Chemisorbed pyridine on Ni(001): A high resolution electron energy loss study of vibrational and electronic excitations

Abstract

High resolution angle-resolved electron energy loss spectroscopy has been applied to the study of the vibrational and electronic losses of pyridine chemisorbed on Ni(001). The evaluation of the contributions of in-plane vs out-of-plane vibrational modes of the chemisorbed molecule in the dipole scattering regime shows that orientational phase transitions occur as a function of coverage and temperature. In particular, the molecular plane is parallel to the surface at low coverages at ∼170 K corresponding to a π-bonded species; at higher coverages the molecular plane tilts vertically with respect to the surface which is indicative of a bonding interaction through the N lone pair. Heating to room temperature or room temperature exposures produce an approximately vertically oriented species with some rotation about the N end. This species might be assigned to a chemisorbed α-pyridyl species. The different types of bonding at the surface are manifested in the characteristics of the electronic loss spectra taken with electron primary energies less than 20 eV. A Ni surface state excitation at ∼0.7 eV observed in the clean spectra is quenched when bonding is through the N-lone pair; this can be related to the localization of the surface electronic state. Intramolecular π→π* excitations are not observed when the molecule is π bonded; this is due to a combination of the broadening of the π levels by interaction with the substrate and the screening of the (parallel) dynamic dipole by the metal surface in the dipole contribution to the loss cross section. Off-specular collection geometries show a broad angular distribution for the inelastically scattered electrons as expected for dipole scattering when the loss energy is a significant fraction of the impact energy. A strong charge transfer excitation centered at ∼3 eV is observed for the n-bonded configurations, and this suggests a Ni d→pyridine 3B1 transition which would be maximized in these cases when the overlap between the respective wave functions is greatest. Electronic loss spectra following the thermal evolution of pyridine initially exposed at room temperature show that the molecular species persists up to ∼220 °C, whereupon characteristic electronic losses for C are observed indicating thermal decomposition. © 1984 American Institute of Physics.