AN INFRARED STUDY OF THE EPITAXIAL LAYER FORMATION IN THE NH3/CO/NI(111) SYSTEM

The adsorption of NH3 on a Ni(111) surface at 85 K pre-dosed with CO below saturation coverage (theta-CO = 0.15 and theta-CO = 0.5) as well as the adsorption of NH3 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 theta-CO = 0.15 and theta-CO = 0.5, respectively when exposed to NH3, indicating a strong interaction between the molecules via "through substrate" effect. From the relative intensites of the symmetric and degenerate NH3 deformation bands at 1211 and 1645 cm-1, respectively, it is concluded that in the theta-CO = 0.15 case the NH3 molecules are adsorbed in an upright position whereas for the theta-CO = 0.5 case the NH3 molecules are tilted with respect to the surface normal. Adsorption of NH3 on a CO saturated layer (theta-CO = 0.57) causes a large red shift of the "on top" CO from 2049 to 1467 cm-1 showing that the NH3 molecules form a stable surface complex with the underlaying "on top" CO molecules. Due to the increased size of the CO-NH3 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.