Raman Shiftcml

Fig. 11.14. (a)Unpolarized Raman spectrum near 1470 cm-1 of a frozen solution of Cm in CS2 at 30 K. The solid line is a three-Lorentzian fit to the experimental data. The highest frequency peak is assigned to the totally symmetric pentagonal pinch Ag(2) mode in 12CW. The other two lines are assigned to the pentagonal pinch mode in molecules containing one and two 13C atoms, respectively. The inset shows the evolution of these peaks as the frozen solution is heated. The three components for the Raman feature are no longer resolved as the frozen solution melts above 150 K. (b) Comparison between the mass spectra and the Raman spectra for a 13C-enriched CM sample [11.25].

mode frequencies for the vibrational modes that are not observed in the first-order Raman and infrared spectra. The two advantages of using the second-order Raman and infrared spectra to find mode frequencies for the silent modes are the symmetry information for the silent modes that is contained in these spectra and the sensitivity of the higher-order Raman and infrared spectra for determining all the silent modes to within ~5 cm-1 (see §11.5.3 and §11.5.4). In general, it is not possible to observe sharp higher-order Raman and infrared spectra in solids because of dispersion arising from interatomic and/or intermolecular interactions in the crystalline phase. Because of the highly molecular nature of fullerenes in their crystalline phases, higher-order Raman and infrared spectra can be observed, in analogy to studies of molecules in solution or in the gasphase.

Surface-enhanced Raman spectroscopy (SERS) could also provide information on silent modes. Such studies carried out for CM on noble-metal surfaces show substantial shifts of the Ag(2) pentagonal pinch mode, and these shifts are associated with charge transfer [11.97] (see §17.7). More recently, SERS experiments for C60 on Ag and In polycrystalline surfaces have also been carried out, showing large differences in the SERS spectra upon room temperature annealing of samples grown at low temperature. Different behaviors were observed for the Ag and In surfaces upon annealing [11.98].

11.5.7. Luminescence Studies of Vibrations in C60

Singlet oxygen photoluminescence (PL) has been shown to provide a powerful probe of the intramolecular vibrational modes of C^, independent of their symmetry, as shown in Fig. 11.15 [11.14]. The applicability of the photoluminescence technique to probe fundamental vibrational excitations of C60 is based on the weak vibronic coupling of 02 molecules intercalated into the C60 lattice (see §13.3). Raman scattering studies had shown previously that this coupling is indeed weak, since no shifts in the Raman-active C60 mode frequencies were detected upon oxygen intercalation [11.75]. This

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