Quantum efficiencies of gold and copper photocathodes in the VUV and X-ray range

The quantum efficiency, eta, of gold and copper photocathodes has been measured in the photon-energy range from 50 eV to 10 keV. The experiments were performed with monochromatized synchrotron radiation at the radiometry laboratories of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage rings BESSY I and BESSY II. Relative standard uncertainties of eta lower than 0.036 were achieved using a cryogenic electrical-substitution radiometer as the primary detector standard. The samples were cleaned, stored and examined in an ultra-high-vacuum chamber. The quantum efficiency of cleaned gold and copper was found to be reproducible within 2 parts in 10(2) to 3 parts in 10(2). A simple model assumes eta to be proportional to the product of absorption coefficient, mu, and photon energy, h nu. The relative course of the absorption coefficient was also measured with relative standard uncertainties below 0.04. As proportionality was only locally fulfilled, a more sophisticated model for quantum efficiency was developed.