Spectroscopic Properties of Excited States

The elementary excitations of a conjugated polymer chain can be described within the mono-electronic approach as electron and hole quasiparticles [74] in a one-dimensional band structure, possibly weakly bound into extended Wannier-type excitons [71,75]. Within this framework, electron-phonon interactions lead to a peculiar family of exotic excitations including solitons, polarons, polaron pairs and bipolarons. In many cases, however, disorder is so significant that the polymer films are better described as an ensemble of relatively short conjugated segments [76], essentially behaving 

Since all these excitations possess clear spectroscopic fingerprints, a very powerful tool for studying their photophysics is the so-called pump-and-probe technique, which can be related to the optical nonlinearities of the material. As far as optical properties are concerned, the change in refractive index upon excitation (An or Ak), either optical or otherwise, is the nonlinear part of the optical response. Traditionally two approaches have been developed to measure nonlinearities, based on different techniques  

A clear-cut distinction between the two processes is the time response, which is ultrafast in the former (essentially instantaneous), and finite (associated with the lifetime of real excitations) in the latter. Here we limit our discussion to resonant nonlinearities, which are crucial for describing the fundamental working mechanisms of organic photovoltaic cells. Examples will be reported concerning isolated molecules and the condensed phase. 

---