100 ISO 200

Temperature (°C)

100 200 TCC)

100 ISO 200

Temperature (°C)

Fig. 11.35. Temperature dependence of the integrated intensity / obtained from analysis of the Raman spectra in Fig. 11.34 for the 1459 cm"1 (□) line (Cm polymer) and the 1469 cm-1 (o) line (Qq monomer) in comparison to rate equation model calculation (solid curves). Inset: the calculated fractional monomer population (FMP) in photopoly-merized Cm as a function of temperature for several values of the incident light flux <l>0 at 488 nm [11.156],

Arrhenius plot of the Raman intensity ratio (/1459//1469) yields an activation energy of 1.25 eV for the thermal dissociation process [11.156],

Additional radial and tangential molecular modes are activated (Fig. 11.33) by the apparent breaking of the icosahedral symmetry, resulting from bonds that cross-link adjacent molecules (see Fig. 7.20). A new Raman-active mode is also observed at 118 cm"1 in phototransformed C6{) [11.80], and this new mode is identified with a stretching of the cross-linking bonds between molecules [see Fig. 11.36] [11.156]. This frequency falls in the gap between the lattice and molecular modes of pristine Qq (see Fig. 11.1), and the value of this mode frequency is in good agreement with theoretical predictions for this mode at 104 cm"1 [11.157], Figure 11.36 shows the temperature dependence of the intensity of the 118 cm"1 mode and the suppression of this mode as the thermal detachment of Qq monomers from oligomer clusters proceeds with increasing temperature. The decrease in intensity of the 118 cm"1 mode is closely correlated with the attenuation of the 1459 cm-1 Raman feature, discussed above and shown in Fig. 11.34 [11.156].

Because of the characteristic features of the Raman and IR spectra of phototransformed C60, these spectroscopic tools are often used to characterize the phototransformed material, including the amount of phototransformation that has occurred when studies of other properties of phototransformed C60 are carried out.

Infrared spectroscopy has also been used to characterize C70 films regarding photoinduced oligomer formation [11.122]. Once again, the onset of additional spectral features and line broadening is used for the characterization of the polymerization process.

Fig. 11.36. Temperature dependence of the Raman-active 118 cm-1 intermolecular mode in a photopolymerized Cm film [d « 4500 A, Si (100) substrate]. (a) Raman spectra taken in the 110-133 cm~' range at the indicated temperatures. The solid lines are the results of a Lorentzian lineshape analysis in which the peak frequency and width were fixed at the values obtained at 25°C. (b) Integrated intensity /118 of the photoinduced intermolecular mode at 118 cm"1 vs temperature. The circles are obtained from a Lorentzian line-shape analysis and the solid curve is calculated from a rate equation model. The inset shows the intermolecular mode schematically [11.156],

11.9. Vibrational Spectra for C^ under Pressure

The pressure dependence of the vibrational spectra has been used in two ways to elucidate the phase diagram and stability of fullerenes. One approach has involved study of the effect of pressure on the ambient vibrational spectra of the fullerenes (see §11.5.1 and §11.5.2) and the second has involved use of spectroscopy to characterize some of the novel high-pressure phases of fullerenes (see §7.3).

A number of studies of the pressure dependence of the vibrational spectra have been carried out [11.78,149], confirming the stability of the fullerene phase of carbon, providing information on the pressure dependence of the most prominent Raman-active modes, and identifying possible phase boundaries at higher pressures. Some of these studies were carried out with sufficient laser intensity to cause some phototransformation of the C60 films (see §11.8), resulting in photoinduced modification of the spectra in addition to pressure-induced effects.

By tracking the pressure dependence of the room temperature Raman spectra, the stability of the C60 phase up to 22 GPa has been confirmed [11.149], in comparison to graphite [11.158,159]. C^ films show evidence for a phase transition in the 15-18 GPa range. Although C60 is a molecular solid with vibrational frequencies similar to those for the free molecule in solution, and the compressibility of the molecule itself is expected to be low, the pressure dependence of the vibrational modes is readily mea-

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