Raman Heterodyne Interference of Inequivalent Nuclear Sites

A novel interference effect that is unique to the recently introduced technique of Raman heterodyne detection of NMR is illustrated for the impurity-ion crystal ${\mathrm{Pr}}^{3+}$: YAl${\mathrm{O}}_3$. Experiments confirm theoretical arguments that the two inequivalent Pr nuclear sites in the host lattice generate coherent Raman signals of opposite sign that interfere. A necessary requirement, which can now be examined in detail, is that the principal nuclear quadrupole $Z$ axis of Pr be incongruent in the ground and optically excited electronic states.