Electric Field Effects in Electron-Nuclear Double-Resonance Spectroscopy ofFCenters

The change in the magnetic hyperfine interaction of the $F$-center electron with its nearest-neighbor nuclei, due to an external uniform electric field, has been measured by ENDOR spectroscopy. These perturbations were observed for the first-, and fourth-nearest-neighbor nuclei in three host lattices: NaCl, KCl, and KBr. An effective-spin Hamiltonian is developed to predict theoretically the observed line shapes and splittings. Using the theoretical interpretation of the data, the values of the change in the hyperfine interaction with applied electric field, $\frac{\partial a}{\partial E}$, were determined for all three host crystals.