Selective resonant Raman enhancement in polyiodide chains

A selective resonant Raman enhancement is observed in solid films of the blue complex formed between starch and iodine. The intensity of the 162-${\mathrm{cm}}^{-1\mathrm{}}$ phonon peak is strongly enhanced when the laser frequency is resonant with the main electronic absorption band at ≃ 590 nm, whereas the 112-${\mathrm{cm}}^{-1\mathrm{}}$ phonon is strongly enhanced when the laser frequency is tuned in the blue region in resonance with the shoulder at ≃ 455 nm. Similar behavior has been observed in a number of compounds containing polyiodide chains such as iodinedoped polyvinyl alcohol, sulfur polynitride, and polyacetylene. A theoretical explanation of this phenomenon is given in terms of the theory of resonant secondary emission, and explicit analytical expressions for the Raman cross sections as a function of the exciting wavelength are derived for the case of resonance with broad and structureless absorption bands (i.e., a rapidly relaxing system with strong electron-vibration interactions). A structural model for the chromophore of these compounds is also proposed.