Abstractive chemisorption in the initial oxidation of Al(111) has been experimentally verified using variable incident energy O2. Scanning tunneling microscopy images show a transition between single O-adatom reaction products to more pairs of O-adatom reaction products as the O2 incident energy is raised from 0.025 to 0.8 eV. The ejected O atoms have been detected in the gas phase with resonant enhanced multiphoton ionization. The observations that both abstractive and dissociative chemisorption are activated processes are in contrast to current adiabatic models of the absorption process.
The dissociative and abstractive chemisorption dynamics of NO on Al͑111͒ were studied. A higher sticking probability for the N end-on of NO onto Al͑111͒ was measured. In contrast, Auger electron experiments reveal stepped surfaces to be oxygen rich at low coverage after exposure to NO. Density functional theory calculations show ͑i͒ a few angstroms from the surface, an N end-on first collision geometry results in electronic structures consistent with charge transfer; ͑ii͒ there is stabilization on the surface for N end-on or side-on orientations; ͑iii͒ dissociation is enhanced by a partial or full flip of the molecule.
The adsorption of monoenergetic IBr molecules on the Si(111)-7×7 surface has been studied using scanning tunneling microscopy, mass spectrometry, Auger electron spectroscopy, and supersonic molecular beam techniques. The adsorption proceeds predominantly via the direct abstractive adsorption mechanism and preferentially occurs at the center Si adatoms. The IBr abstraction probabilities at the incident energies of 0.15 and 0.82 eV have been determined to be 0.90±0.03 and 0.77±0.03, respectively. The minor dissociative adsorption channel of IBr can be enhanced at the expense of the abstractive adsorption channels by raising the incident energy. Most importantly, no atomic selectivity for iodine or bromine was observed. A reaction mechanism involving two types of transition states, Si⋯I⋯Br(s) and Si⋯Br⋯I(s), has been proposed to interpret the experimental observations. The attractive interaction between the nearly symmetric highest occupied molecular orbitals (HOMO, π* antibond) of IBr and the partially-filled Si adatom dangling bonds governs the surface site selectivity and the atomic selectivity of IBr adsorption on Si(111). Comparison with the adsorption of ICl on the surface has also been made to clarify the role of the asymmetric molecular bonding in adsorption dynamics.
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