Lateral interactions in coadsorbate layers: Vibrational frequency shifts

The effect of coadsorbed argon, hydrogen, and oxygen on the internal vibrat ion of CO on Ru(001) has been studied by infrared absorption spectroscopy i n order to disentangle electrostatic and chemical frequency shifts. Ar is e xpected to lead only to the former, H only to the latter, and O to a combin ation. In all cases, intermolecular interactions among CO molecules are avo ided by working at very low CO coverages (0.01-0.03 ML). Interestingly, the observed frequency shifts are discrete rather than continuous which is att ributed to a local interaction. Density functional calculations for suitabl e clusters have been used to model the frequency shifts, arriving at good a greement with experiment. Analysis of these theoretical results is then use d to quantify the contributions of electrostatic fields and of chemical eff ects on these shifts. It is shown that, despite very different signatures o f the various coadsorbate species, the observed C-O frequency shifts are la rgely of electrostatic origin, provided one uses the electrostatic field ge nerated by the coadsorbate and not an effective constant field. (C) 1999 Am erican Institute of Physics. [S0021-9606(99)70729-4].