Phonon line shape in disordered A₃C₆₀ (A=K, Rb)

We present a calculation of the H_g phonon self-energy for a model of A₃C₆₀ (A=K, Rb). The orientational disorder of the C₆₀ molecules is included, and the lowest order self-energy diagram is considered. The calculations illustrate that due to the violation of momentum conservation by the orientational disorder, Raman scattering can measure the decay of a phonon in an electron-hole pair, allowing the estimate of the electron-phonon coupling from such experiments. Comparison with experimental line shapes further provides support for a local correlation of the molecular orientations, with neighboring molecules preferentially having orientations differing by a 90° rotation.