INFRARED-SPECTROSCOPY OF OVERTONES AND COMBINATION BANDS

We present a detailed discussion on infrared spectroscopy of vibration al combination bands and overtones of adsorbate systems. For the case that the (dynamical) lateral coupling between the adsorbates is domina ted by dipole coupling, we present general results for the absorption spectra which can be used to analyze experimental data and deduce the bond anharmonicity delta omega. The theoretical results are used to an alyze experimental line shape data for the combination band of the C-O and the Ru-CO stretch vibrational modes of CO adsorbed on Ru(001), as well as the overtone of the C-O stretch vibration for the same adsorb ate system. It is found that for strong lateral coupling land weak anh armonicities) asymmetric line shapes are common; strong anharmonicitie s lead to the formation of localized two-phonon bound states besides a continuum of delocalized two-phonon states. However, even then the ex traction of anharmonic parameters can be severely impeded by dynamic l ine shifts of the localized overtone band through virtual transfers of vibrational quanta to neighboring oscillators. We further consider th e mechanisms which contribute to the broadening of overtone excitation s and show that the energy relaxation rate for the overtone is twice a s large as for the fundamental, while the (pure) dephasing rate is app roximately four times higher than that of the fundamental excitation. (C) 1998 American Institute of Physics. [S0021-9606(98)70643-9].