That is, at C 1.2 ! 1, it is close to k gk .
So, the model is straightforward. It is only necessary to establish when it can be
applied. This work was carried out by its creators in the 70-s of the last century. It
was shown that for its applicability, the following requirements must be met:
1. The matrix element of the interaction of the considered levels V 1,2 should be so
small that
2psV 1;2 h
ð5:5:6Þ
(here s is the time of the AB and M collision). Since the collision time is of the
order of 10
–12 s, ħ = 5.3 10
–12 cm
−1 s, then V 1.2
1 cm
−1 should be. If this is
not so, then we must take into account the evolution of the complex during its
existence, i.e., consider the motion of an image point along the PESs of the
complex. The task immediately becomes difficult, and the transition
cross-section cannot be calculated using (5.5.4). It is said that the collision
should be ‘sudden’, i.e., during the collision, the molecule should not rotate (this
is not a very strong condition since the rotation period is approximately two
orders of magnitude longer than the oscillation period).
2. Collision should be ‘weak’; in other words, the PESs corresponding to states 1
and 2 have to be equidistant, the gap between them should not depend on the
distance AB
… M. Otherwise, at least the coefficient C 1,2 depends on R, the C 1,2
value calculated for a free molecule cannot be used. In short, in the event of a
‘strong’ collision, we must consider the transition in the AB
… M complex.
Let us discuss the gateway model as it has been studied for the NO
(a
4 P ! B
2 P, b
4 R) transitions in [24, 25]. The scheme of the experimental set-up
utilized is shown in Fig. 5.6. In general terms, it consists of a device forming a
supersonic molecular beam (a pulse valve, skimmer, diaphragms) and a chamber
with a gas target placed in a large chamber equipped with pumping systems,
pumping speed is 3400 l/s.
Excited molecules are generated by means of dc discharge between a nozzle and
filament. Charged species can be deflected by a field (electric or magnetic) so that,
if necessary, they do not enter into the chamber. The important thing is that one can
obtain a supersonic molecular beam of excited metastable (having a sufficiently
long lifetime for radiation decay or predissociation) molecules, study the processes
that occur when an excited molecule collides once (this is very important) with the
target species, excites or ‘takes away’ it by laser radiation, etc. Of course, there are
several excited states in a beam, but sometimes this fact does not interfere.
So, what will happen if one forms in this way a NO molecule molecular beam?
The NO(X
2 P,v X ) and NO(a
4 P,v a ) metastable (long-lived) states get in the camera
with target species (Fig. 5.7); the A
2 R
+ , B
2 P, C
2 P, and many other states decay
due to rather fast radiative transitions.
What will happen if molecules excited in this way collide with a target species?
In principle, in this case, a collision-induced transition to the A
2 R
+
, B
2 P, C
2 P states
5.5 Collision-Induced Nonadiabatic Transitions
171
So, the model is straightforward. It is only necessary to establish when it can be
applied. This work was carried out by its creators in the 70-s of the last century. It
was shown that for its applicability, the following requirements must be met:
1. The matrix element of the interaction of the considered levels V 1,2 should be so
small that
2psV 1;2 h
ð5:5:6Þ
(here s is the time of the AB and M collision). Since the collision time is of the
order of 10
–12 s, ħ = 5.3 10
–12 cm
−1 s, then V 1.2
1 cm
−1 should be. If this is
not so, then we must take into account the evolution of the complex during its
existence, i.e., consider the motion of an image point along the PESs of the
complex. The task immediately becomes difficult, and the transition
cross-section cannot be calculated using (5.5.4). It is said that the collision
should be ‘sudden’, i.e., during the collision, the molecule should not rotate (this
is not a very strong condition since the rotation period is approximately two
orders of magnitude longer than the oscillation period).
2. Collision should be ‘weak’; in other words, the PESs corresponding to states 1
and 2 have to be equidistant, the gap between them should not depend on the
distance AB
… M. Otherwise, at least the coefficient C 1,2 depends on R, the C 1,2
value calculated for a free molecule cannot be used. In short, in the event of a
‘strong’ collision, we must consider the transition in the AB
… M complex.
Let us discuss the gateway model as it has been studied for the NO
(a
4 P ! B
2 P, b
4 R) transitions in [24, 25]. The scheme of the experimental set-up
utilized is shown in Fig. 5.6. In general terms, it consists of a device forming a
supersonic molecular beam (a pulse valve, skimmer, diaphragms) and a chamber
with a gas target placed in a large chamber equipped with pumping systems,
pumping speed is 3400 l/s.
Excited molecules are generated by means of dc discharge between a nozzle and
filament. Charged species can be deflected by a field (electric or magnetic) so that,
if necessary, they do not enter into the chamber. The important thing is that one can
obtain a supersonic molecular beam of excited metastable (having a sufficiently
long lifetime for radiation decay or predissociation) molecules, study the processes
that occur when an excited molecule collides once (this is very important) with the
target species, excites or ‘takes away’ it by laser radiation, etc. Of course, there are
several excited states in a beam, but sometimes this fact does not interfere.
So, what will happen if one forms in this way a NO molecule molecular beam?
The NO(X
2 P,v X ) and NO(a
4 P,v a ) metastable (long-lived) states get in the camera
with target species (Fig. 5.7); the A
2 R
+ , B
2 P, C
2 P, and many other states decay
due to rather fast radiative transitions.
What will happen if molecules excited in this way collide with a target species?
In principle, in this case, a collision-induced transition to the A
2 R
+
, B
2 P, C
2 P states
5.5 Collision-Induced Nonadiabatic Transitions
171