Effect of strain on the chemisorption of CO on ultrathin Ni films on Cu(001) - art. no. 075401

The chemisorption of CO on ultrathin epitaxial Ni films on a Cu(001) surfac e was investigated by high resolution electron energy loss spectroscopy (HR EELS). CO molecules adsorb on both on-top and bridge sites of the Ni film a t room temperature (RT) as is the case on a bulk terminated Ni(001). The vi bration energies of CO stretching modes on both sites monotonically increas ed and grew beyond those on a Ni(001) surface with increasing Ni thickness. The relative population of the two bonding sites gradually changed: the on -top site is preferred by CO for submonolayer Ni coverages while the bridge site is slightly favored for the Ni films thicker than 3 monolayers (ML). We found an existing model, the "pillow model," for the strain effect on th e vibrational energy incompatible with our spectroscopic results.