No abstract

No abstract

The adsorption probability for NO/Pt(lll) is highest for an initial orientation with the N end of the molecule towards the surface. This steric effect in the adsorption brings about a large averaged steric effect in the direct scattering, which is amplified by the fact that the adsorption probability is very high. In case of direct scattering an N-end collision results in a smaller reflection angle than an O-end collision. PACS numbers: 79.20.Rf, 34.50.Lf, 82.65.MyRecently, the first experimental investigations of the vector properties of the initial or final angular momentum J in molecule-surface interactions have been performed. Alignment and orientation of J in the case of diatomic-surface scattering have been demonstrated. 1 " 5 A preferential orientation of the molecular axis for CHF3 desorption from Ag(lll) has been claimed. 6 For adsorption of NO on Ni(100) a dependence on initial orientation has been observed. 7 In reactive gas-phase collisions orientation and polarization effects are also intensively investigated. 8,9 In our group the influence of initial molecular orientation on direct scattering and on adsorption of NO at Ag(lll) have been demonstrated. 10,1! The measured effect in the adsorption was unexpected. One would expect that approaching the surface with the orientation for which the attractive potential is stronger, the N-end towards the surface, would give a higher adsorption probability. This expectation was also confirmed by classical trajectory calculations. 12 But the experiment showed that the approach in the least attracted orientation, the O-end towards the surface, results in the highest adsorption probability. This result could be explained by assuming a strong anisotropic or orientation-dependent repulsion. By means of rotationally mediated adsorption this can lead to the highest adsorption probability for the least attractive orientation. 13 The weaker anisotropy in the attraction eventually causes a trapped molecule to equilibrate with the most attracted end towards the surface.If the anisotropy in the attraction were larger than NO/Ag(l 11), it could overrule the effect of the anisotropy in the repulsion in the adsorption dynamics. Therefore, it is interesting to study steric effects for NO adsorption at Pt(lll). For this system the binding energy (1.13 eV) is five times larger than for NO/Ag(ll l). 1415 The anisotropy in the attraction is likely to be much larger as well. So, in the case of NO/Pt(lll) one expects the highest adsorption probability for approach with the most attracted orientation (the N-ends towards the surface). 16 A steric effect in the adsorption probability can be investigated in two different ways. One possibility is the measurement of the desorption flux as a function of the orientation of the incoming molecules. A second possibility is a measurement of the dependence of the total directly scattered flux on orientation. Since one distinguishes only two ways of reflection, direct scattering and adsorption-desorption, a steric effect in one reflection chann...

A pulsed supersonic and cold oriented beam of NO molecules is incident upon the (111) face of clean Ag and Pt single crystal surfaces. The steric effect in the scattered density distributions is determined by a quadrupole mass spectrometer. It is found that the steric effect in the peak in the distribution of direct inelastically scattered molecules depends linearly on the reflection angle. In all circumstances O-end collisions lead to scattering angles more inclined towards the surface than N-end collisions. For the Pt(111) surface a much stronger steric effect is measured than for the Ag(111) surface. The steric effect seems to scale with the incident normal velocity. These strong steric effects can be explained by the larger trapping probability for the N-end orientation and a leverage effect due to the high trapping probability.

No abstract