INFLUENCE OF ORIENTATION ON COADSORPTION DYNAMICS - CO DISPLACEMENT FROM A C(2X2) PRECOVERED NI(100) SURFACE BY FREE ORIENTED NO

Collision experiments between two different molecules both oriented wi th their axis are presented: The kinetics of the CO desorption and NO sticking probability by oriented NO has been investigated using supers onic molecular beam and work function techniques. The work function me asurements exhibit mainly molecular adsorption of NO on a CO precovere d Ni(100) surface at T=300 K. The desorption and the sticking depend s trongly on the initial orientation of the NO molecules in the gas phas e. The sticking probability is higher for preferential N-end collision s and the sticking asymmetry is constant, whereas the desorption asymm etry changes its sign from initially negative to positive values, depe nding on the exposure time or NO coverage. We explain the asymmetries by two different mechanisms, namely direct and indirect molecular exch ange. N-end collisions lead preferentially to NO sticking and NO induc ed CO desorption, whereas O-end collisions enhance the direct CO displ acement.