GIANT VIBRATIONAL RESONANCES IN A6C60 COMPOUNDS

Formation of alkali-C. compounds A(n)C60 (A = K, Rb) induces very large enhancements in the optical strength of two intra-C60 vibrational resonances and the emergence of a strong electronic transition centered at 1.2 eV. The vibrational spectra are particularly well defined for the A6C60 semiconductor comPounds, and it is proposed that the ca. ninetyfold enhancement of the 1428-cm-1 mode and its large shift (to 1342 cm-1 ) are generated by coupling to electronic excitations located on the molecule, consistent with recent theoretical descriptions of C60(6-) and the ionic solid bcc-A6+C60(6-).