Jl

200 400 600 800 1000 1200 1400 1600 Raman Shift ( cm"1 ) Fig. 11.31. continued weaker electron-phonon coupling in K4C70 relative to K3C60 is believed to be responsible for the absence of superconductivity in K4C70 down to 1.35 K [11.148].

11.8. Vibrational Spectra for Phototransformed Fullerenes

Because of the symmetry lowering of the phototransformed fullerene phases, Raman and infrared spectroscopies provide powerful tools for the characterization of the phototransformation process. Although Brillouin

Raman shift (cm1)

Fig. 11.32. Room temperature Raman spectra of pristine C70 and K^Cjo at the maximum-conductivity phase (nominally x = 4 in KXC70). The film thickness is ~2500 A [11.148].

Raman shift (cm1)

Fig. 11.32. Room temperature Raman spectra of pristine C70 and K^Cjo at the maximum-conductivity phase (nominally x = 4 in KXC70). The film thickness is ~2500 A [11.148].

scattering can in principle be used for characterizing phototransformed C60, the power levels required to carry out Brillouin scattering are so large that the Brillouin scattering probe itself introduces phototransformation of C60 [11.58]. Thus far, most of the vibrational spectra on phototransformed films have been taken on phototransformed Qq.

The overall effect of phototransformation on the vibrational spectra of solid C60 is shown in Fig. 11.33 [11.80], where the infrared transmission and Raman spectra of solid C50 films can be compared for the pristine fee phase (upper panel (a)) and the phototransformed phase (lower panel (b)). Phototransformation occurs at a moderate optical flux (> 5 W/cm2) in the visible and in the ultraviolet (UV) regions of the spectrum when light is incident on thin solid fullerite films [11.80]. One of the most important effects of phototransformation on the Raman spectrum of C^ is the quenching of the intensity of the Ag(2) pentagonal pinch mode at 1469 cm-1 in pristine C60, with a new mode appearing close by at 1458 cm-1 in the phototransformed material. The various effects observed in the Raman and infrared spectra in Fig. 11.33 have been interpreted [11.80] to be due to a pho-topolymerization of the lattice in which the C60 molecules become cross-linked by covalent bonds, rather than by the weaker van der Waals forces that normally bond C60 monomer molecules in the solid phase (see ยง7.5.1). The shift to lower frequencies of the pentagonal pinch mode has been

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