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 studi ed by infrared reflection absorption spectroscopy (IRAS). The occupati on ratio of terminal CO to bridged CO species on Pt(111) at 25 K range s 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 range s 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 smal l coverages, suggests that the substrate-mediated interaction between CO molecules operates al a relatively long range (>10 Angstrom). The i sotope experiments suggest that there is dynamic interaction between p readsorbed (accommodated) CO species and incoming (mobile) CO species.