Electron-hole interaction in a polyene

Summary form only given. The electron-hole interaction in a polyene is studied both for a short chain and at its thermodynamic limit. We found that there exists a competition between the on-site interaction U and the nearest neighbor interaction V for the interaction between the electron and hole. The former serves as a repulsion force of the electron and hole for a finite-long polyene while the latter as an electron-hole atttractive interaction. The excitonic states should appear in the system with the electron-hole attractive interaction and then the optical absorption is excitonic while it is interband with an electron-hole repulsive interaction.