4.8 Dissociation of Polyatomic Molecules. Long-Lived
States of Polyatomic Molecules
Dissociation of polyatomic molecule excited states has a significantly different
character than that of diatomic; the possibility of their consideration in one state
framework is common only. It is true if there are no nonadiabatic processes during
the decay of these states, see below). We have already discussed them in Sect. 3.5,
and here we will dwell on anomalies of the dissociation processes.
Dissociation of a diatomic molecule, i.e., decay of a molecule in a state corresponding to a continuous spectrum (purely repulsive or bound states, but with an
energy higher than the dissociation limit, taking into account potential barriers)
proceeds in half of one vibration, i.e., for a time of the order of 10
–13 s.
The dissociation of a polyatomic molecule, even in a purely repulsive state, as is
seen in Sect. 3.5, cannot be considered as one vibration. Let us examine a dissociation of the bound state of a linear symmetric triatomic CO 2 molecule to O + CO
(valley c) or OC + O (valley d) (Fig. 4.26). One can describe any vibration of a
polyatomic molecule, including dissociation or vibrational predissociation as a
motion of an image point on multidimensional PES. The vibration described by the
motion of an image point along the bisector of the OCO angle (straight line a-a in
Fig. 4.26) is symmetric valence: the OC and CO distances, in this case, are equal.
If an image point motion is carried out along the other half-axis of the ellipse
(straight line b-b), then this is an antisymmetric stretching vibration (see Sect. 3.5).
Let the absorption in CO 2 molecule from the minimum of the ground state corresponds to point C lying higher than the dissociation limits O–CO and OC–O. The
CO 2 dissociation corresponds to the exit of an image point into the valleys c or
d. Therefore, an image point motion when describing the dissociation cannot be
described as straight lines a-a or b-b. It has to correspond to Lissajous figures, C-D
Fig. 4.26 A dissociation of a linear bound state of CO 2 molecule (the bending vibration is frozen)
4.8 Dissociation of Polyatomic Molecules …
141
States of Polyatomic Molecules
Dissociation of polyatomic molecule excited states has a significantly different
character than that of diatomic; the possibility of their consideration in one state
framework is common only. It is true if there are no nonadiabatic processes during
the decay of these states, see below). We have already discussed them in Sect. 3.5,
and here we will dwell on anomalies of the dissociation processes.
Dissociation of a diatomic molecule, i.e., decay of a molecule in a state corresponding to a continuous spectrum (purely repulsive or bound states, but with an
energy higher than the dissociation limit, taking into account potential barriers)
proceeds in half of one vibration, i.e., for a time of the order of 10
–13 s.
The dissociation of a polyatomic molecule, even in a purely repulsive state, as is
seen in Sect. 3.5, cannot be considered as one vibration. Let us examine a dissociation of the bound state of a linear symmetric triatomic CO 2 molecule to O + CO
(valley c) or OC + O (valley d) (Fig. 4.26). One can describe any vibration of a
polyatomic molecule, including dissociation or vibrational predissociation as a
motion of an image point on multidimensional PES. The vibration described by the
motion of an image point along the bisector of the OCO angle (straight line a-a in
Fig. 4.26) is symmetric valence: the OC and CO distances, in this case, are equal.
If an image point motion is carried out along the other half-axis of the ellipse
(straight line b-b), then this is an antisymmetric stretching vibration (see Sect. 3.5).
Let the absorption in CO 2 molecule from the minimum of the ground state corresponds to point C lying higher than the dissociation limits O–CO and OC–O. The
CO 2 dissociation corresponds to the exit of an image point into the valleys c or
d. Therefore, an image point motion when describing the dissociation cannot be
described as straight lines a-a or b-b. It has to correspond to Lissajous figures, C-D
Fig. 4.26 A dissociation of a linear bound state of CO 2 molecule (the bending vibration is frozen)
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141