Magneto-optical studies of photoexcitations in C61.

Abstract

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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and E=9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. 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 \ensuremath{\mu}sec, and a phosphorescence emission (PL) band with onset at 1.85 eV and a lifetime of 10 \ensuremath{\mu}sec. 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\ensuremath{\ne}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.