Role of electron-phonon interaction in resonant x-ray Raman scattering by polymers and solids

We present a theory of radiative resonant x-ray Raman scattering by solids and polymers beyond the Born-Oppenheimer approximation. The role of electro n-phonon coupling of the intermediate core excited and final electronic sta tes is analyzed in detail for pi-electron systems and solids assuming the f rozen-orbital approximation. The momentum exchange processes between valenc e electrons and phonons and with x-ray photons are analyzed, showing the si milarities and differences between the two processes. The resonant inelasti c x-ray scattering by a pi-electron system is quenched up to zero in the Bo rn-Oppenheimer approximation with frozen orbitals if the momentum exchange between valence electrons and x-ray photons is neglected. The electron-phon on and electron-photon interactions open the scattering channels for all oc cupied states in pi systems. The frequency dependence of these effects is a nalyzed, showing that when the duration of the scattering is shortened by a large detuning of the excitation frequency, the role of electron-phonon co upling of both core-excited and final states is suppressed, depleting the c ross section for pi systems up to zero. The detuning quenches the symmetry breaking of the core-excited electronic states and results in a restoration of the selection rules and a conservation of electron momentum. Specific s election rules for the zero-phonon line in x-ray Raman spectra of linear po lyenes are found. A detailed investigation of the narrowing, or collapse, o f the electron-vibrational bands is given. When the detuning is large, the spectral profile is described by a joint density of states. It is predicted that the singularities of this joint density of states follows the Raman-S tokes dispersion law, something that allows a mapping of the band structure . We found that the phonon broadening of these singularities is completely quenched by detuning. A detailed investigation of the spectral shape versus detuning of the so-called excitonic band is given. [S0163-1829(99)07913-8] .