74
INTERPRETATION
OF THE
ELECTRON
SPECTRUM
displayed
as
differentials)
to
atomic concentrations.
In
this section,
quantification
will
be
primarily concerned with homogeneous samples.
The
situation
is
more complicated
for
samples which have
films at
their
surface
which
are
either thinner than
the
information depth
of the
tech-
nique
or
discontinuous.
3.3.1 Factors
affecting
the
quantification
of
electron
spectra
There
are
many factors which must
be
considered when attempting
to
quantify
electron
spectra,
these
are
either sample-related factors
or
spectrometer-related factors.
Sample
related factors include
the
following.
• The
cross-section
for
emission, which
is the
probability
of the
emission
of
an
electron
due to the
effect
of the
incoming radiation
(X-ray
photon
in
XPS or
electron
in
AES).
The
cross-section depends upon
a
number
of
factors
such
as
o
the
element under investigation,
o
the
orbital
from
which
the
electron
is
ejected,
o
the
energy
of the
exciting radiation.
In
general,
for XPS
measurements,
the
cross-section
for
photoelectron
emission
from
a
given orbital quantum number increases with
atomic
num-
ber for a
given series
of
core levels, such
as 1s, 2p,
etc.
For AES the
cross-
section
depends upon atomic number
in a
more complex manner, passing
through
a
maximum
for
each transition type (KLL, LMM, MNN, etc.).
• The
escape depth
of the
electron emitted
from
the
atom which depends
upon
o
its
kinetic energy (the escape depth passes through
a
minimum with
increasing
kinetic energy,
the
minimum occurs
in the
region
of 20 to
50
eV),
o
the
nature
of the
sample.