Chemistry Reference
In-Depth Information
Depending on the pressure, excited rare gas molecules in vibrationally excited
states are produced in three-body collision processes of the lowest excited rare gas
atoms with two rare gas atoms in their ground state via the reaction
Rg
∗
+
→
Rg
2
(
)
+
2Rg
v
0
Rg.
(8.17)
In a next step, the highly vibrationally excited molecules either decay radiatively
toward the unstable ground state connected with the dissociation into two rare gas
atoms according to
Rg
2
(
v
0
)
→
2Rg
+
h
ν
1
(8.18)
or undergo vibrational relaxation
Rg
2
(
Rg
2
(
v
0
)
+
Rg
→
v
≈
0
)
+
Rg
(8.19)
with subsequent radiative dissociation of the relaxed lowest vibrational state into two
rare gas atoms
Rg
2
(
v
≈
0
)
→
2Rg
+
h
ν
2
.
(8.20)
These radiative transitions are attributed to bound-free transitions, and they result
in the first (8.18) and second (8.20) continuum. Depending on the pressure, the
radiation process (8.18) competes with the vibrational relaxation process (8.19).
In addition, the radiation process (8.20) is in competition with the quenching of
Rg
2
(
≈
)
by rare gas atoms and by itself, which leads to a decrease of the efficiency
of the emission at high pressures.
As an example, the pumping mechanism involved in the formation of xenon
excimers in DBDs is illustrated in Figure 8.18. The reaction scheme includes elec-
trons, ground state xenon (Xe
v
0
(
1
S
0
)
) atoms, the excited atomic (Xe
(
3
P
2
)
,Xe
(
3
P
1
)
,
Xe
∗
,Xe
∗∗
) and molecular (Xe
2
(
1
u
,0
u
;
v
≈
0
)
,Xe
2
(
0
u
;
v
≈
0
)
,Xe
2
(
1
u
,0
u
;
v
0
)
,
Xe
2
(
) states, and the atomic (Xe
+
) and molecular (Xe
2
) ions, where Xe
∗
and Xe
∗∗
represent the lumped 6s' and 6p levels, respectively. Electrons generated
in the microdischarge excite and ionize the xenon atoms according to the reactions
given by (8.13) through (8.16). The main electron loss in the discharge results from
the electron-ion recombination
0
u
;
v
0
)
Xe
2
e
−
→
Xe
∗∗
+
+
Xe
(
1
S
0
)
,
(8.21)
where the molecular ions are the predominant positive charge carrier. They are rapidly
produced via the three-body reaction
Xe
+
1
S
0
)
→
Xe
2
1
S
0
)
+
2Xe
(
+
Xe
(
.
(8.22)
The xenon excimer radiation results from the reaction processes given by (8.17)
through (8.20), where Rg
∗
denotes Xe
(
3
P
2
)
and Xe
(
3
P
1
)
, respectively. At pressures
