Charged-phonon absorption in doped ${\mathrm{C}}_{60}$

We interpret the giant infrared resonances observed in ${\mathit{A}}_6$${\mathrm{C}}_{60}$ (A=K, Rb) by Fu et al. as being due to a classical charged-phonon effect in which the ${\mathit{T}}_{1\mathit{u}}$ molecular vibration modes of ${\mathrm{C}}_{60}$ acquire electronic oscilator strength via couplng to virtual ${\mathit{t}}_{1\mathit{u}}$→${\mathit{t}}_{1\mathit{g}}$ electronic transitions. For the compounds ${\mathit{A}}_{\mathit{x}}$${\mathrm{C}}_{60}$ (x=3, 4, and 6), we predict, for weak electron–molecular-vibration coupling, that the softenings of the ${\mathit{T}}_{1\mathit{u}}$ modes are proportional to x, but that the integrated oscillator strengths of their absorption bands are proportional to the square of x.