Covalent Effects in Molecule-Surface Charge Exchange: O2 ON Ag(111).

摘要

The formation of negatively charged ions (oxygen) in the scattering of oxygen from silver surfaces is difficult to explain in the Brako-Newns scheme. To understand these processes, we have performed extended Hueckel tight binding calculations on model silver(111) slabs, placing O2 adsorbates at various sites and molecular orientations. As the survival probability of a negatively charged ion leaving the surface is determined by the back-tunneling frequency, the molecular ion survival probability should increase as the O2-Ag interaction decreases, and vice versa. On the other hand, the dissociation probability will increase with increasing interaction strength. The strength of the O2-Ag interaction can be gauged by the dispersion found in the projected DOS of the O2 affinity level 1 pi g. The second moment serves as a measure of relative dispersion, and in addition, can be related to the tunneling frequency. We find that bridging and hollow sites have the strongest O2-Ag interaction, thus the lowest survival probability of negative ions and the highest degree of dissociation. The interaction at top sites is much less. These results can be used to explain the higher O(-)/(O(-) + O2(-)) ratio observed at more grazing incident beam angles. Keywords: Surfaces, Reprints, Covalent effects, Molecule-surface change, Oxygen, Density of states (DOS), Scattering, Charge transfer, Silver surfaces. (JG)