Photoemission broadening of Fermi-liquid systems, and its relevance to high-temperature superconductors

We measured at high energy and angular resolution the spectral width of photoemission features near the Fermi energy for Ag(100) and for the nearly-two-dimensional metal 2H-NbSe₂. The results do not agree with the quadratic width-energy relationship predicted for the lifetime broadening of a Fermi liquid: We found a nearly inverse linear relationship for the sp band of Ag(100), and a similar behavior in the case of NbSe₂. These results indicate that the broadening is primarily affected by extrinsic factors rather than by the lifetime---and also that the ``anomalous'' broadening of the photoemission spectra found for the normal state of the high-temperature superconductors cannot be automatically interpreted as evidence of non-Fermi-liquid behavior.