Vibrational analysis of the 1×1-O overlayer on Ru(0001)

A densely packed 1×1-O overlayer with an oxygen coverage of 1.0 (relative to the number of substrate surface atoms) was prepared on Ru(0001) by sequential dissociation of ${\mathrm{O}}_2$ and ${\mathrm{NO}}_2$ and studied by means of high-resolution electron-energy-loss spectroscopy. Two dipole-active modes were observed at 81 and 7.3 meV due to a ${\nu }_{\mathrm{\perp }}$(Ru-O) stretch mode and a new surface mode, respectively. The value of ${\nu }_{\mathrm{\perp }}$(Ru-O) is shifted to higher energy compared to that for the known 2×2-O and 2×1-O adlayers at smaller oxygen coverages. These measurements indicate that the ${\mathrm{O}}_2$ dissociation is kinetically hindered for oxygen coverages larger than 0.5.