Environmental Engineering Reference
In-Depth Information
where
A
is a constant, we have for the cross section of atom quenching resulting
from collision with a slow electron of energy
ε
D
E
Δ
ε
Δ
ε
Δ
ε
g
p
E
g
0
σ
D
A
.
(2.55)
q
Δ
ε
From this it follows that the rate constant for atom quenching by a slow electron
(
m
e
is the electron mass)
Δ
ε
p
2
g
p
m
e
A
g
0
k
q
D
v
σ
D
,
(2.56)
f
q
and the rate constant
k
ex
for atom excitation by electron impact is
r
k
q
g
g
0
ε
Δ
ε
Δ
ε
k
ex
D
.
(2.57)
It is of importance that the quenching rate constant
k
q
does not depend both on
the electron energy and on the energy distribution function for slow electrons. It
depends only on the parameters of the transition between atomic states, so the
quenching rate constant is a convenient parameter characterizing also excitation of
atoms by electron impact near the threshold. In particular, in the case of quenching
of a resonantly excited state when this process is effective, the quenching rate con-
stant within the framework of the perturbation theory using formula (2.49) gives
approximately [92]
g
0
f
0
g
(
k
0
k
q
D
const
)
3/2
D
,
(2.58)
)
7/2
(
Δ
ε
Δ
ε
τ
0
where
f
0
is the oscillator strength for this transition,
τ
0
is the radiative lifetime
of the resonantly excited state, and
λ
is the wavelength of the emitted photon; const
Ta b l e 2 . 7
Parameter
k
0
in formula (2.58) obtained from data in the references cited. This pa-
rameter is expressed in units of 10
5
cm
3
/s if
Δ
ε
is in electronvolts and
τ
0
is in nanoseconds.
T
e
,10
3
K
6
8
10
12
K(4
2
P
) [90]
-
4.1
4.2
3.9
Rb(5
2
P
) [90]
5.5
5.7
4.4
3.8
Cs(6
2
P
) [90]
3.4
3.1
2.8
3.4
K(4
2
P
) [79]
5.2
4.8
5.0
5.4
Rb(5
2
P
) [79]
4.6
4.8
5.0
5.0
Cs(6
2
P
) [79]
4.3
4.4
4.5
4.8
K(4
2
P
) [91]
3.4
3.7
4.1
4.2
Rb(5
2
P
) [91]
3.7
3.7
4.0
4.0
Cs(6
2
P
) [91]
3.9
4.3
4.6
4.9
