Photoemission from Clean and Cesium-Covered Nickel Surfaces

Abstract

Photoemission energy distributions (PED) have been measured for clean and cesium-covered singlecrystal Ni(lll) surfaces. The surface condition was monitored during the experiments with a low-energy electron-diffraction (LEED) apparatus. Photoemission results for the clean surface are similar to those previously reported for polycrystalline material, except that minor structure resulting from direct transitions was observed, and a much higher work function of 5.42=fc0.04 eV was obtained for the (111) surface than has been reported for polycrystalline surfaces. The relative intensity of the anomalous peak at -4.5 eV with respect to the upper edge of the PED was found to vary with surface condition. With monolayer cesium coverage, very strong peaks are observed in the PED at -4.0 and -5.4 eV. These peaks appear when cesium is applied in excess of that required to lower the work function to its minimum value. A LEED pattern indicating ordering over twice the Ni lattice spacing is correlated with the cesium-related peaks. We argue that a fixed energy-loss mechanism is operating that causes structure to be displaced in the external PED by the amount of the fixed energy loss. We present evidence that this loss does not occur during transport of a photoexcited electron across the surface region, but that it accompanies the optical absorption process. It is suggested that peaks at -4.5 eV on clean Ni and at -4.0 and -5.4 eV on cesium-covered Ni result from the simultaneous excitation of interband transitions and a surface plasmon of 4.0-4.5 eV. Further experiments are suggested to check this hypothesis. For cesium coverages in excess of about three monolayers, a shoulder appears in the PED at -2.8 eV that we interpret as resulting from the excitation of the bulk plasmon in cesium.

Dipolar-energy-activated magnetic domain pattern transformation driven by thermal fluctuations

“…The reason for the shift of the SRT is the modification of the magnetic surface anisotropy, which depends on the chemical composition of the surface layer. Cs deposition changes the occupation of the atomic orbitals of the surface layer owing to hybridization 14,15 . However, the domain patterns and the evolution of the patterns relative to the SRT remain unaffected motivating the introduction of an effective temperature, where the real temperature as well as the thickness are incorporated on an equal footing.…”

Characterization of dye molecules and carbon nanostructures by tip‐enhanced Raman spectroscopy

“…Of particular interest to nano‐optics are the plasmons localized at a surface or in a particle, both of which may be accompanied by a field enhancement 1–4 of up to 100 times compared to the amplitude of the exciting wave. Surface plasmons play a key role in the optical properties of small structures as well as in electron tunnelling 5, field ion and photoemission 6, 7, van der Waals forces 8, surface‐enhanced Raman scattering (SERS) 9 and recently in tip‐enhanced Raman scattering (TERS) 1–4.…”

O- and Na-Induced Surface States on Paramagnetic Ni

“…I propose that these peaks are associated with Cs-induced surface states instead of surface plasmons as proposed by Callcott and McRae. 6 Application of LCMTO to thin films proceeds exactly as in solids except that the layer structure constants of Kambe 7 are used in place of the bulk structure constant of Ham and Segall. 8 The method of constructing wave functions and potentials has already been described in an application 9 to Cu.…”

“…6 They find their spectra to depend on the amount of Cs deposited on the Ni(lll) surface. I suggest that the peak at -4.5 eV for what they refer to as clean Ni is actually due to small amounts of contaminant such as O or Cs.…”