  The spectroscopic effects of electron-phonon coupling in mixed-valence
chlorine-bridged Pt chains complexes are investigated through a parallel
infrared and Raman study of three compounds with decreasing Pt-Pt distance
along the chain. The e-ph interaction is analyzed in terms of the
Herzberg-Teller coupling scheme. We take into account the quadratic term and
define a precise reference state. The force field relevant to this state is
constructed, whereas the electronic structure is analyzed in terms of a simple
phenomenological model, singling out a trimeric unit along the chain. In this
way we are able to account for all the available optical data of the three
compounds, and to estimate the relevant microscopic parameters, such as the
e-ph coupling constants and the CDW amplitude.
