An adiabatic model of chemisorbed molecules: electron spectroscopy and excited-state potential-energy surfaces

We review models that have been used to understand excited states of chemisorbed species, focussing on CO and NO, and encounter problems in attempting to fit all observables. We then introduce a new model. We show that a purely-electronic adiabatic approximation leads to an accurate solution for the system wavefunction in the limit that the molecule-substrate interaction is weak. This produces a configuration-interaction theory that has a Hubbard-like form. We derive semi-empirical parameters for the NO:Pt(111) system and find that the transfer integral for the 2π-substrate interaction is small for all choices of the screened electron-electron interaction, U. This suggests, for this system, that the substrate indeed adiabatically follows the fluctuations in adsorbate charge which are inherent in a covalent bond. We propose that our model is robust and applies to many adsorbed molecules. We then investigate low-lying excited-states of the metal-molecule bond.