Resonance Raman Scattering of a Peierls-Hubbard Dimer

The resonance Raman scattering spectra of a Peierls-Hubbard dimer with explicit inclusion of electron coupling to both intramolecular vibration and intermolecular phonon modes are discussed theoretically and investigated experimentally. When the exciting wavelength is in resonance with the charge transfer band of a symmetric dimer the intermolecular phonon modes which modulate the transfer integral are selectively enhanced through both Albrecht's A and C mechanisms. No enhancement occurs for the intramolecular modes. This prediction is strikingly demonstrated by reporting the low temperature resonance Raman spectra of crystalline (TTF)Br (TTF = tetrathiafulvalene) whose structure consists of almost isolated (TTF+)2 dimers. Fourteen bands are observed at low frequencies whereas the intramolecular mode region is silent. The observed bands are tentatively assigned to intermolecular modes in which the TTF+ cations in a dimer translate antiphase and to one low frequency out-of-plane molecular vibration. It is argued that a detailed study of the resonance enhancement mechanism may provide information on the electron correlation in a Peierls-Hubbard dimer.