Fig. 11.20. The reflectance of a single-crystal Cm sample at 295, 200, 140, and 80 K in the vicinity of the four infrared-active modes. The fourfold splittings of the Flu(l), Fi„(3) and Flu(4) infrared-active modes are indicated by the arrows, but no splitting is observed for the Flu(2) mode [11.106], modes due to disorder. The absence of splitting in the Flu(2) feature and the narrowing of the Flu(2) line with decreasing temperature have not yet been explained [11.106].
If the crystal field effect were associated with zone folding of the phonon dispersion relations, it is unlikely that the dispersion over the entire fee Brillouin zone would be as small as a few cm-1, nor does the crystal field effect easily account for the fourth line in the quartet. Another possible explanation for the low-temperature structure is to associate the observed split IR modes with the effect of the local environment on the vibrational modes. In the high-temperature crystalline phase we would expect the molecules to see an average local environment. Below Tm the rotations of the molecules become restricted to a few axes [11.73] (see, for example, §7.1.5), and the molecules jump (or ratchet) between a few (e.g., five) orientational minima. We then identify each of the components of the low-temperature multiplet with a specific site orientation of the molecule relative to its neighbors. As the temperature is lowered, these specific sites become better defined and therefore the splittings are expected to become better resolved. The effect of the local molecular environment should be more pronounced for some atomic displacements (i.e., certain Tu modes), in agreement with experiment. Furthermore, the magnitude of the mode splittings arising from local environment effects is expected to be only a few cm-1, also in agreement with observations. Crystal field effects associated with the local environment of C60 molecules also seem to be important in explaining temperature-dependent structure observed in the Raman spectrum (see Fig. 11.19).
Since the appearance of the quartet structure in the infrared spectrum occurs gradually as T is decreased, these experiments do not provide an accurate method for the determination of T0l. The results of Fig. 11.18, however, show that the Raman-active modes more sensitively determine T01, since they are more sensitive to local environment changes near T01.
The appearance of additional structure in the infrared-active spectra below T01 has also been verified [11.92] through temperature-dependent infrared spectroscopy studies on single-crystal C60. This study [11.92] was focused on identifying which features in the infrared spectra are due to isotope effects, crystal field effects, and combination modes, by monitoring the change in lineshape for the many lines observed in the 200 or more resolved features in the high-resolution infrared spectra (see §11.5.4). For example, features associated with the isotope effect narrow at rather low temperatures (see §11.5.5 for the Raman analog). Crystal field effects are expected to be turned on below T0l as preferential orientational ordering sets in.
11.5.11. Vibrational States Associated with Excited Electronic States
Because of the metastability of the lowest-lying triplet electronic state above the Fermi level (see §13.2.3), it is possible to observe vibrational features [11.107] associated with the metastable triplet state [11.108,109], Experimental results are available for the Ag(2) pentagonal pinch mode in the excited triplet electronic state. This mode is found to be downshifted relative to that for the vibration in the electronic ground state, because of the weaker binding and consequently reduced force constants for the excited state [11.107], Above a laser excitation intensity of ~50 W/cm2, a line downshifted from the 1468 cm-1 Ag(2) mode appears in the Raman spectra taken at a temperature of 40 K and 514 nm laser excitation wavelength, as shown in Fig. 11.21 [11.107]. At these low temperatures, the probability for molecular alignment to induce polymerization is very low. Furthermore, the reversibility of the observations on lowering the laser intensity provides supporting evidence that the observed phenomena are not connected to polymerization. The identification of the downshifted feature with Raman scattering from the excited metastable triplet state is further supported by a quenching of this feature by the introduction of oxygen, which greatly decreases the lifetime of the excited triplet state (see §13.2.3). This excited state Raman line grows in intensity with increasing laser intensity (see Fig. 11.21) until about 300 W/cm2; above this laser intensity, the process becomes irreversible [11.107].
The effect of 514 nm laser intensity on the Raman spectrum of C60 has also been studied at room temperature [11.109], showing a downshifted
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