W. Eberhardt

The adsorption of N2: Chemisorbed on Ni(110) and physisorbed on Pd(111)

Abstract The bonding of molecular N 2 has been investigated with angle resolved photoelectron spectroscopy and inelastic electron scattering. The spectra obtained from N 2 chemisorbed onto a Ni(110) surface are compared to CO chemisorbed onto Ni(110) and to N 2 physisorbed onto Pd(111). The N 2 molecular axis was found to be normal to the crystal surface for the chemisorbed state on Ni(110) and random for the physisorbed state on Pd(111). The NN and NiN 2 stretching frequecies indicate that the N 2 molecule is terminally bonded to a single Ni atom on Ni(110). The binding energies of the two outer σ states and one π state of chemisorbed N 2 were measured, indicating that the bonding of N 2 to a metal surface is different than CO. Both σ states drop in energy compared to the π level due to the fact that both of them are involved in the N 2 substrate bond. The symmetry of the gas phase N 2 molecule is reduced upon adsorption. The consequences of this are seen in the dipole active NN vibrational mode, the large intensity of the Ni-N 2 vibrational mode and the coupling of the adsorbate 4σ(2σ u ) level to the final state resonance which is forbidden by symmetry in the gas phase. Many electron excitation satellite lines are observed in the valence spectra of both the chemisorbed and physisorbed N 2 . The physisorbed satellite lines are nearly identical to those seen in gas phase N 2 , while the chemisorbed N 2 spectra has new satellite structure, due to the interaction with the substrate.

The characterization of surface acetylene and ethylene species on Pt(111) by angle resolved photoemission using synchrotron radiation

Abstract Angle resolved photoemission using synchrotron radiation was employed to elucidate the molecular structure of the species present in the low and high temperature phases of ethylene and acetylene on Pt(111). The plane polarized nature of synchrotron radiation allows the use of simple symmetry arguments to determine the orientation of an adsorbed species relative to the surface. In the low temperature phases of acetylene and ethylene the data are consistent with the carbon-carbon bond axis being parallel to the surface in agreement with earlier work. The high temperature phases of both molecules were found to consist of identical surface complexes where the carbon-carbon bond axis is normal or nearly normal to the surface. The orbital symmetries determined from this study favor the ethylidyne structure originally proposed by Kesmodel et al.

A comparison of surface electron spectroscopies

Abstract The objective of all surface electron spectroscopic techniques is to illucidate the nature of the surface and surface-adsorbate bond by probing the electronic states of the systems. All spectroscopies sample the system by measuring the response to an incident probe, yet the desired information is usually the ground state of the system. This paper will discuss and compare several types of spectroscopies: photoemission, photoadsorption or electron loss. Auger spectroscopy and inverse photoemission. The adsorption of CO will be used as the model system because much data exists for gas-phase CO, carbonyls and adsorbed CO.

High energy final bands in Cu

Abstract We probe final energy bands of Cu in the kinetic energy range 20 eV ⩽ E ⩽120 eV by angle resolved photoemission techniques. We observe deviations from free electron like behaviour in agreement with a LEED-type KKR calculation (Jepsen [5]). The lifetime broadening of the final bands at 60 eV kinetic energy is ∼5 eV (FWHM).

The bandstructures of graphite and graphite intercalation compounds as determined by angle resolved photoemission using synchrotron radiation

Abstract Angle resolved photoelectron spectra and yield spectra of graphite and “stage one” intercalated graphite (LiC6 and RbC8) show that the graphite bands are shifted to higher binding energies. This shift is a direct result of the charge transfer from the alkali atoms to graphite. The measured energy bands of graphite and LiC6 are in good agreement with theoretical calculations except that the measured carbon bands do not shift rigidly upon intercalation.

New experiments using a soft X-ray undulator☆

Abstract We discuss some of the exciting new experimental possibilities offered by soft X-ray undulators as a radiation source. In addition to the general discussion we present two specific experiments on isolated molecules for which the undulator source is extremely important. The first one of these experiments is a coincidence experiment between Auger electrons and ions generated in the events following the absorption of a soft X-ray photon by an isolated molecule. This gives a detailed picture of the involvement of individual valence electrons into the molecular bond. The second experiment is a study of the electronic decay of a resonant core electron excitation into a bound molecular orbital. The decay of these states yields new information about the localization of the molecular valence orbitals around the atom where the core hole was created.

Photoemission from laser excited semiconductors: Dynamic response of the electronic structure in Si

Abstract Using angle-resolved photoemission in coincidence with a pulsed Nd:Yag laser beam (h ν L = 2.33 eV), we have studied the dynamic rearrangement of the electronic states in Si in the presence of a large number of electron hole pairs created by laser photon excitation. We observed considerable changes in the valence band structure, a renormalization of the bandgap, and a satellite peak in the 2p core level emission due to more effective core hole screening.

Atomic and solid-state effects in the angle resolved photoemission cross section from an adsorbate: 4d levels of c(2 × 2)Te on Ni

Abstract The photon energy dependence of the normal emission from the Te - 4d levels of an ordered Te c(2 × 2) overlayer on Ni(100) shows two distinct types of structure. The main feature is due to an atomic effect, a delayed onset, a peak followed by a Copper minimum. Superimposed on this atomic profile are intensity modulations of 30 to 40% due to coherent scattering from the substrate. The atomic contribution is in qualitative agreement with theoretical calculations, while the modulations due to solid-state effects are not reproduced by a LEED final state type calculation.