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

In-situ scanning tunneling microscopy (STM) combined with infrared reflection absorption spectroscopy (IRAS) has been used to investigate the structural 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 coverage of 0.33 ML formed, where all the CO molecules occupy on-top sites. A further 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 image clearly demonstrates that one of the threefold hollow sites remains unoccupied in the c(2 x 2)-2CO saturation adlayer. (C) 2000 Elsevier Science B.V. All rights reserved.