[en] Er₃N rate at C₈₀ exhibits sharp optical emission lines in the near-infrared attributed to fluorescence from the Er³⁺ ion. Here we demonstrate that high magnetic fields cause this spectrum to split, corresponding to transitions from the lowest field-split Kramers doublet of the ⁴I_13/2 manifold to the four lowest field-split levels of the ⁴I_15/2 manifold. The internal structure of these fullerenes can be spatially aligned with a preferred orientation under high magnetic field; the effect of alignment is to reduce the broadening associated with the isotropic spatial averaging characteristic of powder or frozen-solution spectra. Using a tunable 1.5 μm laser, we directly observe non-cage-mediated optical interactions with the Er³⁺ ion. This spectroscopic method provides the opportunity to map the energy level structure of the incarcerated ion and to coherently control its quantum state. These qualities suggest that rare-earth endohedral fullerenes have characteristics that could be employed as a readout pathway for fullerene-based quantum information processing. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)