Molecular beam scattering of CO on iron oxide clusters supported on graphite (HOPG)

The adsorption dynamics of CO has been studied by measuring adsorption probabilities by means of the molecular beam scattering technique. Iron has been vapor deposited on ordered pyrolytic graphite (HOPG) with the vacuum chamber at a base pressure below 3 × 10−10 mbar. However, the Fe clusters partially oxidize, according to Auger electron spectroscopy (AES). Indications for a transition from two-dimensional to three-dimensional cluster growth modes with increasing Fe exposure have been obtained by AES and thermal desorption spectroscopy (TDS). The initial adsorption probabilities, S0, decrease with increasing impact energy and are systematically larger for larger clusters, consistent with the capture-zone model (CZM) and molecular, non-activated adsorption. S0 initially increases with adsorption temperature, Ts, and decreases above Ts = 100 K. The latter is not fully consistent with the simplest version of the CZM. The coverage dependence of the adsorption probabilities obeys the Kisliuk precursor model, as expected in the framework of the CZM.