Electron energy loss spectroscopy of ammonia on Ru(001)

Abstract

The adsorption of ammonia on Ru(001) has been studied using high-resolution electron energy loss spectroscopy. Multilayer, second-layer, and monolayer ammonia have been characterized vibrationally. These three states desorb near 115, 130 and between approximately 150 and 350 K, respectively. The symmetric deformation mode of chemisorbed ammonia shifts down in frequency continuously with increasing coverage from approximately 1160 cm−1 in the low-coverage limit to approximately 1070 cm−1 at (monolayer) saturation. The frequency of this mode in coordination compounds of ammonia is sensitive to the charge on the metal atom (increasing with increasing positive charge), and the frequency shift of this mode on the Ru(001) surface can be correlated with the work function decrease that this surface undergoes as the ammonia coverage increases. Off-specular EEL spectra allow the weak NH3 rocking mode and the frustrated translation of the ammonia perpendicular to the surface (i.e., the metal–nitrogen stretch) of chemisorbed ammonia to be resolved near 625 and 340 cm−1, respectively. These modes have not been identified in previous EELS studies of chemisorbed ammonia on hexagonally close-packed metal surfaces. Second-layer and multilayer ammonia yield EEL spectra similar to those observed on other metal surfaces. In agreement with previous results, the adsorption of ammonia on Ru(001) at 80 K, followed by annealing, leads only to reversible desorption.