Vibrational analysis of the 1x1-O overlayer on Ru(0001)

A densely packed 1X1-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 O-2 and NO2 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 v(perpendicular to)(Ru-O) stretch mode and a new surface mode, respectively. The value of v(perpendicular to)(Ru-O) is shifted to higher energy compared to that for the known 2X2-O and 2x1-O adlayers at smaller oxygen coverages. These measurements indicate that the O-2 dissociation is kinetically hindered for oxygen coverages larger than 0.5.