Fig. 13.25. Eigenvalue spectrum of C^ in lh symmetry (left) and distorted D5d symmetry (right) together with the irreducible representation labels. Only spin-up orbital levels are shown. Some of the calculated levels remain approximately degenerate and are shown in the figure as degenerate. Dashed lines correspond to unoccupied levels [13.142]. The Fermi level lies in the flu level (left) and in the a2u level (right).

distortion, which, if allowed, would split the tiu orbitals as discussed for the C'd0 ions in §12.4.2 and §13.4.1. Electric dipole transitions between the HOMO (ilu) and the LUMO (ilg) of the hexanion are dipole allowed, and a coupling between these dipole transitions and IR-active (Flu) intramolecular phonons has been proposed to explain the unusually strong Flu-mode oscillator strengths observed in the M6C50 füllendes (see §11.6.2). The smaller Na+ ion also forms a compound with the M6C60 stoichiometry (see §8.5.3), but this fulleride crystallizes in the fee phase and would be expected to have different optical properties from those for M = K, Rb, Cs, which crystallize in the bcc structure.

In Fig. 13.26, we display the T = 300 K experimental e2(w) data for optically thin M6C60 (M = K, Rb, and Cs) films on transparent Suprasil quartz (Q) substrates [13.77]. The films were prepared by exposing vacuum-deposited C60 films to alkali metal vapor in a sealed quartz tube. The spectra were taken in situ through the ampoule walls after removing the ampoule from the doping furnace. The dielectric function

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