Environmental Engineering Reference
In-Depth Information
An excimer molecule is an excited molecule
A
B
such that the corresponding
molecule
AB
in the ground state of atom
A
does not have a stable bound state,
as usually occurs for an inert gas atom. Excimer molecules are used in excimer
lasers, which operate on bound-free molecular transitions. Atoms of inert gases
neon, argon, krypton, and xenon in the ground state have a filled electron shell (p
6
)
and cannot form any chemical bonds. The lowest excited states of these atoms have
the electron shell configuration p
5
s, so their valence electron is found in an
s
state,
as are valence electrons of alkali metal atoms. Therefore, an alkali metal atom is a
suitable model for an excited inert gas atom.
Excited states of inert gas atoms have ionization potentials that are similar to
those of alkali metal atoms in the ground state. For example, the lowest resonantly
excited states of the inert gas atoms Ar(
3
P
1
)andKr(
3
P
1
) have ionization potentials
of 4.14 and 3.97 eV, whereas the corresponding alkali metal atoms in the ground
states K(4
2
S
)andRb(5
2
S
) have ionization potentials of 4.34 and 4.18 eV. The anal-
ogy between excited atoms of inert gases and atoms of alkali metals in their ground
states means that excited inert gas atoms form strong chemical bonds with halo-
gen atoms. For example, the dissociation energy of the lowest state of the excimer
molecule KrF(
2
B
1/2
)is5.3eV.
The radiative lifetime of excimer molecules is of the order of that for excited
atoms of inert gases, specifically, in the range 10
7
10
8
s. Therefore, generation
of excimer molecules requires short pulses of energy, and electron beams or ultra-
high-frequency gas discharges are commonly used for this purpose. In these cases
a pulse of electric energy is transformed into the UV radiation energy emitted by
excimer molecules, and the efficiency of this transformation reaches tens of per-
cents. Figure 2.22 contains radiative parameters of diatomic excimer molecules
consisting of inert gas and halogen atoms.
The transformation of the initial electron energy to energy of radiation starts
from the formation of excited atoms as a result of excitation by electron impact. In
the following stage of the process, these excited atoms react with molecules con-
taining a halogen or oxygen. This chemical reaction proceeds according to the so-
called harpoon mechanism (see Figure 2.23). This refers to a mechanism in which
an atom loses an electron during a collision, with that electron becoming attached
to the other participant in the collision - a molecule that contains a halogen atom.
The Coulomb attraction of the resulting oppositely charged ions causes a close ap-
Ta b l e 2 . 13
The rate constants for pro-
cess (2.93) given in units of 10
33
cm
6
/s at
room temperature [27]. The accuracy class-
es are as follows: A - the accuracy is better
than 10%, B - the accuracy is better than 20%,
C - the accuracy is better than 30%, D - the
accuracy is worse than 30%.
A
Ne(
3
P
2
) (
3
P
0
) (
3
P
1
) (
3
P
2
) (
3
P
0
) (
3
P
1
) (
1
P
1
) r(
3
P
2
)
K
0.23(B)
0.5(C)
5.8(C)
10(A)
11(D)
12(D)
14(D)
36(C)
A
Kr(
3
P
0
) r(
3
P
1
) r(
1
P
1
) (
3
P
2
) (
3
P
0
) (
3
P
1
) H (
3
P
0
) H (
3
P
1
)
K
54(D)
30(C)
1.6(A)
55(D)
40(D)
70(C)
250(A)
160(C)
