Vibrational interaction between molecules adsorbed on a metal surface: The dipole-dipole interaction

We have developed a theory for the properties of vibrational excitations in molecules adsorbed on a metal surface. The coherent potential approximation (CPA) is used in the treatment of the vibrational interaction between the molecules. We show, by interpreting infrared spectra of substitutionally disordered systems consisting of isotopic mixtures of CO on Cu(100), that the molecules interact mainly through their dipole fields. We also show that in interpreting the integrated absorptance in infrared spectroscopy or the relative loss intensity in electron-energy-loss spectroscopy it is necessary to take into account the screening due to the electronic polarizability of the adsorbed molecules. A simplified version of the CPA result is used for a discussion of the absorption spectra of partial monolayers of one isotope. With the assumption that the CO molecules are randomly distributed, comparison between theory and experiment indicates that the dipole-dipole interaction alone is responsible for the coverage-dependent frequency shift for CO adsorbed on a transition metal [Ru(001)], whereas there is an almost equally large counteracting chemical shift on a noble metal [Cu(100)]. The meaning and origin of the dynamical dipole moment of adsorbed CO molecules are discussed. We find that the increase of the dynamical dipole moment (by a factor 2-3) upon adsorption probably is due to charge oscillations between CO $2{\pi }^{*}$ molecular orbitals and the metal. Finally, we outline how the theory developed here can be applied to a fundamental step in photosynthesis.