Observation of a novel high density 3O(2x2) structure on Ru(001)

High-resolution electron energy loss spectroscopy and scanning tunneling microscopy have been applied to identify and characterize a novel high density phase of oxygen adsorbed on Ru(001). Three oxygen atoms per (2 x 2) unit cell are arranged such that a (2 x 2) vacancy superstructure is formed. Atomically resolved STM images clearly establish the existence of such a phase. The ordered layer is characterized by a sharp (2 x 2) superstructure as seen by low energy electron diffraction. Two dipole active energy losses were round for the 30(2 x 2) layer at 0.75 ML and interpreted as the in-phase oxygen to metal stretching vibration and the totally symmetric eigenmode consisting of frustrated translations of three next-neighbor oxygen atoms towards each other. The vibrational modes of this novel structure are compared to data for oxygen coverages in the range 0.5-1 ML. (C) 1997 Elsevier Science B.V.