An infrared study of the epitaxial layer formation in the NH 3/CO/Ni(111) system

Abstract

The adsorption of NH 3 on a Ni(111) surface at 85 K pre-dosed with CO below saturation coverage ( θ CO = 0.15 and θ CO = 0.5) as well as the adsorption of NH 3 on a CO pre-saturated Ni(111) surface has been studied by infrared spectroscopy. Downward shifts of 169 and 52 cm −1 are observed in the CO bands for θ CO = 0.15 and θ CO = 0.5, respectively when exposed to NH 3, indicating a interaction between the molecules via “through substrate” effect. From the relative intensities of the symmetric and degenerate NH 3 deformation bands at 1211 and 1645 cm −1, respectively, it is concluded that in the θ CO = 0.15 case the NH 3 molecules are adsorb in an upright position whereas for the θ CO = 0.5 case the NH 3 molecules are tilted with respect to the surface normal. Adsorption of NH 3 on a CO saturated layer ( θ CO = 0.57) causes a large red shift of the “on top” CO from 2049 to 1467 cm −1 showing that th molecules form a stable surface complex with the underlaying “on top” CO molecules. Due to the increased size of the CONH 3 surface complex, the surrounding “bridge” CO molecules are forced into new positions halfway between a “bridge” and an “on top” position, generating a new CO stretching band only 14 cm −1 above the original CO bridge frequency.