The inelastic mean free path (IMFP) is an index of how far an electron on average travels through a solid before losing energy.
If a monochromatic primary beam of electrons is incident on a solid surface, the majority of incident electrons lose their energy because they interact strongly with matter, leading to plasmon excitation, electron-hole pair formation, and vibrational excitation. The intensity of the primary electrons,
I
0
, is damped as a function of the distance, d, into the solid. The intensity decay can be expressed as follows:
I
(
d
)
=
I
0
e
−
d
/
λ
(
E
)
where
I
(
d
)
is the intensity after the primary electron beam has traveled through the solid. The parameter
λ
(
E
)
, termed the inelastic mean free path (IMFP), is defined as the distance an electron beam can travel before its intensity decays to
1
/
e
of its initial value. The inelastic mean free path of electrons can roughly be described by a universal curve, which is the same for all materials.
