Theoretical treatment of excited electronic states of adsorbates on metals: Electron attachment to CO2 adsorbed on Pt(1 1 1)

Photochemistry involving adsorbates on metals often proceeds by photoexcitation of the metal followed by transient attachment of photoemitted electrons to the adsorbate. First principles theoretical methods suitable for describing electronic states embedded in a near continuum of metal to metal excitations are described and an application to electron attachment to CO2 adsorbed on Pt(1 1 1) is reported. Wavefunctions are constructed by ab initio configuration interaction methods which allow a rigorous resolution of states and differentiation between competing pathways of molecular desorption and dissociation. An embedding theory is used to achieve high accuracy in the adsorbate-surface region. The energy required to form the electron attached state is 5.2 eV for excitation to bent CO2 and 6.8 eV for excitation to linear CO2, hence both energies are near the work function of the metal (5.7 eV). The process also involves localization of the metal hole and attraction of the charged adsorbate to the metal. Optimum geometries are calculated and pathways that results in desorption, dissociation by bond rupture directly in the excited electronic state, or dissociation after return to the ground state potential energy surface via vibrational processes are explored.