Raman phonon spectra of isotopic mixed naphthalene crystals: Librational exciton model and the amalgamation limit

We present an experimental Raman study of lattice modes in neat and mixed (10%-90%) crystals of napnthalene-As and naphthalene-da at 100°K with 1 cm^-1 resolution. The spectral features of the neat crystals are preserved in the heavily doped mixed crystals, with small shifts and broadenings characteristic of an amalgamation limit that assumes weakly coupled excitation bands in the restricted Frenkel-Davydov limit. Rotation-translation interaction does not affect the mixed crystal spectra, thus making the Raman technique uniquely suited for the investigation of the librational (rotational) phonon band structure. The evidence is against localized or pseudolocalized phonons in these isotopic mixed crystals.