Further studies of Ni(001)c(2×2)CO: Evidence for back donation in the chemisorption bond

We report new results for the chemisorption system Ni $\mathrm{}\left(001\right)c(2\mathrm{}\times 2)\mathrm{CO}$ using angle-resolved polarization-dependent photoemission with synchrotron radiation ($10\le h\nu \le 40$ eV). Adsorbate-induced structure with $\pi$- like symmetry is observed in normal emission at $E_i\simeq -2.2\mathrm{}$ eV, and in the (100) mirror plane off normal at $E_i\simeq -1.3\mathrm{}$ eV. Using the back-bonding scheme of Blyholder, an interpretation is given in terms of the mixing of metal $d\pi$ orbitals with the empty $2\pi$ orbital of molecular CO.