Infrared absorption by charged phonons in doped C60

The giant infrared activity observed in A6C60 (A=K, Rb) by Fu et al. is interpreted 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. The theory we introduce for the compounds AxC60 (x=3, 4, and 6), describes the A6C60 data extremely well and enables values of the electron-molecular vibration coupling constants and the unsoftened T1u frequencies to be deduced directly from the IR spectra. We note that the T1u vibrations participate in the phonon mechanism of superconductivity in the A3A60 compounds. (orig.)