Depth Dependence

The conversion and Auger electrons produced by the resonant event have a finite range in the material of the absorber due to interactions with other charged particles. The range is dependent on both the mean free path of the electron in the absorber and the initial kinetic energy of the electron. This is illustrated in Figure 2.8. The dependence is essentially an inverse exponential one with the majority of the signal coming from the uppermost 1000Å.

Figure 2.8: Probability of a 7.3keV K-conversion electron reaching the absorber surface in metallic iron.

This depth dependency is a double-edged sword in that the quite shallow detection depth may prevent the probing of deeper structures and that surfaces have to be kept relatively clean but it also prevents such things as buffer and seed layers in multilayer samples contaminating the signal. In the samples studied in this thesis using CEMS the whole of the thin film or multilayer is 2000Å or less and deposited onto thick, opaque substrates making this technique a practical solution.

The depth dependency can also be combined with the selection of electron energies to obtain separate spectra for different depths into the sample. This is known as Differential CEMS or DCEMS.