Direct and indirect mechanisms in site occupation of CO molecules on Ni(100) and Pt(111)

Abstract

Adsorption of CO onto a cold surface at 20 K has been studied by infrared reflection absorption spectroscopy (IRAS). The occupation ratio of bridged CO to terminal CO species on Ni(100) at 20 K ranges from ∼3.1 to 0.8 at the total coverage from 0.003 to 0.15 ML. Two adsorption processes of direct adsorption of CO molecules on the bare metal surface and indirect collisional site conversion are discussed. At zero-coverage limit, CO tends to accommodate on the site with the larger adsorption energy. Strong coverage dependence of the occupation ratio even at small coverages suggests that the interaction between CO molecules operates at relatively long range ( > 10 A ̊ ). The isotope experiments suggest that there is substantial interaction between preadsorbed (accommodated) CO species and incoming (mobile) CO species.