integral spins.