Measurement of Pair Interactions and $1.5\mathit{\mu }m$ Emission from ${\mathrm{Er}}^{3+}$ Ions in a $C_{82}$ Fullerene Cage

Erbium ions encapsulated in the metallofullerene ${\mathrm{Er}}_2@{\mathrm{C}}_{82}$ emit near infrared fluorescence about 1.5 μm, typical for ${}_{}{}^4{}_{}{}^{}I_{13/2}^{}{}_{}{}^{}\to {}_{}{}^4{}_{}{}^{}I_{15/2}^{}{}_{}{}^{}$ transitions of ${\mathrm{Er}}^{3+}$. The similarity of spectra measured in glassy and polycrystalline media at 1.6 K, and the very narrow inhomogeneous linewidth $(<1\mathrm{}{\mathrm{cm}}^{-1\mathrm{}})$, show that the fullerene cage provides a locally ordered environment for the two erbium ions, well isolated from the host medium. Spectral fine-structure is observed with line splittings of $\sim 10{\mathrm{cm}}^{-1\mathrm{}}$. It is proposed that these splittings are dominated by exchange interaction between the two ${\mathrm{Er}}^{3+}$ ions involving a novel superexchange pathway through the ${\mathrm{C}}_{82}$ cage.