INTRAMOLECULAR-VIBRATIONAL-MODE SOFTENING IN ALKALI-METAL-SATURATED C-70 FILMS

A comparative study on the Raman-active modes of solid C70 and alkali- metal-saturated M(x)C70 (x approximately 6;M = K, Rb, Cs) has been car ried out. The effect of saturation doping of alkali metals into the C7 0 fullerene lattice leads to significant changes in the vibrational sp ectra that are found to be insensitive to the radius or mass of the al kali-metal species. Whereas the high-frequency tangential modes (omega > 1000 cm-1) decrease in frequency by approximately 50 cm-1 with M do ping, the radial modes (omega < 800 cm-1) are found to be relatively i nsensitive to M doping, some of them increasing slightly in frequency. The effect on the vibrational modes of C70 due to electron-charge tra nsfer from the alkali-metal dopant to the fullerene molecule is in mos t respects similar to previous observations on M-doped C60.