We present a detailed discussion about optical absorption and exciton motion in extended systems. The model considered consists of an ordered structure of molecules with arbitrary lateral interactions and a local process for phase relaxation. The model is solved accurately: The lateral interaction is treated by the use of the coherent-potential approximation, and a projection-operator formalism is used in treating the phase-relaxation process. The theory is in good agreement with infrared light-absorption data for CO adsorbed on Ni(111) and Ru(001).

• Received 9 December 1985

DOI:https://doi.org/10.1103/PhysRevB.34.2266