Excitation Spectra of Charge-Transfer Complexes in Molecular Crystals

Abstract

The excitation spectra of charge-transfer (CT) complexes at low temperature have been studied in the visible and uv region of frequencies of molecular crystals. CT complexes in the excited and in the ground state have been considered, which consist of four particles (two electrons and two holes, respectively). The CT complex in the excited (or ground) state is formed by the resonance interaction between an excited electron (or a positive hole) and a positively (or negatively) charged bound-state ion. The formation of the positively or negatively charged bound-state ion is described by the physical process, where a Frenkel exciton and a hole are annihilated or a Frenkel exciton is created and an electron is annihilated, respectively. Favorable conditions for the existence of the positively (or negatively) charged boundstate ion are found to be when the exciton and the hole (or the electron) are tightly bound to one another and the hole (or the electron) state is occupied. Using a self-consistent approach, expressions for the one-electron and the one-hole Green's functions are derived, which are correct in the generalized random-phase approximation. There is a splitting in the excitation spectrum of the CT complexes in the excited and ground state, respectively. The coupling functions responsible for the formation of the charged bound-state ions as well as for the CT complexes are due to the Coulomb and exchange CT-type interactions. The derived expressions for the energies of excitation and for the CT energies are discussed in terms of Mulliken's theory of molecular CT complexes. Optical transitions that occur between the CT complex in the ground state and that in the excited state are discussed and expressions for the absorption coefficient for the processes in question are derived.