The sticking probability of NO at Ni(100) was examined using a beam of orientated NO molecules. It is found to be higher for N-end collisions. The asymmetry of the sticking probability has been measured to be a linear function of the molecular degree of orientation.It was determined to be A = 0.7 f 0.1 and nearly independent of coverage when normalized to the degree of orientation. The orientational dependence of the sticking probability as a function of coverage shows that the adsorption of the molecules cannot be described by a precursor model that neglects direct chemisotption.NO molecules are bond to most surfaces perpendicularly with the nitrogen atom directed towards the surface. The dynamics of the adsorption process should therefore be orientation-dependent due to anisotropic molecule-surface interactions. This leads to the question whether the sticking probability of the molecules can be influenced by a specific orientation of the molecular axis prior to adsorption. In theoretical studies a higher sticking coefficient was calculated for molecules approaching the surface in the favourable bonding configuration for NO/Pt(lll) and NO/Ag(lll) [l]. Recent experiments were focused on the effects of orientation in scattering and trapping/ desorption. Steric effects were observed in scattering experiments of orientated NO from Ag(ll1) [2] and of oriented CH,F from graphite [3]. Recently, Kuipers et al. [4] have measured steric effects in scattering and trapping/desorption of NO at Pt(ll1) from which they determined a higher trapping probability for an initial orientation with the N-end towards the surface. complemental to these scattering experiments we have examined steric effects in the sticking probability, which probes directly the chemisorption potential. As reported briefly [S], in the case of NO/Ni(lOO) for molecules with one particular degree of orientation we have observed a higher initial sticking probability when the surface is approached with the N-end first. However, to assess the order of magnitude of the effect measured one has to take into account the actual degree of orientation achieved. Therefore, we present here measurements of the sticking probability of nitric oxide on Ni(lOO) at different orientation field strengths. In addition, to study the influence of the molecular orientation on the adsorption kinetics we have examined the sticking probability as a function of coverage.Ni(100) was selected to study asymmetries in the sticking process because it is known that the NO molecules are chemisorbed perpendicularly with the N-end to the Ni surface (61 and have an initial sticking coefficient markedly different from unity and from zero (7]. We work at low target temperatures where the desorption probability from the chemisorbed state has been found to be negligibly small.Using the electrostatic hexapole technique [ 81, NO molecules can be state-selected for subsequent orientation in homogeneous electric fields 193 due to their permanent dipole moment. In our experimental apparatus a molecula...

The interactmn between gas-phase ortented NO molecules with Nl(100) has been mvesUgated by means of pulsed supersomc molecular beam, hexapole and time-of-flight techmques With TOF we examined the NO molecules leaving the clean N~(100) surface. A ,,eloclty dlstltbuhou t.ommen~utate wtth the surface temperature was obsc,ved The ttme-mtegrated s~gnal of the scattered NO beam shows an onentatmn asymmetry of As~ = 0 38 + 0 05 wtth respect to the NO and ON mltml orientation An lmtial sticking probabdity of 0 27 and 0 30 for preferential O-end and N-end colhsmns, respechvely, at a c(2 x 2) saturated layer of CO was also obtained. Taking the degree of orientation of the NO molecules into account, this leads to an asymmetry of Ast = 0 5 ___ 0 1 independent of coverage The CO molecules dtsplaced by NO were simultaneously recorded The CO signal shows an asymmetry with respect to the NO onentahon, as well. Two different d~splacement mechamsms are discussed to explain thts behavtor.

In a molecular beam experiment we have measured the sticking probability of NO at Ni(lO0)

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.

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