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 multiply 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 vibrational 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 must 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.