HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY AND ANGLE-RESOLVED ULTRA-VIOLET PHOTOEMISSION SPECTROSCOPY STUDIES OF CARBON-MONOXIDE ON A CS-PRECOVERED RU(10(1)OVER-BAR-0) SURFACE

There are two molecular adsorption states of CO in the CO + Cs/Ru(<10( 1)over bar 0>) system. One is alpha(1)-CO, with lower C-O frequency, a nd the other is alpha(2)CO, with higher C-O frequency. The latter is s imilar to that on a clean Ru(<10(1)over bar 0>) surface. The two state s of CO are occupied sequentially during the exposure to CO. The C-O s tretch frequency of both states shifts to a higher value with the expo sure to CO and to a lower value with higher Cs precoverage. The electr onic structure of the alpha(1)-CO molecule is different from that of t he alpha(2)-CO molecule and is of the same character as CO in a tilted mode.