Structure of theFCenter in NaF

The electron-nuclear double resonance (ENDOR) method is applied to the study of the structure of the $F$ center in NaF. Hyperfine interactions between the $F$-center electron and a number of the surrounding nuclear shells are presented and compared with the results of earlier experiments and with theory. It is found that the resolved hyperfine structure is not due to the predominance of the first-shell interaction, as has been thought, but it arises from the fortuitous equality of the first- and second-shell isotropic hyperfine interactions. Electron paramagnetic resonance (EPR) absorption measurements of the resolved structure confirms an expected 31-line resolved pattern. The exceptional resolution found in the ENDOR spectrum permits the identification of interactions with nuclei well beyond the eighth shell. Although the samples also contained large numbers of $M$ centers, no evidence for a paramagnetic $M$ center was found in either the EPR or ENDOR spectra.