Chemistry Reference
In-Depth Information
A detailed view on the dynamics of particle densities, charge density, plasma
potential, and the electron velocity distribution resulting from the PIC-MCC simula-
tion gives a good understanding of the processes underlying the observed excitation
patterns. Respectively, the axial profile of the plasma potential (a), the axial density
distribution of the electrons, molecular oxygen ions, negative oxygen ions and charge
density (b), the profile of the electron axial velocity component distribution (c), and
the spatiotemporally resolved dissociative electron impact excitation rate profile for
the corresponding emission at 844.6 nm (d) from the simulation for the oxygen rf
discharge at
p
800 V are shown in Figures 9.23 through 9.27,
corresponding to five different phases of the rf cycle. The white bar in the graph of the
spatiotemporally resolved excitation rate profile indicates the actual phase position.
The dissociative electron impact excitation is dominant in oxygen, because its cross
sections and the density of O
2
molecules in comparison with atomic oxygen are large,
even if the threshold values for dissociative excitation are about 5eV larger than those
for direct excitation [128]. An evidence for that can be found in measurements of the
rise time of excited atomic oxygen, which show that the emission appears already
before the ground state atomic oxygen density reaches its constant value of saturation
[119,129].
=
60 Pa and
U
RF
=
800
600
Hot energy tails
400
200
15
10
0
X
(mm)
(a)
0
5
X
(mm)
10
15
5
v
x
(
v
the
)
0
-4
-2
1×10
10
0
2
4
O
2
+
(c)
e
-
15
10
5×10
9
Charge density
O
-
5
0
0
0
0
20
40
60
80
100
120
t
(
ns
)
(b)
5
X
(mm)
10
15
(d)
FIGURE 9.23
The axial profile of the plasma potential (a), the axial density distribution of
the electrons, molecular oxygen ions, negative oxygen ions and charge density (b), the profile
of the electron axial velocity component distribution (c), and the spatiotemporally resolved
excitation rate profile for the corresponding emission at 844.6 nm (d) from the simulation
for the oxygen rf discharge at
p
=
60 Pa and
U
RF
=
800 V. The white bar in the graph of
the spatiotemporally resolved excitation rate profile indicates the actual phase position. The
temporal position within the rf cycle is 0ns.
