Initial adsorption sites of CO on Pt(111) and Ni(100) at low temperature

Adsorption of CO on Pt(lll) at 25 K and Ni(100) at 18 K has been studied by infrared reflection absorption spectroscopy (IRAS). The occupation ratio of terminal CO to bridged CO species on Pt(111) at 25 K ranges from similar to 2.3 to 1.2 at total coverages From 0.03 to 0.13 ML, and that of bridged CO to terminal CO species on Ni(100) at 18 K ranges from similar to 3.2 to 0.7 at total coverages from 0.003 to 0.15 ML. Such strong coverage dependence of the occupation ratio, even at small coverages, suggests that the substrate-mediated interaction between CO molecules operates al a relatively long range (>10 Angstrom). The isotope experiments suggest that there is dynamic interaction between preadsorbed (accommodated) CO species and incoming (mobile) CO species.