Uv/HREELS measurements of the excited states of adsorbed CO: Benchmarks for ab initio calculations

Recently there has been some controversy about CO adsorption on platinum, with some researchers supporting the Blyholder model and others saying that corrections are needed. We have used UV/HREELS to examine the electronic structure of adsorbed CO to distinguish between the various models. We have found that the 5σ to 2π* triplet peak shifts from 6.2 to 5.6 eV upon adsorption while the 5σ to 2π* singlet peak shifts from 8.3 to 8.2 eV. Both peaks also broaden, as expected from the Blyholder model. A comparison of these results to previous calculations shows that only Hückel calculations predict a shift that is within 0.5 eV of the experimental shift. Most of the previous calculations predict shifts that are in the wrong direction.