Chemistry Reference
In-Depth Information
In the special case that the particle concentration
n
C
is sufficient high against the
concentration of the other participating particles, e.g., the particle concentration
n
C
represents the neutral background gas in nonthermal atmospheric pressure plasmas,
the production rate
n
C
=
k
AB
ABC
n
C
·
dn
AB
dt
=
n
B
=
k
AB
ABC
k
AB
ABC
·
n
A
·
n
B
·
·
n
A
·
·
n
A
·
n
B
(3.172)
can be described as a quasi-binary collision with an effective rate coefficient
k
AB
ABC
involving the constant concentration
n
C
. Following, the effective rate coefficient is a
function of the neutral gas concentration or pressure at constant temperature.
Taking into consideration the backward reaction of the electron impact ionization
(3.156) the following three-particle recombination reaction has to be considered
k
ee
A
+
−→
e
+
e
+
A
+
A
+
e
,
(3.173)
dn
A
dt
=−
dn
A
+
dt
=−
dn
e
dt
=
k
ee
A
+
·
n
e
·
n
A
+
.
(3.174)
Contrary to the detailed equilibrium in CTE, in the nonequilibrium plasma the rate
coefficients for forward and backward reactions, e.g., electron impact ionization
k
A
+
e
A
and three-body recombination
k
ee
A
+
, and the corresponding reaction velocities are
quite different, see Section 3.2.1.
3.5.6 R
EACTION
R
ATE
C
OEFFICIENT OF
C
HEMICAL
R
EACTIONS
In the general case of chemical reactions in nonthermal plasmas the rate coefficient
k
AB
ofareactionbetweentwoparticlesAandBrepresentsthereactionratecoefficient.
k
AB
−→
A
(
a
)
+
B
(
b
)
C
(
c
)
+
D
(
d
)
.
(3.175)
Taking into consideration the binary collisions of particles at different states of
internal energy (
a
,
b
,
c
,
d
) the reaction rate of the product concentration
n
C
depends
on the concentration of the reagents
n
A
and
n
B
as well as the reaction rate coefficient
k
AB
characterizing the probability of this chemical reaction [30].
dn
C
(
c
,
d
|
a
,
b
)
=
k
AB
(
C,D;
c
,
d
|
A, B;
a
,
b
)
·
n
A
(
a
)
·
n
B
(
b
)
.
(3.176)
dt
The averaging
over the distribution function
f
AB
, and the sum over all
internal energy states of the reaction partners provide the total reaction rate coefficient
of the chemical reaction between the particles A and B which produce the particles
C and D.
v
AB
·
σ
AB
k
(
C, D
|
A, B
)
=
x
A
(
a
)
·
x
B
(
b
)
·
k
(
C,D;
c
,
d
|
A,B;
a
,
b
)
.
(3.177)
abcd
