
80
Laser-Induced Damage in Optical Materials
is required. Itisgiven by the inverse product of density of colliding electrons n
e
, colli-
sion cross section σ, and average relative speed v
e
of colliding electrons (Ashcro and
Mermin 1976):
τ
ν
ec
ec
n
==
, (4.2)
where ν
ec
is the frequency of the collisions. Since electron–electron collisions are
impossible if the conduction band is totally empty and the valence band is com-
pletely occupied (Ashcroft and Mermin 1976), the density of colliding electrons of
Equation 4.2 equals the density of excited electrons departed from the valence band.
The collision time τ
ec
of Equation 4.2 reaches a lower li ...