2π*-derived states for CO on Ni(111) studied by uv bremsstrahlung spectroscopy

Angle-resolved and polarization-dependent uv bremsstrahlung spectroscopy (inverse photoemission) is used to determine the energy and symmetry of the $2{\pi }^{*}$-derived states for CO on Ni(111). We observe a 3-eV-wide band centered at 3 eV above the Fermi level. This shows that the electron-hole interaction for transitions C $1s\to 2{\pi }^{*}$ is reduced from 10 eV in the gas phase to 1 eV for chemisorbed CO due to screening by metal electrons. Compared with physisorbed and gas phase CO, chemisorption on Ni(111) lowers the energy of the $2{\pi }^{*}$ orbital and increases its width.