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The ionization spectrum characterizes the pre-surface layer, the effective depth of which
 depends on:
mean free pass of the electrons forming the ionization line;
geometry of experiment (primary electrons angle of incidence and secondary electrons escape angle);
primary electron energy Ep and the energy Ep-
of the electrons forming the ionization line.
Using approximate 
dependency (the Handbook's software application provides a possibility to estimate this dependency
for simple substances), it is possible to estimate in the wide range
of primary electron energies. If Ep>2Ec, and the energy of IL electrons E>100 eV,
then can be calculated from the following expression:
The dependency allows to investigate the following sample
characteristics with the IS technique:
homogenity of the elemental composition in the pre-surface layer [Ref 2, Ref 3];
segregation of one or several elements;
compositional changes during adsorption and chemical reactions in the pre-surface layer [Ref 3];
continuity of thin layers and others
[Ref 2].
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