Lateral interactions in coadsorbate layers: Vibrational frequency shifts

The effect of coadsorbed argon, hydrogen, and oxygen on the internal vibration of CO on Ru(001) has been studied by infrared absorption spectroscopy in order to disentangle electrostatic and chemical frequency shifts. Ar is expected to lead only to the former, H only to the latter, and O to a combination. In all cases, intermolecular interactions among CO molecules are avoided by working at very low CO coverages (0.01-0.03 ML). Interestingly, the observed frequency shifts are discrete rather than continuous which is attributed to a local interaction. Density functional calculations for suitable clusters have been used to model the frequency shifts, arriving at good agreement with experiment. Analysis of these theoretical results is then used to quantify the contributions of electrostatic fields and of chemical effects on these shifts. It is shown that, despite very different signatures of the various coadsorbate species, the observed C-O frequency shifts are largely of electrostatic origin, provided one uses the electrostatic field generated by the coadsorbate and not an effective constant field. (C) 1999 American Institute of Physics. [S0021-9606(99)70729-4].