Raman study of the polymerized state of RbC60 and CsC60

We report a room-temperature Raman investigation of C-60 and orthorombic AC(60) phase (o-AC(60)). The o-AC(60) phase is present when the sample is slowly cooled from the AC(60) fee disordered phase (T > T-c of about 370 K) to room temperature. In the o-AC(60) phases large changes in the intensity of intramolecular modes not active in C-60 as well as splitting of some modes of C-60 are observed. In addition, new modes at 342, 632, and 967 cm(-1) are evidenced. Nevertheless, from these data the length and the geometry of the oligomers phase cannot be unambiguously defined. Only the significant differences between the o-AC(60) Raman spectrum and the Raman spectrum of photopolymerized C-60 and pressure-induced polymerized phases of C-60 indicate some differences in the structural organization of C-60 monomers in the o-AC(60) phase with respect to these systems. In both o-RbC60 and o-CsC60 phases the low-frequency Raman spectrum as excited at 776 nm exhibits a well-defined peak at around 31 cm(-1). The assignment of this mode is discussed and we propose to attribute it to the interball stretching mode in the relatively long oligomer (longer than dimer). This assignment supports the mechanism of polymerization via a (2+2) cycloaddition reaction between C-60 molecules in the o-AC(60) phase.