Infrared and Raman studies of pressure-polymerized ${\mathrm{C}}_{60}$s

We discuss the room-temperature infrared and Raman spectra for the pressure-induced rhombohedral, tetragonal, orthorhombic ${\mathrm{C}}_{60}$ polymers and the ${\mathrm{C}}_{60}$ photopolymer. The interfullerene bonds are found to not only split the ${\mathrm{H}}_{1\mathrm{g}}$ - and ${\mathrm{F}}_{1\mathrm{u}}$ -derived modes, but also to soften these intramolecular modes relative to those in pristine solid ${\mathrm{C}}_{60}$ . The observed mode softening is attributed to the loss of C=C bonds in the molecular cages, which are broken to reform as intermolecular four-membered rings. Lorentzian line-shape analysis of the Raman data in the vicinity of the pentagonal pinch mode frequency in these polymerized structures reveals six distinct Raman lines at very nearly the same frequencies. However, the relative intensities of these lines were found to be sensitive to the phase of the fullerene polymers. The frequency of the ${\mathrm{A}}_{\mathrm{g}}$ (2)- and the ${\mathrm{F}}_{1\mathrm{u}}$ (4)-derived modes is found to depend linearly on the intermolecular coordinate ${\mathrm{a}}_{\mathrm{I}}$ of the polymeric phases. A frequency softening of Δω/Δ${\mathrm{a}}_{\mathrm{I}}$ ∼19 and 58 ${\mathrm{cm}}^{\mathrm{-}1}$ /Å was deduced for the ${\mathrm{A}}_{\mathrm{g}}$ (2)- and the ${\mathrm{F}}_{1\mathrm{u}}$ (4)-derived modes, respectively. In addition, the experimental spectra are compared to the vibrational spectra obtained from a first-principles molecular dynamical calculation for a ${\mathrm{C}}_{60}$ dimer and an infinite chain of polymerized ${\mathrm{C}}_{60}$ .