approximation to the real, accurate picture of the process; we are not given to know
the latter. Hereinafter, we will discuss various relaxation models in which nonadiabatic processes take place, and the author will convince you: ‘… and then a
spontaneous process occurs in a molecule’ or ‘… and then a spontaneous process
occurs in a complex.’ But we must remember all the time: the adiabatic process, the
non-adiabatic process are all approximations with which we create models that help
us get closer to understanding the essence of the process and somehow describe it in
a consistent (at this stage of our development). Of course, a more accurate model is
the one in which we consider the transition in a complex, rather than in an isolated
molecule. But the statement is also obvious that it is much simpler to describe
processes in a diatomic molecule than in a triatomic complex. And if we are able to
describe the process of energy transfer, assuming that the partner that collides with
the molecule produces a perturbation, and the process occurs in a free (isolated)
molecule, and not in a complex, i.e., if the conclusions from our theory describe the
experimental data well, then this is what should be done. Naturally, all models of
vibrational relaxation, which we have discussed earlier, and which fit into the
framework of the adiabatic principle, are also approximate.
And now we turn to the consideration of various models that describe the processes
of energy transfer and do not fit into the framework of adiabatic approximations.
5.5 Collision-Induced Nonadiabatic Transitions
The first systematic research in this area of chemical physics was carried out more
than 40 years ago. Almost simultaneously, the first attempts were made to create
theoretical models describing the data obtained in experiments. Several models of
collision-induced nonadiabatic transitions (CINATs) have been created, but even
now, only a few of them have at least some predictive power. Most of the models,
at best, explain the results of experiments, for which they were created.
The CINATs in different molecules obey laws that can be roughly divided into
several groups. In some cases, non-adiabatic transitions occur between rovibronic
states which mixing is also allowed in a free molecule, in others, the transitions
between the states combining in the CINAT is strictly forbidden in a free molecule;
an example is CINATs in some diatomic homonuclear molecules between electronic states of different g, u parities. Sometimes, it is assumed that large FCFs of
combining levels (of course, in a free molecule) facilitate the transition in collisions,
sometimes this correlation is absent.
CINAT model, in the author’s opinion, can be considered truly successful if it
has predictive power. It should make it possible to calculate the CINAT
cross-sections or to show in which cases these cross-sections can be zero or very
small, i.e., establish the CINAT selection or propensity rules. The term ‘propensity
rule’ is undoubtedly more suitable for many collisional transitions because the
mechanisms of these processes are not well studied, and the rules themselves often
have a phenomenological character.
5.4 The In uence of Nonadiabatic Effects on the Vibrational Relaxation Rate
167
the latter. Hereinafter, we will discuss various relaxation models in which nonadiabatic processes take place, and the author will convince you: ‘… and then a
spontaneous process occurs in a molecule’ or ‘… and then a spontaneous process
occurs in a complex.’ But we must remember all the time: the adiabatic process, the
non-adiabatic process are all approximations with which we create models that help
us get closer to understanding the essence of the process and somehow describe it in
a consistent (at this stage of our development). Of course, a more accurate model is
the one in which we consider the transition in a complex, rather than in an isolated
molecule. But the statement is also obvious that it is much simpler to describe
processes in a diatomic molecule than in a triatomic complex. And if we are able to
describe the process of energy transfer, assuming that the partner that collides with
the molecule produces a perturbation, and the process occurs in a free (isolated)
molecule, and not in a complex, i.e., if the conclusions from our theory describe the
experimental data well, then this is what should be done. Naturally, all models of
vibrational relaxation, which we have discussed earlier, and which fit into the
framework of the adiabatic principle, are also approximate.
And now we turn to the consideration of various models that describe the processes
of energy transfer and do not fit into the framework of adiabatic approximations.
5.5 Collision-Induced Nonadiabatic Transitions
The first systematic research in this area of chemical physics was carried out more
than 40 years ago. Almost simultaneously, the first attempts were made to create
theoretical models describing the data obtained in experiments. Several models of
collision-induced nonadiabatic transitions (CINATs) have been created, but even
now, only a few of them have at least some predictive power. Most of the models,
at best, explain the results of experiments, for which they were created.
The CINATs in different molecules obey laws that can be roughly divided into
several groups. In some cases, non-adiabatic transitions occur between rovibronic
states which mixing is also allowed in a free molecule, in others, the transitions
between the states combining in the CINAT is strictly forbidden in a free molecule;
an example is CINATs in some diatomic homonuclear molecules between electronic states of different g, u parities. Sometimes, it is assumed that large FCFs of
combining levels (of course, in a free molecule) facilitate the transition in collisions,
sometimes this correlation is absent.
CINAT model, in the author’s opinion, can be considered truly successful if it
has predictive power. It should make it possible to calculate the CINAT
cross-sections or to show in which cases these cross-sections can be zero or very
small, i.e., establish the CINAT selection or propensity rules. The term ‘propensity
rule’ is undoubtedly more suitable for many collisional transitions because the
mechanisms of these processes are not well studied, and the rules themselves often
have a phenomenological character.
5.4 The In uence of Nonadiabatic Effects on the Vibrational Relaxation Rate
167