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 electroni c states and to distinguish states involving the adsorbate from those corre sponding to metal-metal excitations. A large number of excited electronic s tates occur in the range 3-9 eV above the ground state. Potential curves fo r singlet and triplet states are reported. Many of the states are charge-tr ansfer states arising from Pt 6s and 5d transitions to CO pi* levels. Altho ugh 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 adso rption that excitations from it lie very high in energy. It is concluded th at the broad band observed in HREELS experiments is due to the charge-trans fer excitations. (C) 2001 Elsevier Science B.V. All rights reserved.