  Attosecond spectroscopy is currently restricted to photon energies around 100
eV. We show that under these conditions, electron-electron scatterings, as the
photoelectrons leave the metal give rise to a tail of secondary electrons with
lower energies and hence a significant background. We develop an analytical
model based on an approximate solution to Boltzmann's transport equation, to
account for the amount and energy distribution of these secondary electrons.
Our theory is in good agreement with the electron spectrum found in a recent
attosecond streaking experiment. To suppress the background and gain higher
energy resolution, photon sources of higher energy could be advantageous.
