Interpretation of empty states spectrum is usually started with comparison of
measured  values with the values of Ec. Let us remind that
 |
(1.6.1) |
while Ec>> . The Ec values are normally taken from XPS (Ec, measured by this
technique is referenced to the Fermi level). As it follows from (1.6.1), usually >Ec. The inequality  >Ec is indeed true for many investigated substances.
Therefore, the energetic structure of empty states, the value and position of the empty
states maximum are all easily determined by IS with the help of (1.6.1). Small
deviations, which are rarely observed have not yet found a detailed explanation.
The dependency =f(Ep)
can be partially caused by the differences in the
core level binding energies for surface and bulk atoms and by the influence of the
empty surface states at the solid - vacuum interface. With Ep decreasing, the
measured values of will become more and more characteristic for surface atoms,
because of reduction of the effective analysis depth (paragraph 1.3).
It is problematic to compare and Ec in some elements, though it does not
impede qualitative identification and elemental quantitative analysis by IS technique.
But interpretation of the empty states spectrum and determination of both suffer from
complications in these cases. One of such problem's solutions may be a comparison of
data from different types of threshold spectroscopy and numeric techniques of
empty states spectrum recovery from nearthreshold fine structure.
In appendix 1 in [Ref 8] a table is given showing binding energies
of core levels for isolated atoms relative to the vacuum level. It can be used for
clarifying of ionization spectra. It is discussed in more details in paragraph 7.1
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