Optical Absorption Spectra of Bipolarons
  The absorption of large bipolarons is investigated using the path-integral method. The response of a bipolaron to an external electromagnetic field is derived in the framework of the memory-function approach. The bipolaron optical absorption spectrum consists of a series of relatively narrow peaks. The peculiarities of the bipolaron optical absorption as a function of the frequency of the electromagnetic field may be attributed to the transitions involving relaxed excited states and scattering states of a bipolaron. 
J. T. Devreese, S. N. Klimin, V. M. Fomin (Universiteit Antwerpen -   UIA)