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Pristine Solid

Photopolymer

Fig. 13.18. (a) Optical absorbance si for pristine and phototransformed Cm (called Cm polymer and Photopolymer in the figure) solid films (thickness ~ 500 A) on a Suprasil (fused silica) substrate in the photon energy range 0.5-6.0 eV. The same CM film was used for both spectra, (b) Schematic diagram of the electronic states and optical transitions of pristine and phototransformed solid C60 near the Fermi level using simple molecular orbital notation. The numbers next to the arrows denote the transition energies (in eV) at the center of the optical absorption bands [13.77].

of phototransformed C60 were found to be ~0.1-0.15 eV wider than those in pristine C^. It is expected that the LUMO+1 states are broadened by about the same amount as the HOMO states.

In addition to the photoinduced broadening of the absorption bands in polyfullerene, the peaks at ~3.6, 4.7, and 5.6 eV in pristine solid C60 are noticeably blue-shifted in the polymer. It is difficult to judge whether the ~2.7 eV feature is shifted because the absorption band in the polymer is too broad to locate its center. To visualize the effect of phototransformation on the lowest energy states of crystalline C60, we refer to the schematic diagram in Fig. 13.18(b) that was used to interpret the absorption spectra in Fig. 13.18(a). The variances between the level energies in Fig. 13.18(b) and Table 13.4 are indicative of present uncertainties in the assignments of the optical transitions in Cœ. The solid horizontal lines on the left in Fig. 13.18(b) represent the peak positions of the calculated Gaussian density of state (DOS) for the contributing band of electronic states [13.104], and allowed optical transitions and transition energies (in eV) are indicated in Fig. 13.18(b). To explain the observed OA blue shift upon phototransformation, and to be consistent with the photoemission data on the photopolymer [13.127], the electronic levels of the phototransformed C60 are upshifted with respect to the electronic levels of pristine C60. In the figure, A is an energy shift of the LUMO level in the phototransformed phase which can be estimated from the data in Fig. 13.18(a). Since the optical band in the phototransformed C60 solid that is assigned to electronic transitions between the HOMO-1 and LUMO+1 states (at ~3.7 eV) is blue-shifted by ~0.05 eV, the LUMO+1 states are upshifted by -0.15 eV to be consistent with the photoemission results [13.127], If the LUMO state is not also upshifted by ~0.15 eV (i.e., A ^ ~0.15 eV), the energies of the HOMO LUMO+1 and HOMO-1 LUMO transitions will be different [see Fig. 13.18(b)], and the peak at ~2.7 eV in the pristine C60 spectrum would split into two poorly resolved peaks in the phototransformed C60 spectrum. Since there is no evidence for an unresolved doublet at ~2.7 eV, it is assumed that A — 0.1-0.2 eV.

In Fig. 13.19, we display the 80 K PL spectra near the absorption edge for the same C60 film on a Suprasil fused silica substrate before (solid curve)

I 1.09 eV

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