STM investigation of the Ni(111)−c(4 × 2)−2CO structure

Scanning tunneling microscopy has been used to investigate the Ni(111)-c(4 × 2)−2CO structure at room temperature. From the determination of the local bonding symmetry and molecular separation distances, STM images show that CO occupies both fcc and hcp threefold hollow adsorption sites with the molecular axis tilted with respect to the surface normal. Although this result is in conflict with various vibrational studies, it is in agreement with the assignment based on recent SEXAFS, PED and LEED studies. STM images also show a height difference between adjacent CO molecules which is attributed to an adsorbate-induced buckling of the Ni surface.