Raman line shapes in A3C60 (A = K, Rb) effects of orientational disorder on phonon spectra

We calculate the H-g phonon self-energy for a model of A(3)C(60) (A= K, Rb) using the lowest order self-energy diagram. The model includes the orientational disorder of the C-60 molecules in A(3)C(60) compounds. We find that due to the violation of momentum conservation by the orientational disorder, even q = 0 phonons can very effectively decay in electron-hole pairs. This leads to an additional broadening of these phonons in metallic systems, which can be measured in Raman spectroscopy. The measured additional broadening provides an estimate of the electron-phonon coupling. 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 degrees rotation.