An atom is in a metastable state
if the transition into its ground state via photon
emission is quantum mechanically forbidden. During
its lifetime, the atom carries its excitation
energy in the form of potential energy. The most
commonly used atom in MIES source is He (the 3S1
state at 19.82 eV has a lifetime of ~4000 seconds.)
Collision of the slow noble gas atoms (in excited
metastable states) with the surface gives rise
to neutralization and also electron emission (hence,
the name Metastable Impact Electron Spectroscopy).
See also INS - Ion Neutralisation
Spectroscopy.
In general, during de-excitation,
the 2s-electron tunnels into an empty level of
the surface (target) atoms. This is followed by
an Auger Neutralization or Auger De-excitation
process, in which an electron from the target
atom is ejected (and measured using standard Hemispherical
Analyzers).
- Advantage:
The metastable atoms only carry thermal energies,
and do not penetrate into the solid. The consequence
is excellent surface sensitivity. Only those
surface atoms which come into contact with
the impinging metastable atoms emit electrons.
An MIES spectrum is not convoluted with bulk
information, as in the case of UPS or XPS.
- Disadvantage:
Interpretation is never unambiguous ! due
to the participation of more than one de-excitation
mechanism.
