Adsorption and orientation of NH3 on Ru(001)

The interaction of NH3 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 α1, α2, β and γ) were detected with TDS. At low coverages, NH3 desorbs from the α1 state with a TDS peak maximum at ~ 310 K. The broadening of the TDS peaks and their shift to lower temperature with increasing NH3 coverage are related to repulsive lateral interactions between neighboring NH3 molecules. At higher NH3 coverages (θNH3≳ 0.15), a second desorption peak (α2) develops at T = 180 K, accompanied by a (2 × 2) LEED structure. With further increase of NH3 exposure a sharp desorption peak (β state) is found at T = 140 K, and is interpreted as due to NH3 species desorbing from a second adsorption layer. Finally a desorption peak due to multilayer adsorption (γ state) is found at 115 K. At low NH3 coverages (α1 state), a “halo”-like H+ ESDIAD pattern gives evidence of randomly oriented or freely rotating NH3, monomers, bounded via the N atoms to the surface with the H atoms pointing away from the surface. This orientation of NH3 is supported by work function measurements showing a linear decrease of Δφ in the α1 state. Structural information concerning the adsorption geometry of NH3 in the β state has been obtained from LEED and ESDIAD. During the formation of the second NH3 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 NH3 molecules are bonded via threefold hydrogen bonds to the first layer NH3, is proposed.