Observation of muon-fluorine "hydrogen bonding" in ionic crystals

Abstract

We have found that a positive muon (${\ensuremath{\mu}}^{+}$) implanted into LiF, NaF, Ca${\mathrm{F}}_{2}$, or Ba${\mathrm{F}}_{2}$ pulls two ${\mathrm{F}}^{\ensuremath{-}}$ ions together in a strong "hydrogen bond" until the $^{19}\mathrm{F}$ nuclei are separated by roughly twice the nominal ${\mathrm{F}}^{\ensuremath{-}}$ ionic radius, with the ${\ensuremath{\mu}}^{+}$ midway between. The resultant "$\mathrm{F}\ensuremath{\mu}\mathrm{F}$" center is easily observed via the distinctive behavior of the collinear $^{19}\mathrm{F}:{\ensuremath{\mu}}^{+}:^{19}\mathrm{F}$ spin system (coupled by dipole-dipole interactions between the muon and the fluorine nuclei) in both transverse-field muon-spin rotation and zero-field muon-spin relaxation experiments. We speculate that implanted ${\mathrm{H}}^{+}$ ions may initially form similar hydrogen bonds between adjacent ${\mathrm{F}}^{\ensuremath{-}}$ ions in many metal fluoride crystals.