66
INTERPRETATION
OF THE
ELECTRON SPECTRUM
chemical
shift.
In
most metals there
is a
positive
shift
between
the
elemen-
tal
form
and
mono-,
di- or
trivalent ions
but in the
case
of
cerium
the
very
large
final-state
effects
give rise
to a
negative chemical
shift
of
about
2 eV
between
Ce and
CeCO
2
. This
is,
however,
the
exception
and
most elements
behave
in a
predictable manner.
There
are
various compilations
of
binding energies
and the
most
ex-
tensive
is
that
promulgated
by the
National Institute
of
Standards
and
Technology,
USA
(NIST) which
is
available
free
of
charge over
the
internet
(http://srdata.nist.gov/xps/index.htm).
This provides
a
ready
source
of
standard data with which
the
individual components
of a
spectrum
can be
assigned with
a
high degree
of
confidence.
3.2.2
Electron
induced
Auger
electron
spectroscopy
Historically, electron induced
AES is not
credited with
the
ability
to
yield chemical
state
information. Early examples
of
chemical
effects
in
Auger
spectroscopy
were usually
in
quasi-atomic spectra excited
by X-
rays.
The
reason
for
this neglect
of
chemical
effects
is
twofold.
The
thrust
in the
early development
of AES was the use of
analysers, such
as the
retarding
field or the
cylindrical
mirror analyser, which provided
a
high
level
of
transmission
but at the
expense
of
spectral resolution.
Thus,
the
peaks
from
early Auger spectrometers were very broad
and
superimposed
on a
very intense electron background. This
led to the
practice
of
using phase-sensitive detection
to
acquire
differential
spectra.
Even
if
there were well-defined, chemical information
in the
spectra,
the
practices used
for
spectral acquisition would have
effectively
obliterated
it! The
other reason
is the
superposition
of the
degenerate band struc-
ture
onto
the
shape
of the
Auger peak
in the
case
of WV and CVV
transitions. This
may
lead
to
changes
in
shape
of
Auger transitions
from
different
chemical environments but, generally,
not the
discrete chemical
shift
that
is
observed
in XPS
core levels.
If the two
outer electrons
are
not
valence electrons
(i.e.,
CCC
Auger transitions)
a
sharp peak
may
result
as
observed
for
example
in the KLL
series
of
peaks
of
aluminium
and
silicon,
and the LMM
series
of
copper,
zinc, gallium, germanium,
and
arsenic.
The Ge LMM
Auger spectrum
of
Figure
3.3
show compo-
nents
attributable
to Ge° and
Ge
44+
separated
by
over
8 eV.