Electronically excited states of chemisorbed molecules

Abstract

The electronic excitations of CO, NO, and pyridine chemisorbed on a variety of surfaces have been studied with low energy (10–20 eV) electron energy loss spectroscopy. By comparing the observed electronic transitions of CO on Ni(100), Cu, and Si(111) surfaces, the ∼6 and ∼8.5 eV transitions of chemisorbed CO are reassigned as singlet and triplet coupled 5σ̃→2π̃* excitations, respectively. By simple arguments we show that screening (both image and charge transfer) and spin quenching by a metallic substrate should not significantly affect the transition energies of neutral adsorbate excitations. This behavior is discussed in relation to photoemission measurements and, in particular, the limitations of using photoemission data to estimate electronic excitation energies of adsorbed molecules. The dependence of electronic spectra on adsorbate bonding configurations are considered for NO and pyridine. Vibrational spectra of NO on Ni(100) reveal changes in bonding configuration as a function of NO surface coverage, but only small changes are observed in the electronic spectra. Coverage or temperature dependent bonding phase transitions are found for pyridine on Ni(100). In this case, however, intensities of the π→π* excitations of the aromatic ring are found to be very sensitive to the nature (n vs π bonding) of the chemisorption bond.