Adlayer structures of CO adsorbed on Ru(0001) electrode studied by in-situSTM combined with IRAS

In-situ scanning tunneling microscopy (STM) combined with infrared reflecti on absorption spectroscopy (IRAS) has been used to investigate the structur al changes of the CO adlayers adsorbed on Ru(0001) in solution. At a low CO coverage, we found that a (root 3 x root 30 degrees) structure with a cove rage of 0.33 ML formed, where all the CO molecules occupy on-top sites. A f urther increase in the CO coverage gave rise to the formation of a new c(2 x 2)-2CO structure with a coverage of 0.5 ML as the saturation phase where CO molecules occupy both the on-top and threefold hollow sites. Our STM ima ge clearly demonstrates that one of the threefold hollow sites remains unoc cupied in the c(2 x 2)-2CO saturation adlayer. (C) 2000 Elsevier Science B. V. All rights reserved.