Aluminum hyperfine interactions in ruby

Abstract

ENDOR spectra, consisting of some 200 spectral lines in the region 5–10 Mc/s, have been observed in dilute ruby. The observed lines may be ascribed to 5 inequivalent aluminum nuclear sites neighboring the chromium ion impurity. The spectra are classified and analyzed in terms of the magnetic hyperfine and quadrupole interaction parameters of these nuclei with the chromium electrons. A semi-quantitative model is presented for explaining the magnitude of these interactions. The salient features of the model are (1) the electrons are principally confined to the volume of the chromium ion so that the dipole interaction is approximately that due to a point source; (2) there is a moderate amount of local distortion about the chromium ion, of the order of 2 per cent of a lattice spacing, yielding local variations in the quadrupole coupling constant; (3) there is an appreciable degree of covalency in the bonding of the Al 2O 3 lattice, of the order of 20 per cent, which permits the transmission of the Fermi contact interaction through the bonding states of the crystal.