Measurement of the partial photoionization cross section and the asymmetry parameter of the Ag valence band near the 4d threshold.

Abstract

Measurements of the partial photoionization cross section (\ensuremath{\sigma}) and of the photoelectron asymmetry parameter (\ensuremath{\beta}) have been performed on polycrystalline Ag around the 4d threshold. A description of the technique used to obtain \ensuremath{\sigma} and \ensuremath{\beta} from the photoemission spectra is provided. It is shown that in the condensed phase, the photoelectron asymmetry parameter can be experimentally determined much more accurately than the partial cross section, because many experimental uncertainties, such as the dependence of the intensity of the light source with the photon energy, the variation of the electron mean free path, and the transmission and counting efficiency of the electron analyzer as a function of the kinetic energy of the photoelectron, which introduce a considerable amount of distortion in the measurements of \ensuremath{\sigma}, do not play a role in determining \ensuremath{\beta}. Near the threshold, the \ensuremath{\sigma} from bulk Ag is very close to the atomic results, whereas the \ensuremath{\beta} is severely distorted with respect to the free atom, being essentially equal to zero. We propose that the isotropy of the photoelectron angular distribution in polycrystalline Ag is primarily a consequence of a double randomization by a diffraction process of the final states due to the crystal potential and by random orientation of the crystallites. We also point out that there is an interesting analogy between the results of our photoemission experiment and the ``Debye-Scherrer pattern'' of an x-ray-diffraction experiment from a powder of crystals oriented at random.