The Raman Scattering and Hot Luminescence of Self-Trapping Excitons

Summary

A theoretical consideration of light scattering by an excitation of self-trapping excitons in the absorption band is presented. The whole secondary radiation spectrum, including ordinary and hot luminescence (OL and HL), is studied. Polariton effects are disregarded.

Two different cases of exciton self-trapping are treated as follows: (1) the strong interaction of a localized exciton packet as well as a free exciton with phonons; and (2) the strong interaction of a localized exciton packet with phonons, but weak interaction of a free exciton with phonons. In the first case the secondary radiation spectrum is shown to be analogous to the corresponding spectrum of an impurity centre in a crystal. The main differences are the validity of the quasi-momentum conservation law and a certain motion of an exciton in the hot vibrational state. The former significantly affects only the Raman scattering cross sections of some first orders. The latter leads to an asymmetry in scattering excitation profiles and to an enhancement of the blue part of HL spectrum.

The main feature of the second case is the coexistence of free and self-trapped excitons, which are separated by a potential barrier. It is shown that in this case the resonance scattering of small order can be described by formulas analogous to those for the case of weak exciton-phonon coupling. The multiphonon resonance scattering, however, differs sharply in that it is primarily conditioned by transitions involving a self-trapped state as an intermediate one. Formulas are presented describing two kinds of such resonance scattering: (1) tunnel HL created by the penetration of an exciton through the self-trapping barrier and the subsequent relaxation and (2) HL from the top of the barrier.