Raman spectroscopy of ${\mathrm{La}}_2@{\mathrm{C}}_{80}$ and ${\mathrm{Ti}}_2@{\mathrm{C}}_{80}$ dimetallofullerenes

This paper is devoted to the Raman spectroscopy of two dimetallofullerenes ${\mathrm{La}}_2@{\mathrm{C}}_{80}$ and ${\mathrm{Ti}}_2@{\mathrm{C}}_{80}.$ Previous studies of monometallofullerenes $\left(M@{\mathrm{C}}_{82}\right)$ and dimetallofullerenes ${(M}_2@{\mathrm{C}}_{82})$ have demonstrated that Raman spectroscopy is a useful tool to probe the cage-metal bond strength. The latter is a fingerprint of the metal to fullerene charge transfer, which plays an important role in the stabilization of the metallofullerene. In the present study, the metallic ions are trapped inside a ${\mathrm{C}}_{80}$ hollow cage for each metallofullerene, but their group and weight differ. This provides the opportunity to probe in the same study the ability of Raman spectroscopy to determine the ionization state of ions trapped in a cage, which eventually differs only by isomerization. A detailed analysis of the low-frequency part of the Raman spectrum is proposed for both ${\mathrm{La}}_2@{\mathrm{C}}_{80}$ and ${\mathrm{Ti}}_2@{\mathrm{C}}_{80}.$ Two low-frequency modes at 161 ${\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathrm{La}}_2@{\mathrm{C}}_{80}$ and at 196 ${\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathrm{Ti}}_2@{\mathrm{C}}_{80}$ are tentatively assigned to a fingerprint of the interaction between the trapped ions and cage. The corresponding values for the metal-cage valence force constant are in favor of an effective charge transfer of three electrons per lanthanum to the cage and of two electrons at most per titanium to the cage. These results are in good agreement with theoretical predictions and with electron energy loss spectroscopy or x-ray observations.