Pure Nuclear Electric Quadrupole Resonance in Impure Copper

The quadrupole interaction of copper nuclei which are near neighbors of Ag and Zn impurities in metallic copper has been measured by means of pure quadrupole resonance, using the field cycling method of Ramsey and Pound in which resonance is detected by subsequence remagnetization and Zeeman resonance of spins far from the impurities. Coupling between spin systems is enhanced by applying two frequencies simultaneously separated by 20 kc/sec; this is a variant of the phase-reversal method of Hartmann and Hahn. For Ag solute, resonances are observed (due to ${\mathrm{Cu}}^{63}$) at 620, 475, 200, and 75 kc/sec, due to first, second, third, and fourth or fifth neighbors, respectively. For Zn a search up to 5 Mc/sec shows resolved resonances only at 1980 and 100 kc/sec which are tentatively ascribed to second and third neighbors; this assignment is based in part on Rowland's earlier data. The anomalously large interaction of near neighbors of Zn is ascribed to virtual excitation of $d$ electrons to above the Fermi level by the impurity perturbation.