HYDROGEN CHEMISORPTION ON CESIUM-PREDOSED RU(001) SURFACES

The dissociative chemisorption of hydrogen on Cs-predosed Ru(001) surf aces at 90 K has been investigated using high-resolution electron ener gy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS ). The sticking coefficient of H-2 decreases strongly with Cs coverage and the bonding energy increases. A saturation coverage of about one monolayer of H can be chemisorbed independent of Cs coverage. Coadsorb ed Cs does not change the vibrational properties of the H layer which are determined by the Ru(001) surface: for small H coverages there is one perpendicular mode at 85 meV, and at large coverages there are the parallel modes at 100 meV and the perpendicular mode at 140 meV compa tible with a threefold hollow site. There is no evidence of hydride co mplexes at all Cs and H coverages.