Dynamic and spectroscopic studies of single molecules physisorbed on graphite substrates. 1. Theoretical approach

The homogeneous line shapes of the infrared spectrum of single small molecules adsorbed on a graphite substrate are determined by applying a renormalization technique to the Hamiltonian of the system. The absorption profile can be determined by using a cumulant series expansion theory to treat the dynamic couplings between the optical system (formed by the vibration-orientation modes of the molecule) and the bath modes (characterized by molecular translations and substrate phonons). The various causes of homogeneous broadening of the lines are studied, with emphasis on phase relaxations through orientation-phonon transfers.