Inverse-photoemission studies of adsorbed diatomic molecules

We present inverse-photoemission observations of the unoccupied 2pi level for ${\mathrm{N}}_2$, CO, and NO adsorbed on Ni(001) and CO adsorbed on Pd(111). We also present results from a study of the coadsorption system K and CO on Ni(001). We find that the position of the 2π level is deeper (higher binding energy) the stronger the chemisorption bond. This is opposite to expectations based on the conventional picture of σ donation and π back donation. We suggest that this is due to final-state screening. We also compare our inverse-photoemission studies with earlier near-edge x-ray-absorption fine-structure studies of the same systems and find that the principal screening of the core hole in the latter technique is determined by the occupation of the 2π orbital in the excited state. Finally, we find that for the coadsorbed system the 2π orbital is more occupied and that it lies at a higher binding energy: 2.8 eV rather than 4.0 eV above the Fermi level.