Nuclear Electric Quadrupole Interactions in Crystals

A study is made of the effects of the presence of nuclear electric quadrupole moments on the nuclear magnetic resonance absorption in solids. Necessary theoretical background is followed by descriptions of experiments in which the spectra obtained with an r-f spectrograph, described elsewhere, are found to show marked splitting resulting from the quadrupole interaction with the electric field gradient. Both powder patterns and spectra of single crystals are reported, for ${\mathrm{Li}}^7$, ${\mathrm{Na}}^{23}$, and ${\mathrm{Al}}^{27}$, with splittings varying from about 20 kc to 718 kc, depending on the crystal and the nucleus. Unambiguous indication of the spin values is obtained. The possibility of the detection of a resonance resulting from the electric quadrupole splittings in the absence of a static magnetic field is discussed and an unsuccessful attempt to find such a resonance in ICl is described and the failure tentatively explained. Available signal-to-noise ratios are calculated for such resonances. A method is described of determining the frequency of a large electric quadrupole splitting, by observation of the dependence of the frequency of the magnetic splitting of the levels $m=\pm \frac{1}{2}$, for an odd half-integral spin, on the angle between the direction of a magnetic field and the symmetry axis of the crystalline field. The electric quadrupole moment is shown probably to account for relaxation times of nuclei of spin greater than ½ even in cubic crystals and, as evidence, resonances of ${\mathrm{Na}}^{23}$, ${\mathrm{Li}}^7$, ${\mathrm{I}}^{127}$, and ${\mathrm{Br}}^{81}$ are reported. Assuming partial saturation and electric quadrupolar relaxation, calculations are made to account for a departure of the intensity ratios of the lines, in the ${\mathrm{Na}}^{23}$ spectrum in NaN${\mathrm{O}}^3$, from the ratios of the transition probabilities for magnetic dipole radiation. The possibility of determining nuclear electric quadrupole moments from splittings or relaxation times in ionic crystals is discussed.