Influence of orientation on coadsorption dynamics: CO displacement from a c(22) precovered Ni(100) surface by free oriented NO

Collision experiments between two different molecules both oriented with their axis are presented: The kinetics of the CO desorption and NO sticking probability by oriented NO has been investigated using supersonic molecular beam and work function techniques. The work function measurements exhibit mainly molecular adsorption of NO on a CO precovered Ni( 100) surface at T=300 K. The desorption and the sticking depend strongly on the initial orientation of the NO molecules in the gas phase. The sticking probability is higher for preferential N-end collisions and the sticking asymmetry is constant, whereas the desorption asymmetry changes its sign from initially negative to positive values, depending on the exposure time or NO coverage. We explain the asymmetries by two different mechanisms, namely direct and indirect molecular exchange. N-end collisions lead preferentially to NO sticking and NO induced CO desorption, whereas O-end collisions enhance the direct CO displacement.