P.S. Wehner

Photoemission from Noble Metals and Adsorbates using Synchrotron Radiation

With the advent of synchrotron radiation in the 32-280 eV range at the Stanford Synchrotron Radiation Project, it has become possible to elucidate the transition from ultraviolet to X-ray-induced photoemission. This has been accomplished by studies of noble metals. Polycrystalline copper shows a valence-band (VB) profile that approaches the X-ray induced shape at hv ~ 100 eV. In polycrystalline silver, the 4d cross section follows the atomic curve, with a reversal of VB peak intensities near hv = 110 eV. Strongly anisotropic behavior is observed in copper single crystals, using angle-resolved photoemission (ARP). Normal ARP spectra from Cu(100), (110), and (111) crystals follow the band dispersion through the Brillouin Zone, including a dramatic resonance between the Fermi level (EF) and 2 eV binding energy for hv = 43-52 eV. High temperature and high photon energy studies demonstrate the importance of the Debye-Waller factor in photoemission leading to a breakdown of the direct transition model. In adsorption studies of CO on Ni and Pt, CO is shown to stand up with oxygen out. For Pt, electrons are found to flow from t2g orbitals near EF to CO, and the CO 1π and 5σ binding energies are reversed relative to the gas phase. At higher photon energies, hv = 150 eV, the CO orbitals are very prominent on a Pt substrate. An inversion of the angular distribution of these orbitals and energy-dependent resonances in their intensities provide evidence for final state scattering effects at photon energies above 40 eV.

Angle- and energy-dependent core-level photoelectron energy loss studies in Al and In

Abstract Electron energy loss structures of Al and In core-level photoemission spectra, in particular surface and bulk plasmon losses, have been investigated as functions of photon energy (i.e., photoelectron kinetic energy). These studies utilized synchrotron radiation to provide a variable photon source in the ultra-soft X-ray region, thus allowing these loss processes to be studied at photoelectron kinetic energies for which the mean free path of the electrons is minimal. The Al plasmon loss structure was also studied with soft X-ray radiation in an angle-resolved mode, allowing the variation of effective photoelectron sampling depth with different electron take-off (collection) angles. These results for the relative intensity of the bulk and surface plasmons as a function of electron kinetic energy and electron exit angle are in qualitative agreement with the predictions of Sunjic and Sokcevic. The core-level binding energies of surface atoms have also been studied with the result that no significant shift has been observed with respect to bulk-atom core levels.

Adsorbate sensitivity enhancement in photoemission: CO on Pd

Abstract Photoelectron spectra for CO on a Pd substrate have been measured in the photon energy range 40–180 eV. A dramatic resonance in the intensity ratio of the CO-derived peaks compared to the Pd valence band (VB) was found for photon energies near 130 eV, where the ratio is ca. nine times larger than at hv = 40 eV. This increased spectral sensitivity to the CO molecular orbitals results from a Cooper minimum in the photoemission cross section of the Pd 4d valence level. Because such spectral minima are present in all 4d and 5d VB materials, adsorbate studies on these substrates at photon energies near 130 eV (for which laboratory sources are potentially available) should benefit greatly from the decreased background and the increased surface sensitivity. This point is further illustrated by comparing the present results to previously reported photoemission data from CO on Ni and Pt substrates.

EVIDENCE FOR THE ITINERANT ELECTRON MODEL OF FERROMAGNETISM AND FOR SURFACE PHOTOEMISSION FROM ANGLE-RESOLVED PHOTOEMISSION STUDIES OF IRON

Abstract Angle-resolved HeI photoemission spectra of Fe(001) are reported and interpreted within the framework of a direct transition model using Callaways ferromagnetic band structure. The generally good agreement between predicted and experimental peak positions is taken to be strong support for the itinerant electron theory of ferromagnetism. Spectra taken with nearly grazing incidence p -polarized light, however, are not interpretable in terms of the direct transition model. The importance of electron refraction is noted, as is the value of interpolation calculations for interpreting ARP spectra.