  Knowledge of the electron sampling depth and related saturation effects is
important for quantitative analysis of X-ray absorption spectroscopy data, yet
for oxides with the perovskite structure no quantitative values are so far
available. Here we study absorption saturation in films of two of the
most-studied perovskites, LCMO and YBCO, at the L2,3 edge of Mn and Cu,
respectively. By measuring the electron-yield intensity as a function of photon
incidence angle and film thickness, the sampling depth d, photon attenuation
length, lambda, and the ratio lambda/d have been independently determined
between 50 and 300 K. The extracted sampling depth dLCMO=3 nm for LCMO at high
temperatures in its polaronic insulator state (150-300K) is not much larger
than values reported for pure transition metals (d Co or Ni=2-2.5nm) at room
temperature, but it is smaller than dYBCO=3.9 nm for metallic YBCO that is in
turn smaller than the value reported for Fe3O4 (d=4.5 nm) The measured dLCMO
increases to 4.5 nm when LCMO is in the metallic state at low temperatures.
These results indicate that a universal rule of thumb for the sampling depth in
oxides cannot be assumed, and that it can be measurably influenced by
electronic phase transitions that derive from strong correlations.
