Determination of Cr-Al Hyperfine and Electric Quadrupole Interaction Parameters in Ruby Using Spin-Echo Electron-Nuclear Double Resonance

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Abstract
Electron-spin-echo-electron-nuclear-double-resonance (ENDOR) spectra have been observed in dilute ruby crystal at magnetic fields of 1.7, 3.3, 5.8, and 7.5 kG. Analysis of the data yields the Cr-Al hyperfine and electric quadrupole interaction parameters for 34 Al nuclear neighbors, and shows the Al nuclear quadrupole Hamiltonian must include an asymmetry parameter and a tilt of the principal axis of the electric field gradient away from the direction of the crystal optic axis. The parameters are used to calculate electron-spin-echo behavior, and the comparison with experimentally determined spin-echo modulation data is excellent. Explicit expressions for the double-resonance effect are derived under limiting conditions and show the sinusoidal dependence of the double-resonance amplitude on the time separation of the first two excitation pulses. Various properties of the echo-ENDOR effect and its capability to untangle overlapping resonances are experimentally demonstrated.