line, and superposition of the symmetric and antisymmetric stretching vibrations. If
the CO 2 PES is considered in other coordinates, then one can notice that in the
general case, bending vibrations should also be observed. This feature leads to a
relatively strong assertion that collinear decay of linear molecules is unlikely, i.e., it
is a small fraction of noncollinear. Therefore, the decay of any triatomic molecule is
described by the laws of the C 1 or C s symmetry point group. Therefore, correlation
diagrams between the electronic states of linear molecules and their dissociation
products should be built not in the C ∞v and D ∞h symmetry groups but in C 1 or C s
groups, respectively. Due to the non-crossing rule, a decrease in symmetry during
the decay of an excited state can lead to the appearance of potential barriers along
the decay coordinate, which are absent during collinear decay of a molecule
fragments. A similar effect is observed in CO 2 , N 2 O excited states, and we will
discuss this feature below.
So, a polyatomic molecule can make quite a large (tens or more) oscillations
before dissociation, during which stochastization of vibrational energy can occur,
i.e., its pumping from one vibrational mode to another, followed by a statistical
equilibrium (in this electronic state) distribution. If the coupling between the different modes in the AB 2 molecule is strong enough and excitation energy is close to
the dissociation threshold, the stochastization time should be no more than 10
–11 s
(see [72, 73, 74] and references).
As is known, the absorption spectrum of a diatomic molecule, if it corresponds
to a transition to the purely repulsive part of the bound state, is well approximated
by a Gaussian or a Gaussian corrected by the frequency factor (see Sect. 4.3). In
polyatomic molecules, this feature also takes place. Examples are A absorption
bands of alkyl- and perfluoroalkyl iodides RI, R = CH 3 , CF 3 , etc. However, this is
not always observed. According to calculations, for some properties of the upper
repulsive state of linear symmetric triatomic molecules AB 2 (at least), a structure
can be superimposed on the smooth dissociative absorption spectrum corresponding
to the transition to this state due to vibrational excitation of the AB 2 fragment
during dissociation (see [3, 75–77] and references).
A careless approach to the interpreting the decay processes of polyatomic
molecule excited states with energy higher than the adiabatic energy of their dissociation can lead to errors. An example is the photochemistry of N 2 O molecule.
When interpreting photoprocesses in N 2 O molecule in the spectral region shorter
than the corresponding first allowed decay channel
N 2 O e
X
1 R
þ
À
Á þ hv ! N 2 X
1 R
þ
g
þ O
1 D
À Á
ð4:8:1Þ
several authors considered a priori that since luminescence is absent, then the
quantum yield of N 2 O photodecay is u N 2 O ðkÞ = 1 at any pressure (see [4, 9] and
references). As it turned out, this is not true. At p N 2 > 1.5 Torr, the u N 2 O ðkÞ value
decreases with photon energy at k < 180 nm, and close to zero in the k = 158–
162 nm spectral range Fig. 4.27.
142
4 Photolysis of Free Molecules
the CO 2 PES is considered in other coordinates, then one can notice that in the
general case, bending vibrations should also be observed. This feature leads to a
relatively strong assertion that collinear decay of linear molecules is unlikely, i.e., it
is a small fraction of noncollinear. Therefore, the decay of any triatomic molecule is
described by the laws of the C 1 or C s symmetry point group. Therefore, correlation
diagrams between the electronic states of linear molecules and their dissociation
products should be built not in the C ∞v and D ∞h symmetry groups but in C 1 or C s
groups, respectively. Due to the non-crossing rule, a decrease in symmetry during
the decay of an excited state can lead to the appearance of potential barriers along
the decay coordinate, which are absent during collinear decay of a molecule
fragments. A similar effect is observed in CO 2 , N 2 O excited states, and we will
discuss this feature below.
So, a polyatomic molecule can make quite a large (tens or more) oscillations
before dissociation, during which stochastization of vibrational energy can occur,
i.e., its pumping from one vibrational mode to another, followed by a statistical
equilibrium (in this electronic state) distribution. If the coupling between the different modes in the AB 2 molecule is strong enough and excitation energy is close to
the dissociation threshold, the stochastization time should be no more than 10
–11 s
(see [72, 73, 74] and references).
As is known, the absorption spectrum of a diatomic molecule, if it corresponds
to a transition to the purely repulsive part of the bound state, is well approximated
by a Gaussian or a Gaussian corrected by the frequency factor (see Sect. 4.3). In
polyatomic molecules, this feature also takes place. Examples are A absorption
bands of alkyl- and perfluoroalkyl iodides RI, R = CH 3 , CF 3 , etc. However, this is
not always observed. According to calculations, for some properties of the upper
repulsive state of linear symmetric triatomic molecules AB 2 (at least), a structure
can be superimposed on the smooth dissociative absorption spectrum corresponding
to the transition to this state due to vibrational excitation of the AB 2 fragment
during dissociation (see [3, 75–77] and references).
A careless approach to the interpreting the decay processes of polyatomic
molecule excited states with energy higher than the adiabatic energy of their dissociation can lead to errors. An example is the photochemistry of N 2 O molecule.
When interpreting photoprocesses in N 2 O molecule in the spectral region shorter
than the corresponding first allowed decay channel
N 2 O e
X
1 R
þ
À
Á þ hv ! N 2 X
1 R
þ
g
þ O
1 D
À Á
ð4:8:1Þ
several authors considered a priori that since luminescence is absent, then the
quantum yield of N 2 O photodecay is u N 2 O ðkÞ = 1 at any pressure (see [4, 9] and
references). As it turned out, this is not true. At p N 2 > 1.5 Torr, the u N 2 O ðkÞ value
decreases with photon energy at k < 180 nm, and close to zero in the k = 158–
162 nm spectral range Fig. 4.27.
142
4 Photolysis of Free Molecules