Adsorption and orientation of NH 3 on Ru(001)

Abstract

The interaction of NH 3 with clean Ru(001) surfaces has been studied using LEED (low energy electron diffraction), ESDIAD (electron stimulated desorption ion angular distribution), TDS (thermal desorption spectroscopy), and work function changes (Δφ). Four different binding states (denoted as α 1, α 2, β and γ) were detected with TDS. At low coverages, NH 3 desorbs from the α 1 state with a TDS peak maximum at ~ 310 K. The broadening of the TDS peaks and their shift to lower temperature with increasing NH 3 coverage are related to repulsive lateral interactions between neighboring NH 3 molecules. At higher NH 3 coverages (θ NH 3≳ 0.15), a second desorption peak (α 2) develops at T = 180 K, accompanied by a (2 × 2) LEED structure. With further increase of NH 3 exposure a sharp desorption peak (β state) is found at T = 140 K, and is interpreted as due to NH 3 species desorbing from a second adsorption layer. Finally a desorption peak due to multilayer adsorption (γ state) is found at 115 K. At low NH 3 coverages (α 1 state), a “halo”-like H + ESDIAD pattern gives evidence of randomly oriented or freely rotating NH 3, monomers, bounded via the N atoms to the surface with the H atoms pointing away from the surface. This orientation of NH 3 is supported by work function measurements showing a linear decrease of Δφ in the α 1 state. Structural information concerning the adsorption geometry of NH 3 in the β state has been obtained from LEED and ESDIAD. During the formation of the second NH 3 layer (β) a (2√3 × 2√3)R30° LEED pattern is observed and is accompanied by an ESDIAD pattern with a hexagonal outline. A structural model of the β-state bonding, in which second layer NH 3 molecules are bonded via threefold hydrogen bonds to the first layer NH 3, is proposed.