THE ADSORPTION OF NO ON RU(001) AND ON O(2X1) RU(001) REVISITED/

The infrared absorption spectra of NO adsorbed on Ru(001) and on O(2 x 1)/Ru(001) have been re-examined to clarify several open questions in these systems, including an old puzzle regarding the insensitivity of IRAS to detect NO/Ru(001) on threefold bridge sites despite its clear presence in (specular) HREELS spectra. We find no indication for such an inconsistency in vibrational spectra as we observe both, the multi ply coordinated v(1)-NO and the linear v(2)-NO species; we are able to detect their internal stretching modes with signal-to-noise ratios up to 1000:1. We attribute this to better instrumentation as well as to improved crystal perfection and layer homogeneity. These improvements reflect themselves in unparalleled 3 cm(-1) linewidths of the vibratio nal bands and very narrow peaks in thermal desorption spectroscopy. As in the case of CO/O(2 x 1)/Ru(001), a (2 x 2) honeycomb structure mus t form for NO/O(2 x 1)/Ru(001) after annealing to 400 K. A consistent new picture of the processes occurring upon coadsorption and annealing can be obtained.