Angle-resolved photoelectron spectroscopy has been used to study the changes in the bonding of CO when coadsorbed with alkali-metal atoms. On metal surfaces [Cu(100) and Ru(0001)] various features in the photoemission spectra, including reduced site symmetry and a large shift of the $1\pi$ molecular orbital of CO, indicate a $\pi$-bonded configuration of the CO molecule. $\pi$ bonding involves bonding through both the $1\pi$ and $2{\pi }^{*}$ molecular orbitals of CO. All of the data indicate a short-range interaction between the K and CO.

• Received 10 September 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.1856