Phonons and electron-phonon interaction in halogen-fullerene compounds

We have investigated the optical spectra of different halogen-fullerene compounds: C₆₀I_4-x, C₇₀I₂, C₆₀Br₂₄, C₆₀Cl₂₄, and C₇₀Cl₁₇. Two types of carbon-halogen bonding have been established: (a) C₆₀I_4-x and C₇₀I₂ compounds are formed by a C₆₀ or C₇₀ molecule sublattice and an I₂ molecule sublattice that weakly interact via van der Waals forces; (b) C₆₀Br₂₄, C₆₀Cl₂₄, and C₇₀Cl₁₇ compounds are characterized by covalent bonds between C and Br/Cl atoms. We have studied in detail the resonance effects in C₆₀Cl₂₄ using the methods of Raman scattering, infrared absorption, and absorption in the visible region. The effect originates from the interactions between the phonon subsystem and the electron band at 2.33 eV and manifests itself in a resonant enhancement of the Raman line intensities and in the repetition of the phonon and the luminescence spectra shifted by the frequency of Raman-active phonon at 1508cm^-1. The group-theory analysis of phonon symmetries in rigid and nonrigid C₆₀Br₂₄ and C₆₀Cl₂₄ crystals has been performed.