Photoemission from Cu valence bands using 50-175-eV synchrotron radiation

Photoemission spectra of the $3d$ valence band of polycrystalline Cu were studied using synchrotron radiation of energy $50\le h\nu \le 175$ eV. The detailed shape of the spectrum was found to change distinctly with photon energy. The observed energy dependence was compared to calculated photoemission energy distributions (PED's) assuming a direct transition model. PED's obtained with this model predicted the experimental intensity distribution quite well for $h\nu <70\mathrm{}$ eV and $h\nu >120\mathrm{}$ eV but failed in the region $h\nu \approx 90$ eV. Fair agreement between experiment and theory was obtained when momentum broadening in the final state was included. The largest broadening was required around $h\nu =90\mathrm{}$ eV. A minimum in the photoelectron mean free path at this energy is discussed as a possible source of broadening. The observed changes in spectral shape for $50\le h\nu \le 70$ eV are attributed to direct transitions; the changes are found to arise mainly from the angular part of the transition matrix element.