We have used a variety of cw optical and optically detected magnetic-resonance techniques to characterize long-lived photoexcitations in methanofullerene (${\mathrm{C}}_{61}$) as a dispersion in polystyrene glass, at temperatures below 10 K. We identify triplet excitons with zero-field-splitting (ZFS) parameters D=105×${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$ and E=9×${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$. The triplet excitons are characterized by two photoinduced absorption (PA) bands in the triplet manifold at 1.5 and 1.7 eV, both with a lifetime of 160 μsec, and a phosphorescence emission (PL) band with onset at 1.85 eV and a lifetime of 10 μsec. The difference in lifetimes between PA and PL is explained as being due to different recombination kinetics of the triplet sublevels. This and the finite axial symmetry ZFS parameter E≠0 compared to E=0 for triplets in ${\mathrm{C}}_{60}$ show the lower symmetry of triplets in ${\mathrm{C}}_{61}$, consistent with its postulated distortions from icosahedral symmetry in the ground-state structure.

• Received 24 May 1993

DOI:https://doi.org/10.1103/PhysRevB.48.18250