E. W. Plummer

The orientation of CO adsorbed on Ni(100)

Abstract We demonstrate the utility of angular resolved photoemission with synchrotron radiation to determine the bonding configuration of adsorbed molecules. This procedure depends on an understanding of the gas phase phase photoexcitation of the molecule and makes specific use of the polarized and continuum nature of the photon source to investigate the angle resolved emission from the adsorbate.

Photoelectron spectroscopy of iodine‐doped polyacetylene

X‐ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been used in a study of the electronic structure of iodine‐doped polyacetylene. Core level and valence band spectra are used to assess the nature and the consequences fo the iodine doping. The spectra are compared with our previous results on pure and AsF5‐doped polyacetylene, and other published work on iodine‐doped polyacetylene.

Atomic Binding of Transition Metals on Clean Single‐Crystal Tungsten Surfaces

The binding energy of single atoms of the Period‐6 transition elements on atomically perfect single‐crystal planes of tungsten was measured by utilizing low‐temperature‐pulse field desorption, in a ultrahighvacuum field‐ion microscope. For any given adatom, the dependence of the binding energy on surface crystallography was observed to be inconsistent with a pairwise interaction model. A new model incorporating a surface‐charge redistribution into a pairwise potential is shown to be consistent with the data. The variation in binding energy with electronic configuration of the adatom is interpreted in terms of an electron‐energy band model. The binding energy depends upon the electron occupancy of two subbands assumed to exist in the d levels of the atom, when it is adsorbed on the surface. Finally, the abnormal field‐desorption properties of adatoms on the (110) plane are discussed in terms of its anomalous work function.

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.

Angle‐resolved and variable impact energy electron vibrational excitation spectroscopy of molecules adsorbed on surfaces

Discrete energy losses occur when a beam of low energy electrons (<30 eV) is scattered from surfaces. For a gas–solid interface, inelastic scattering can occur through excitation of molecular vibrations. Chemical identification, molecular structures and bonding sites can be inferred from the energies of the characteristic losses. The angle and impact energy dependence of the mode intensity can give additional information which in many cases further elucidates the bonding. A consistent picture of the vibrational excitation mechanisms is derived which leads to the identification of helpful selection rules. Discussion will center on the simplest chemical adsorbate of H on W(100) and Ni(111). The application of the basic ideas to diatomic and polyatomic adsorbates will be mentioned briefly.

The chemisorption of CO on Cu(100) studied with angle resolved photoelectron spectroscopy

Abstract Using angle resolved UV photoelectron spectroscopy, coupled with the continuum of polarized light available with synchrotron radiation, we make an interpretation of the photoelectron spectrum of CO adsorbed on Cu(100). We point out that the bonding of CO on Cu as observed by photoemission is considerably different from the bonding of CO on Ni. These differences do not seem to be caused by bonding orientation differences, however, as the CO molecular axis is found to be very close to the surface normal, as was the case for CO on Ni(100). No evidence is found for a second phase of CO on Cu(100).

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.

Short-range polaron correlations in the ferromagnetic La1-xCaxMnO3

We use neutron scattering to demonstrate the presence of lattice polarons and their short-range correlations for several samples of La1-xCaxMnO3 in the Ca doping range 0.15</=x</=0.3. We establish the doping dependence of the orientation, commensuration, and coherence length of the polaron correlations and show that the populations of correlated and uncorrelated polarons are intimately related to the transport properties of the materials.

Electronic structure of polyenes and polyacetylene

Abstract The CNDO/S2 model is applied to evaluate the molecular orbitals and ultraviolet absorption spectra of the series of polyenes C 4 n H 4 n +2 , n = 1, 2, 3, 4; extended to encompass the infinite polyene chain, i.e., polyacetylene; and utilized to interpret the photoemission and absorption spectra of polyacetylene.

c(2 x 2)CO ON Ni(100): Photoemission orientation determination

Abstract Angular resolved photoemission studies of c(2 x 2)CO adsorbed on Ni(100) show that the molecule is bound to the surface with the molecular axis normal to the surface. The uncertainty of this determination is approximately 15°, which is consistent with the expected angular broadening due to vibrational modes. This is in distinct contrast to a bend of 34° proposed to explain LEED data on this system.