Measurement of pair interactions and 1.5 mu m emission from Er3+ ions in a C-82 fullerene cage

Erbium ions encapsulated in the metallofullerene Er-2@C-82 emit near infrared fluorescence about 1.5 mu m, typical for I-4(13/2) --> I-4(15/2) transitions of Er3+. The similarity of spectra measured in glassy and polycrystalline media at 1.6 K, and the very narrow inhomogeneous linewidth (<1 cm(-1)), 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 similar to 10 cm(-1). It is proposed that these splittings are dominated by exchange interaction between the two Er3+ ions involving a novel superexchange pathway through the C-82 cage.