Photoemission and Optical Studies of the Electronic Structure of Palladium

Abstract

Photoemission and optical-reflectivity measurements have been carried out on palladium samples prepared and measured in vacuum of approximately 5×${10}^{-9\mathrm{}}$ Torr for $h\nu \le 11.8$ eV. In addition, photoemission measurements were made on Pd samples in poorer vacuum for $h\nu =16.8\mathrm{} \mathrm{and} 21.3$ eV. The optical functions ${\epsilon }_1$, ${\epsilon }_2$, $\omega \sigma$, $\alpha$, and $\mathrm{Im}\left(\frac{1}{\epsilon }\right)$ have been calculated from reflectivity data by Kramers-Kronig analysis. Optical transitions are found to be predominantly nondirect in the spectral range studied, although relatively weak direct (or nonconstant-matrix-element) transitions are also observed for $11.8 \mathrm{eV}>h\nu >9.8\mathrm{} \mathrm{eV}$. The valence-band optical density of states of Pd, deduced from photoemission and reflectivity data, has peaks at $E-E_f=-0.1\mathrm{} \mathrm{and} -1.1\mathrm{}$ eV and is lacking in strong structure elsewhere. There is no evidence for a high density of states near $E-E_f=-5\mathrm{}$ eV as in Ni, Co, Fe, and Cr. The $d$-band structures in Pd and Ag are found to be related rather well by the rigid-band model. Values for the density of states at the Fermi energy in Pd are obtained, and are found to be in reasonable agreement (within experimental accuracy) with values obtained from band calculations. A strong peak in the loss function obtained from reflectivity data is observed at $h\nu =7.5\mathrm{}$ eV. This correlates with the strong peak observed at 6.8 eV in energy-loss measurements and is probably due to plasma resonance. Detailed analysis of the photoemission and optical data indicate that a peak in the energy-distribution curves, which appears 7.5 eV below the maximum energy in the energy distributions for $h\nu =18.6\mathrm{} \mathrm{and} 21.3$ eV, is due to electrons scattered by this plasma resonance.