Dynamics and instabilities of an exciton in the phonon field and the correlated absorption-emission spectra

Dynamics of an exciton in the phonon field is studied with particular attention to the varying feature of the optical spectra with increasing exciton-phonon coupling. The absorption lineshape varies gradually from a Lorentzian broadened by the scattering rate of the delocalized exciton to a Gaussian broadened by the energy fluctuation of the localized excitation; the latter broadening is narrowed into the former by the motional effect of the exciton. The luminescence is subject to abrupt change from the resonant sharp line of the free exciton to the Stokes-shifted broad band of the self-trapped exciton as coupling constant exceeds a critical value. The steepness constant characterizing the low energy tail of the absorption is inversely proportional to the coupling constant, allowing the prediction on the types of luminescence from the absorption spectrum. Depending on the lattice relaxation energy coping with the optical excitation energy, one can consider three different situations with high, low and no luminescence. As the ultimate instability, one can conceive spontaneous generation and condensation of self-trapped excitons which amounts to the electronic-structural phase transition. A few examples of this catastrophic situation are presented.