CHARGED-PHONON ABSORPTION IN DOPED C-60

We interpret the giant infrared resonances observed in A6C60 (A = K, Rb) by Fu et al. as being due to a classic charged-phonon effect in which the T1u molecular vibration modes of C60 acquire electronic oscillator strength via coupling to virtual t1u --> t1g electronic transitions. For the compounds AxC60 (x = 3, 4, and 6), we predict, for weak electron-molecular-vibration coupling, that the softenings of the T1u modes are proportional to x, but that the integrated oscillator strengths of their absorption bands are proportional to the square of x.