CHEMISORPTION OF CO ON THE IR(111) SURFACE - DOES ADSORPTION-ASSISTEDDESORPTION OCCUR

The kinetics of adsorption and desorption of carbon monoxide on the Ir (111) surface has been investigated using the well-defined IR absorpti on peak position of the intramolecular CO stretching vibration, as mea sured with FT-IRAS, as a nonintrusive surface coverage probe. The adso rption probability depends strongly on the temperature and coverage, w hich is consistent with the mediation of an extrinsic precursor. The r ates of desorption of chemisorbed CO were measured both for the case o f desorption into vacuum and desorption in the presence of an ambient CO background pressure. Data were measured in the range of 390 to 500 K and for CO pressures from 1 X 10(-8) to 1 X 10(-5) mbar. Since the r ates of desorption in both cases are very similar, we exclude the exis tence of ''adsorption-assisted desorption'' for the investigated syste m.