OBSERVATION OF A NOVEL HIGH-DENSITY 3O(2X2) STRUCTURE ON RU(001)

High-resolution electron energy loss spectroscopy and scanning tunneli ng microscopy have been applied to identify and characterize a novel h igh density phase of oxygen adsorbed on Ru(001). Three oxygen atoms pe r (2 x 2) unit cell are arranged such that a (2 x 2) vacancy superstru cture is formed. Atomically resolved STM images clearly establish the existence of such a phase. The ordered layer is characterized by a sha rp (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 vib ration and the totally symmetric eigenmode consisting of frustrated tr anslations of three next-neighbor oxygen atoms towards each other. The vibrational modes of this novel structure are compared to data for ox ygen coverages in the range 0.5-1 ML. (C) 1997 Elsevier Science B.V.