INFRARED PROFILE OF SINGLE MOLECULES ADSORBED ON IONIC SUBSTRATES - RELAXATIONAL MECHANISM FOR THE HOMOGENEOUS LINEWIDTH

The homogeneous line shape of the infrared spectrum of a single molecu le physisorbed on ionic substrates is determined on the basis of a ren ormalization technique of the total system Hamiltonian. The absorption profile can be written in terms of a limited cumulant series expansio n of the dynamical couplings between the optical system formed by the vibration-orientation modes of the molecule, and the bath modes charac terized by molecular translations and substrate phonons. The various c auses of homogeneous broadening of the lines are studied, implying pha se and population relaxations through intramolecular transfers, In thi s latter case, two situations are considered depending on whether the internal vibration modes of the admolecule are localized or resonant ( quasi-resonant) with the substrate phonon modes. These situations are usual for small polyatomic molecules adsorbed on substrates with exten ded optical phonon bands. An application to the calculation of the Q, P and RX branch widths for CO, CO2 and NH3 molecules adsorbed on NaC l(001) and/or MgO(001) is extensively discussed. We show that, at low temperature T less than or similar to 20 K, the homogeneous width is q uite negligible and inhomogeneity must be invoked to interpret the mea sured width. By contrast, for increasing T values (T greater than or s imilar to 50 K), homogeneous broadening increases and contributes sign ificantly (about to 50%) to the measured width for CO and CO2 molecule s adsorbed on clean substrates. (C) 1996 American Institute of Physics .