Phonons and electron-phonon interaction in halogen-fullerene compounds

We have investigated the optical spectra of different halogen-fullerene compounds: C60I42x ,C 70I2 ,C 60Br24, C60Cl24, and C70Cl17. Two types of carbon-halogen bonding have been established: ~a! C60I42x 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 ,C 60Cl24, 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 1508 cm 21 . The group-theory analysis of phonon symmetries in rigid and nonrigid C60Br24 and C60Cl24 crystals has been performed. @S0163-1829~98!04710-9#