unchanged. The mechanisms of such processes have not yet been fully studied.
However, it can be argued that if the energy loss of the excited species is large, then
the deactivation rate constant can be significant only in two cases:
1. In the collision of this species, for example, an excited atom A* or a AB*
molecule with M, an excited complex (A
… M)*, (AB
… M)* with energy not less
than the energy of its dissociation are formed. A complex then either dissociates
or predissociates into species A**, AB** with lower excitation energy, and M
(M*, M
# ). If the (A
… M)*, (AB
… M)* predissociation rate is comparable to the
rate of its dissociation, then the probability of deactivation of A*, AB* in a
collision with M is high. This process does not differ from those that we considered in Sect. 5.6. It should be noted that a deactivation rate constant to be
large, a matrix element of the non-adiabatic interaction of the complex states
that formed in the collision and the one through which the predissociation
occurs has to be large as well. Otherwise, the complex should have a sufficiently
long lifetime, i.e., be triatomic at least (see below).
2. An almost resonant transfer of excitation energy A*, AB* to M,
A
à AB
Ã
ð
ÞþM ! A
ÃÃ AB
ÃÃ
ð
ÞþM
à M
#
À
Á þ DE
ð5:7:1Þ
and the kinetic energy of the scattering partners DE ¼ E A
à AB
Ã
ð
ÞÀ
E A
ÃÃ AB
ÃÃ
ð
Þ
þ E M
à M
#
ð Þ is small, comparable to kT.
Both types of these processes are relatively well understood. The first of them,
once called the complex-predissociation mechanism of deactivation (the term has
not taken root), can be illustrated by the example of deactivation of O(
1 D) by Xe
atoms and CO(X
1 R
+
) molecules.
The deactivation rate constant
O
1 D
À Á þ Xe ! O
3 P
À Á þ Xe
ð5:7:2Þ
is k 7.2 = 7 10
–11 cm
3 /s, and that of
O
1 D
À Á þ CO ! O
3 P
À Á þ CO
ð5:7:3Þ
k 7.3 = 4 10
–11 cm
3 /s. The O(
1 D) atom excitation energy is E = 1.97 eV. The
spin-orbit interaction in the XeO excimer is so large that the XeO complex with a
probability of about 0.3 predissociates during one vibration, although this process is
forbidden due to the Wigner rule [46], and with a probability of 0.7 dissociates
(Fig. 5.21) [47].
In the complex O…CO, i.e., in a CO 2 (
1 B 2 ,
1 A 2 ) molecule with excitation energy
equal to the dissociation energy of O(
1 D) + CO(X
1 R
+
) (Fig. 5.22), the spin-orbit
interaction is orders of magnitude lower, but the lifetime is orders of magnitude
longer with respect to dissociation. During this time, an image point repeatedly
192
5 Energy Transfer in Collisions
However, it can be argued that if the energy loss of the excited species is large, then
the deactivation rate constant can be significant only in two cases:
1. In the collision of this species, for example, an excited atom A* or a AB*
molecule with M, an excited complex (A
… M)*, (AB
… M)* with energy not less
than the energy of its dissociation are formed. A complex then either dissociates
or predissociates into species A**, AB** with lower excitation energy, and M
(M*, M
# ). If the (A
… M)*, (AB
… M)* predissociation rate is comparable to the
rate of its dissociation, then the probability of deactivation of A*, AB* in a
collision with M is high. This process does not differ from those that we considered in Sect. 5.6. It should be noted that a deactivation rate constant to be
large, a matrix element of the non-adiabatic interaction of the complex states
that formed in the collision and the one through which the predissociation
occurs has to be large as well. Otherwise, the complex should have a sufficiently
long lifetime, i.e., be triatomic at least (see below).
2. An almost resonant transfer of excitation energy A*, AB* to M,
A
à AB
Ã
ð
ÞþM ! A
ÃÃ AB
ÃÃ
ð
ÞþM
à M
#
À
Á þ DE
ð5:7:1Þ
and the kinetic energy of the scattering partners DE ¼ E A
à AB
Ã
ð
ÞÀ
E A
ÃÃ AB
ÃÃ
ð
Þ
þ E M
à M
#
ð Þ is small, comparable to kT.
Both types of these processes are relatively well understood. The first of them,
once called the complex-predissociation mechanism of deactivation (the term has
not taken root), can be illustrated by the example of deactivation of O(
1 D) by Xe
atoms and CO(X
1 R
+
) molecules.
The deactivation rate constant
O
1 D
À Á þ Xe ! O
3 P
À Á þ Xe
ð5:7:2Þ
is k 7.2 = 7 10
–11 cm
3 /s, and that of
O
1 D
À Á þ CO ! O
3 P
À Á þ CO
ð5:7:3Þ
k 7.3 = 4 10
–11 cm
3 /s. The O(
1 D) atom excitation energy is E = 1.97 eV. The
spin-orbit interaction in the XeO excimer is so large that the XeO complex with a
probability of about 0.3 predissociates during one vibration, although this process is
forbidden due to the Wigner rule [46], and with a probability of 0.7 dissociates
(Fig. 5.21) [47].
In the complex O…CO, i.e., in a CO 2 (
1 B 2 ,
1 A 2 ) molecule with excitation energy
equal to the dissociation energy of O(
1 D) + CO(X
1 R
+
) (Fig. 5.22), the spin-orbit
interaction is orders of magnitude lower, but the lifetime is orders of magnitude
longer with respect to dissociation. During this time, an image point repeatedly
192
5 Energy Transfer in Collisions