Giant vibrational resonances inA6C60compounds

Formation of alkali-${\mathrm{C}}_{60}$ compounds ${\mathit{A}}_{\mathit{n}}$ ${\mathrm{C}}_{60}$(A=K, Rb) induces very large enhancements in the optical strength of two intra-${\mathrm{C}}_{60}$ vibrational resonances and the emergence of a strong electronic transition centered at 1.2 eV. The vibrational spectra are particularly well defined for the ${\mathit{A}}_6$ ${\mathrm{C}}_{60}$ semiconductor compounds, and it is proposed that the ca. ninetyfold enhancement of the 1428-${\mathrm{cm}}^{\mathrm{-}1}$ mode and its large shift (to 1342 ${\mathrm{cm}}^{\mathrm{-}1}$) are generated by coupling to electronic excitations located on the molecule, consistent with recent theoretical descriptions of ${\mathrm{C}}_{60}^{6\mathrm{-}}$ and the ionic solid bcc-${\mathit{A}}_6^{+}$ ${\mathrm{C}}_{60}^{6\mathrm{-}}$.