Spectral and relaxation features of excitons mixed with excimer quasi-molecular states

For some class of crystals, an excited atomic state gives rise, along with free excitons, to another related type of crystalline excitations also formed by resonance bindings, viz. an excimer two-atomic quasi-molecule. Such an exciton polaron, due to its intrinsic vibrational degree of freedom, rather moves fast through the lattice and mixes with free-exciton states. It is shown that this mixing results in a distortion of the lower part of the exciton band via splitting it into narrow polaronic subbands corresponding to vibrational levels of the excimer quasi-molecule. This entails essential physical consequences involving absorption spectrum and anomalous relaxation phenomena. The theory is in good qualitative agreement with experiment. (C) 1999 Elsevier Science B.V. All rights reserved.