The quantum efficiency, eta, of gold and copper photocathodes has been meas
ured in the photon-energy range from 50 eV to 10 keV. The experiments were
performed with monochromatized synchrotron radiation at the radiometry labo
ratories 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 r
adiometer as the primary detector standard. The samples were cleaned, store
d and examined in an ultra-high-vacuum chamber. The quantum efficiency of c
leaned 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 relativ
e course of the absorption coefficient was also measured with relative stan
dard uncertainties below 0.04. As proportionality was only locally fulfille
d, a more sophisticated model for quantum efficiency was developed.