Photoelectron Escape Depth and Inelastic Secondaries in High Temperature   Superconductors
  We calculate the photoelectron escape depth in the high temperature superconductor Bi2212 by use of electron energy-loss spectroscopy data. We find that the escape depth is only 3 Ang. for photon energies typically used in angle resolved photoemission measurements. We then use this to estimate the number of inelastic secondaries, and find this to be quite small near the Fermi energy. This implies that the large background seen near the Fermi energy in photoemission measurements is of some other origin. 
M. R. Norman, M. Randeria, H. Ding, and J. C. Campuzano