Excited electronic states of CO adsorbed on platinum

Excited electronic states for CO adsorbed on Pt(111) are investigated using a seven-atom monolayer model of the surface in which CO is adsorbed atop a Pt atom. Configuration interaction theory is used to resolve the electronic states and to distinguish states involving the adsorbate from those corresponding to metal-metal excitations. A large number of excited electronic states occur in the range 3-9 eV above the ground state. Potential curves for singlet and triplet states are reported. Many of the states are charge-transfer states arising from Pt 6s and 5d transitions to CO pi* levels. Although the CO sigma to pi* transitions for an isolated CO molecule fall in the 6-9 eV range, the CO sigma orbital is stabilized to such an extent on adsorption that excitations from it lie very high in energy. It is concluded that the broad band observed in HREELS experiments is due to the charge-transfer excitations. (C) 2001 Elsevier Science B.V. All rights reserved.