Cryogenic microwave whispering gallery mode spectroscopy of paramagnetic impurities in high-purity crystalline LiF

A low-noise cryogenic microwave spectroscopy experiment was performed on a high-purity lithium fluoride (LiF) crystal. The spectroscopy data revealed avoided level-crossing interactions in whispering gallery modes, indicative of electron spin resonance (ESR) coupling with paramagnetic impurities. Analysis of the interaction spectra identified distinct spin systems corresponding to (S=3/2,I=7/2), (S=1,I=7/2), and (S=3/2,I=0). The number of hyperfine splittings observed, together with the natural abundance of ions possessing the appropriate nuclear spin values, suggests that V2+ and V3+ impurities, exhibiting orthorhombic distortion, are the most likely sources of narrow interaction features. This interpretation is supported by earlier ESR studies and established manufacturing records for LiF crystal growth. Additionally, a separate set of broader interaction points is consistent with an orthorhombic model involving a (S=3/2,I=0) spin system, although the specific impurity responsible for this interaction remains unidentified.