Trends in sticking and adsorption of diatomic molecules on the Al(111) surface

Abstract

To deepen our understanding of sticking and chemisorption behaviors, a trend study of static and dynamic aspects of the interaction between diatomic molecules, including ${\mathrm{F}}_{2}$, ${\mathrm{O}}_{2}$, NO, CO, and ${\mathrm{N}}_{2}$, and the Al(111) surface is performed. General features of free-electron-like metals are extracted and ramifications to other metals are indicated. With a slight generalization of the common form of density-functional theory, potential-energy surfaces (PES's) are calculated for both the adiabatic ground state and some excited states, where one or several electrons have been transferred from the adsorbate to the substrate. One trend applies to chemisorption (from dissociative, over molecular, to absent), and another one to the sticking probability (from unity, over incidence-energy-dependent, to zero). Ground- and excited-state PES's, local densities of states, and estimated electron-transfer probabilities are utilized. Electron transfer and energy dissipation to electron-hole pairs are identified as key processes in the dissociative adsorption. We apply a simple but general sticking model, where the competition between the electron-tunneling and the nuclear-motion time scales plays a central role [Surf. Sci. 532-535, 126 (2003)], now with a first-principles calculation of excited-state PES's. Measured trends in sticking and chemisorption behaviors are accounted for. Trends can be understood qualitatively in terms of electronegativity, kinetic Pauli-repulsion ranges, bond orders, and asymmetries of the molecules.