Raman scattering from charge-density waves and application to polyacetylene

Oscillations in an electronic gap formed by a charge-density wave are described in terms of Raman-active amplitude modes. In a multi-phonon system we find the Raman shifts and their intensity ratios in terms of a parameter A which contains all effects of electron- phonon, electron-electron and electron-disorder interactions; the single assumption in the theory is the adiabatic limit. The theory is compared with data on polyacetylene where the relation of A with the electronic gap yields information on the type of disorder. For an electron-phonon system an ab initio calculation of the Raman cross section is given and compared with more conventional calculations.