Infrared and Raman Evidence for Dimers and Polymers in Rb1C60

The infrared- and Raman-active vibrational modes of C60 were measured in the various structural states of Rb1C60. According to earlier studies, Rb1C60 has an $fcc$ structure at temperatures above ~100C, a linear chain polymer orthorhombic structure when slowly cooled, and an as yet undetermined structure when very rapidly cooled ("quenched"). We show that the spectra obtained in the polymer state are consistent with each C60 molecule having bonds to two diametrically opposite neighbors. In the quenched state, we find evidence for further symmetry breaking, implying a lower symmetry structure than the polymer state. The spectroscopic data of the quenched phase are shown to be consistent with Rb2(C60)2, a dimerization of C60.