electron penetrating into the electron cloud of an atom is attracted
by the positive potential of the nucleus (Coulombic interaction),
and its path is deflected towards the core as a result. The Coulombic
force F is defined as:
= Q1Q2 / 4πε0r2
r being the distance between the charges Q1 and Q2
and ε0 the dielectric constant. The closer the electron
comes to the nucleus, i.e. the smaller r, the larger is F and consequently
the scattering angle. In rare cases, even complete backscattering
can occur (back scattered electrons BSE).
These interactions can be treated as elastic, which means that no
energy is transferred from the scattered electron to the atom.
simple model helps to explain basic contrast mechanisms in TEM and
STEM. The mass-thickness contrast is somehow
related to the contrast in optical microscopy, but it is the local
scattering power that determines contrast in TEM instead of absorption
of light. The interaction of electrons with heavy atoms having a
high charge Q is stronger than with light atoms so that areas in
which heavy atoms are localized appear with darker contrast than
such with light atoms (mass contrast). In thick areas, more electron
scattering events occur of course; thus, these thick areas appear
darker than thin areas (thickness contrast). In particular, this
mass-thickness contrast is important in bright and dark field imaging.