[11.8,88], The IR spectrum of solid Cm remains almost unchanged relative to that of the isolated C60 molecule, with the most prominent addition being the weak feature at 1539 cm-1 [11.19,31,42], However, more than 60 additional weak lines can be observed in the infrared spectrum in the 400 to 4000 cm"1 range. These weak features can be identified with higher-order processes, as discussed in §11.5.4.
The strong correspondence between the solution and/or gas-phase IR spectrum and the solid-state IR spectrum for C60 is indicative of the highly molecular nature of the crystalline phase of CMI. The infrared spectrum shown in Fig. 11.11 is for a thin film (~5000 A) of C60 on a KBr substrate. We discuss below (§11.6.2) the effect of doping on the mode frequencies and intensities of the infrared-allowed modes in the C«, spectrum.
Raman scattering measurements on a C60 film show a well-resolved second-order Raman spectrum (see Fig. 11.12) [11.20], From group theoretical considerations (see §4.2), the expected number of second-order Raman lines is very large, consisting of a total of 151 modes with Ag symmetry and 661 modes with Hg symmetry [11.20], The total number of second-order modes with Ag symmetry is found by taking the direct product n(T,-® nfYf and counting the number of modes with Ag symmetry in that direct product. Here, the number of modes and nf with r, and symmetries, respectively, that yield modes with Ag symmetry are found in Table 4.6, and the direct products are given in Table 4.7. The same procedure is used to find the number of overtones and combination modes with Hg symmetry. The second-order Raman spectra include both overtones (i.e.,
Table 11.6
Overtone and combination mode frequencies (in cm"1) in the range 250-1710 cm-1 observed in several experiments.
Table 11.6
Assignment |
Model" |
IR, Raman |
PI , NIS, HREELS |
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