Two-band model description of electroabsorption and third-harmonic generation in 1-D MX linear chains

Within a two-band model, we investigate the electroabsorption (EA) and third-harmonic generation (THG) processes in halogen-bridged mixed-valence Pt complexes: PtCl, PtBr and Ptl. For PtCl, the theoretical THG spectrum shows three peaks, corresponding to (i) a three-photon resonance at 0.83 eV originating in a M(etal)-M(etal) transition; (ii) a two-photon resonance at 1.5 eV from an M-M band-edge transition; and (iii) a three-photon resonance at 1.6 eV from an M-X transition. The latter two peaks account well for the twin-peak structure seen experimentally. We show that the twin-peak intensity strongly decreases from PtCl to PtBr and disappears for Ptl. We also discuss the theoretical EA spectra due to localized defects (polarons, bipolarons, kinks, and excitons).